Near Net Shape Preforming by 3D Weaving Process

Transcription

1 Near Net Shape Preforming by 3D Weaving Process A thesis submitted to The University of Manchester for the degree of Doctor of Philosophy In the Faculty of Engineering and Physical Sciences Dhavalsinh Jetavat Textile Composites Group School of Materials

2 Table of Contents List of Figures 2 List of Tables 9 Abstract 10 Declaration 11 Copyright 12 Acknowledgements 13 Chapter: 1 Introduction Introduction Composite manufacturing methods Advanced textile preforms Recognition of New Need Aim and objectives 19 Chapter: 2 3D Weaving Introduction Weaving Braiding Stitching/Nonwoven Knitting D Weaving D weaving by Conventional Weaving Purpose built weaving machine for 3D weaving Critical analysis of existing methods Discussion 47 Chapter: 3 Near net shape manufacturing on conventional 48 weaving machine 3.1 Background Introduction 49 1

3 3.3 Design considerations Selection of the Weaving machine Selection of the structure Selection of yarns Design and development of 3D step woven preforms Design of the 3D fabric with three steps in weft 55 direction Design of the 3D fabric with steps in warp direction Manufacturing Process Warp Beam Preparation Warp let off Shedding Weft insertion Beat-up Take up mechanism Observations and Analysis Prospects of other 3D fabric designs Conclusion 75 Chapter: 4 Design of the purpose built 3D weaving machine Introduction Basic Design Design parameters for weaving processes Yarn feeding device Weft yarn feeding device Selvedge yarn feeding device Knitting needle mechanism Take-up mechanism Single yarn multiple insertion Weft Insertion Beat-up Selection of Drives and controls Shedding Mechanism Weft Insertion mechanism 100 2

4 4.5.3 Take-up mechanism Beat up mechanism Selvedge knitting needle mechanism Control of drives 103 Chapter: 5 Manufacturing Process Introduction Software and Electrical Installation Electrical Installation Programming of SPC Programming of FC Warp supply Weft supply Process cycle Produced Preform Problems and Solutions Warping Shedding Weft insertion Beat up Take-up Conclusion 128 Chapter: 6 Braid Weaving Introduction Braiding process Why Braiding Braiding on complex shapes Braid Parameters Machine specifications Braid Geometry Braiding on 3D near net shape woven preforms Conclusion 138 3

5 Chapter: 7 Consolidation Process Introduction Classification of Consolidation methods Hand Impregnation Preimpregnation Vacuum bagging Autoclave Liquid moulding Selection of resin impregnation method Resin Infusion Process Conclusion 148 Chapter: 8 Mechanical and Physical testing Introduction D glass woven preform Image analysis Flexural test D carbon woven preform X-ray Tomography Fibre Volume Fraction Short Beam test Tensile Test Conclusion 173 Chapter: 9 Conclusion and Further work Conclusion Further work and recommendations 177 References 179 List of Publications 185 4

6 List of Figures Fig 1.1 Growth of composite use in Aircraft 14 Fig 1.2 Composite manufacturing methods 16 Fig 1.3 Near net shape 3D textile structures 17 Fig 2.1 2D fabric structure 21 Fig 2.2 3D fabric structures 21 Fig 2.3 Classification of textile performing 22 Fig 2.4 Angle interlock structure (a) with warp stuffer yarn (b) without warp 26 stuffer Fig 2.5 Orthogonal weave 26 Fig 2.6 Layer to layer weave 27 Fig 2.7 2D and 3D weaving process 29 Fig 2.8 Near net shape woven fabric 31 Fig 2.9 A schematic view of weaving machine according to the invention 33 Fig 2.10 A perspective view of a short length of fabric structure 33 Fig 2.11 A Perspective View Showing a Loom Embodying the Invention 34 Fig 2.12 A diagram showing structure formation according to invention 35 Fig 2.13 A schematic structural diagram of a weaving machine 36 Fig 2.14 A perspective view of resultant fabric 36 Fig 2.15 Resultant Structure 37 Fig 2.16 Apparatus to Produce Fabric 37 Fig 2.17 A schematic view of process of fabric formation cycle 38 Fig 2.18 A schematic view of three-dimensional resultant fabric 39 Fig 2.19 (a) a schematic diagrams of weaving machine and screw shaft 40 mechanism (b) a resultant three dimensional fabric (c) non-uniform bias arrangement from screw-shaft weaving mechanism Fig 2.20 A schematic diagram of the formation of row wise and column wise 42 sheds Fig 2.21 A side view of the resultant fabric 42 Fig 2.22 A perspective view of three dimensional fabric 42 Fig 2.23 Spiral woven fabric 43 Fig 2.24 Shape 3D weaving machine 44 5

7 Fig 2.25 Shaped woven preforms 44 Fig 3.1 2D woven fabrics 49 Fig 3.2 (a) Angle interlock (b) Layer to Layer (c) Orthogonal Hybrid 52 (d) Orthogonal Fig 3.3 A schematic view of 3D fabric with the steps in weft direction 54 Fig 3.4 A schematic view of 3D fabric with steps in warp direction 54 Fig 3.5 Weave Design, Draft And Peg Plan for the Three Step Fabric 56 Fig 3.6 Step formation in weft direction 57 Fig 3.7 2D view of the Fabric 58 Fig 3.8 3D view of the 3D fabric 59 Fig 3.9 Production of Preform on loom 59 Fig 3.10 One step 3D fabric (orthogonal and orthogonal hybrid) 60 Fig 3.11 Weaving process 61 Fig 3.12 Arrangement of Beam in Rapier weaving machine 63 Fig 3.13 Arrangement of beams in shuttle weaving machine 63 Fig 3.14 Binding Yarn Beams 64 Fig 3.15 Shed Creation in shuttle weaving machine 65 Fig 3.16 Weft Insertion by Rapier in Jumberca Loom 67 Fig 3.17 Step Glass 3D Fabric 71 Fig 3.18 Six step fabric on loom 71 Fig 3.19 (a) Orthogonal Hybrid (b) Orthogonal (c) Layer to layer 72 Fig 3.20 Weave design for the taper section 73 Fig 3.21 Double lap joint weave design 73 Fig 3.22 Taper 3D fabric cross section 74 Fig 4.1 A schematic view of Proposed machine 78 Fig 4.2 Vertical and horizontal stack of warp yarns 79 Fig 4.3 Yarn creel arrangement 80 Fig 4.4 Warp beam arrangement 82 Fig 4.5 Adjustable rod arrangement for warp yarns 83 Fig 4.6 Final shed size Geometry 84 Fig 4.7 A schematic view of weft passage 85 Fig 4.8 Rapier design 86 Fig 4.9 Needle insertion into warp shed 87 Fig 4.10 Weft loop catching with needles 88 6

8 Fig 4.11 Knitting needle mechanism 90 Fig 4.12 Yarn compensator mechanism 91 Fig 4.13 Tension roller arrangement 93 Fig 4.14 Single yarn multi insertion system 94 Fig 4.15 Weft insertion system 96 Fig 4.16 Beat-up positions 97 Fig 4.17 Roller for binding yarn movement 99 Fig 4.18 Beat-up process cycle movements 102 Fig 5.1 Machine set-up 105 Fig 5.2 Schematics for drives control 107 Fig 5.3 Control Cabinet for machine process cycle 108 Fig 5.4 SPC 200 with 5 modules 109 Fig 5.5 User interface of Winpisa software 110 Fig 5.6 FC 640 I/O module 111 Fig 5.7 User Interface of FST Fig 5.8 Winding of warp beam 113 Fig 5.9 (a) Weft yarn supply (b) weft yarn rapier 114 Fig 5.10 (a) Overview of weft insertion mechanism 116 (b) Rapier shed entrance (c) Rapier shed exit Fig 5.11 Overview of single yarn rapier system 117 Fig 5.12 (a) Needle bed insertion to warp shed 118 (b) pneumatic cylinders for beat-up process Fig 5.13 Needle beat-up process 119 Fig 5.14 Binder yarn change over process 120 Fig 5.15 Take-up process 121 Fig 5.16 Produced Preform (a) 3D glass preform (b) 3D carbon Preform 122 (c) Multistep Preform (d) 3D narrow carbon preform Fig 5.17 Uneven warp tension 124 Fig 5.18 Warp support rod system 125 Fig 5.19 Proposed rapier head 127 Fig 6.1 Biaxial Yarn packages 133 Fig 6.2 Triaxial Yarn Packages 133 Fig 6.3 Tension setting of package 134 7

9 Fig 6.4 Actual and set tension value in packages 135 Fig 6.5 Multiple step joint 137 Fig 6.6 Braided narrow 3D laminate 137 Fig 7.1 Autoclave process out-line 140 Fig 7.2 Tooling for RTM 141 Fig 7.3 Vacuum bag arrangement 144 Fig 7.4 Vacuum bag lay-up for resin infusion 146 Fig 7.5 Braided 3D woven laminate 147 Fig 8.1 Cross section (a) Binder yarn (b) Warp yarn (c) Weft yarn 152 Fig 8.2 Cross section (a) warp (b) weft 153 Fig 8.3 Loading conditions for flexural test 154 Fig 8.4 Machine set-up for flexural test 155 Fig 8.5 Load (kn) vs Displacement (mm) graph for flexural test 156 Fig 8.6 Comparison of Interlaminar shear strength values 158 Fig 8.7 3D X-ray image of braided woven laminate 159 Fig 8.8 X ray images of braided woven laminate cross section 160 (a) weft cross section (b) weft/warp cross section (c) warp section Fig 8.9 Loading conditions and Sample dimensions 165 Fig 8.10 Load (kn) vs Cross head (mm) graphs for short beam test 166 Fig 8.11 Comparison of maximum load value (kn) for short beam test 168 Fig 8.12 Comparison of Interlaminar shear strength (MPa) values 168 Fig 8.13 Machine set up 169 Fig 8.14 Load (kn) vs Extension (mm) graph for tensile test 171 Fig 8.15 Comparison of Maximum load (kn) values for tensile test 172 Fig 8.16 Comparison of Maximum tensile strength (MPa) 173 8

10 List of Tables Table 8.1 Maximum Flexural Strength (MPa) values 157 Table 8.2 Maximum Load (kn) values 157 Table 8.3 3D Woven Preform specifications 161 Table 8.4 Fibre volume fraction for 3D woven laminate 163 Table 8.5 Fibre volume fraction for 3D braided woven laminate 163 Table 8.6 Maximum Load (kn) values 167 Table 8.7 Interlaminar shear strength (MPa) values 167 Table 8.8 Maximum Load (kn) for tensile test 172 Table 8.9 Maximum Tensile stress (MPa) 172 9

11 Abstract Significant proportion of composite industry is currently produced using prepregs, cured in autoclave which is very expensive and time consuming process. Dry textile preforms in conjunction with liquid molding techniques can lead to significant reductions in material costs, manufacturing costs and cycle times. These dry preforms are typically 2D woven or braided fabrics which also required lay-up and have low interlaminar properties. Through thickness reinforcement provides solution for this problem as it gives better interlaminar properties as well as near net shape performing. Various 3D performing methods are discussed and reviewed in this research where 3D weaving comes out as ideal process to develop near net shape preforms with more efficiency and better material performance. This research highlights the advantages and limitations of conventional 3D weaving processes. A number of approaches for improving the flexibility of 3D weaving process have been presented including changing fiber architecture in different sections of the preform, tapering in the width and thickness directions and finally to change the fiber orientation. It is concluded that multi step and taper fabrics can be produced on conventional weaving by some modifications. Furthermore, a novel 3D weaving machine is designed and developed after reviewing various patents and weaving methods to overcome limitations of conventional weaving machine. Key criterions from limitations of conventional weaving processes are considered and modified such as multiple weft insertion, limited warp stuffer movement, linear take-up to develop 3D weaving machine. In order to achieve isotropic material, two textile technologies are combined to get final requirements. 3D weaving can provide us fibres in 0 and 90 direction with through thickness reinforcement, whereas braiding can satisfy the requirement of bias direction fibres. Near net shape preforms such as taper and multistep are produced and laminated. Preliminary testing is performed on these laminates to evaluate fibre architectures. Further work is required in terms of machine modification which can provide weave design flexibility to explore various multilayer weave architectures. Thorough testing is required to evaluate and define structure performance and effect of fibre damage during weaving process. 10

12 Declaration No portion of the work referred to in the thesis has been submitted in support of an application for another degree or qualification of this or any other university or other institute of learning. 11

13 Copyright I. The author of this thesis (including any appendices and/or schedules to this thesis) owns certain copyright or related rights in it (the Copyright ) and he has given The University of Manchester certain rights to use such Copyright, including for administrative purposes. II. Copies of this thesis, either in full or in extracts and whether in hard or electronic copy, may be made only in accordance with the Copyright, Designs and Patents Act 1988 (as amended) and regulations issued under it or, where appropriate, in accordance with licensing agreements which the University has from time to time. This page must form part of any such copies made. III. The ownership of certain Copyright, patents, designs, trade marks and other intellectual property (the Intellectual Property ) and any reproductions of copyright works in the thesis, for example graphs and tables ( Reproductions ), which may be described in this thesis, may not be owned by the author and may be owned by third parties. Such Intellectual Property and Reproductions cannot and must not be made available for use without the prior written permission of the owner(s) of the relevant Intellectual Property and/or Reproductions. IV. Further information on the conditions under which disclosure, publication and commercialisation of this thesis, the Copyright and any Intellectual Property and/or Reproductions described in it may take place is available in the University IP Policy (see pdf), in any relevant Thesis restriction declarations deposited in the University Library, The University Library s regulations (see and in The university s policy on presentation of Theses. 12

14 Acknowledgments I would like to express my sincere gratitude to my supervisor, Dr Prasad Potluri, for his continuous source of guidance, support and advice throughout the period of this work. He has not only great academic expertise with deep vision in industry, but also the great kindness. He has encouraged an independent style of research while providing feedback on both the theoretical and practical aspects of my work. Special thanks to Department of Textiles for giving me a great opportunity to study here. I am thankful to all members of staff for their direct and indirect help during my PhD study. I would like to express my special thanks to Less Downes and Dr Alan Nesbitt for their kind support in my experimental work. I have great pleasure in expressing my sincere thanks to all students at textile composite group for providing a stimulating and friendly environment. I thank Haseeb, Alper, Tahir, Jasmin, and Marco for their valuable comments during my work. I also thank Paresh and Kamlesh for their support and guidance during my PhD. I am also very much thankful to my family for their love, support and motivation. My warmest gratitude goes to my wife, Heta, for her love, patience and understanding during my stay at Manchester. 13

15 Chapter: 1 Introduction 1.1 Introduction In aerospace, industrial, automotive and marine structure, the use of fibre composite material is increasing quickly due to their properties such as light weight, high specific stiffness, high specific strength and good fatigue resistance compared to most metallic alloys such as steel and aluminium. A key characteristic of composites is they are anisotropic where properties are different along the different directions. This feature can help engineers to design the part according to the requirements. Arranging many fibres in one direction, the specific properties of composites can be superior to those of metals [2]. The key challenge for composite industry is to arrange these fibres in specific direction with the help of manufacturing process which is economical and reliable. Composites have considerable potential in commercial aviation sectors due to its competitive offerings and high strength-to-weight ratio. These composite properties provide fuel efficiency of aircraft and thus, resulted in more growth of composites in aircraft currently (Fig. 1.1) in production and next-generation aircraft as well. Fig 1.1 Growth of composite use in Aircraft (Source: Hexcel- Commercial Aerospace- composite penetration) 14

16 The major composite manufacturing processes of filament winding, fibre placement, pultrusion are reviewed. Since these methods have some drawbacks, it will emphasize techniques of advanced textile preforming that have been under intense development over a number of years and the potential benefits that these processes have. 1.2 Composite manufacturing methods Fibre reinforced composites mainly consist of high modulus fibres embedded in a resin matrix. In this form, both the fibres and the matrix produce the combination of properties that cannot be achieved by individual constituents alone. Fig. 1.2 illustrates manufacturing methods such as filament winding, fibre placement and pultrusion which are used to form the composite parts. The basic principles of these processes are discussed in order to recognise the need for the textile performing methods. Filament winding is very economical specifically when the production volume and level of automation is very high [36]. However, it is very difficult to produce components with significant concave surfaces which restrict the use of filament winding method. In addition, it needs mandrel to fix the components on the machine which needs to be removed after formation of composite part. Fibre placement which is the combination of filament winding and tape placement is primarily used where much greater degree is needed to orientate fibres and cover concave surfaces [48]. This technique consists of robotic control which controls the placement head that allow multiple preimpregnated tapes or tows to be heated, placed and compacted under individual control. However, there is a limitation in placement of tows particularly at the components boundaries which creates gap in between tows or bumpy surfaces in the final part Pultrusion is a process where fibres are bunched and stretched together with the matrix and cured with the heater [36]. Its main advantage is that it provides truly continuous composite part without too much material wastage. Thus, this process has been mainly used in the production of high volume components. Disadvantage of this process is its inability to produce near net shape composite parts. 15

17 Fig 1.2 Composite manufacturing methods All these composite manufacturing method is currently in industrial use, but they have some limitations which need to be addressed. Key challenge is to get throughthickness properties in these composites. In addition, it is very hard to get near net shape structures. One of the solutions is the use of textile techniques to produce the composite part. The main handling advantage of textiles is that they are manufactured as dry fibre preforms that hold together when they leave the textile machinery without any polymer or other matrix. 16

18 1.3 Advanced textile preforms Significant proportion of the aerospace composites is currently produced using prepregs, cured in autoclaves. Dry textile preforms in conjunction with liquid molding techniques can lead to significant reductions in material costs, manufacturing costs and cycle times. Dry fibers are converted into woven, stitched (NCF) broadcloth or braided sleeves using conventional textile techniques. A number of textile layers are draped over a mold surface in preferred orientations, and subsequently bonded or stitched together for ease of handling. It is widely recognized that the preform assembly is the most time-consuming, expensive and labour intensive process. In addition, 2D laminates have poor interlaminar properties. As a result, composites industry has been looking at alternate methods of automating the preform manufacture [49]. There are many ways in which 3D fibre architecture can be produced, however most attention has been given to the textile techniques of weaving, braiding, stitching and knitting. The goal of research remains same in all these techniques to develop processes that can produce in an automated, cost-effective manner near-net, complex shaped preforms with a 3D fibre architecture that will lead to improved component performance. Fig 1.3 shows the recent near net shape 3D textile structures. Fig 1.3 Near net shape 3D textile structures Textile preforms are fibrous assemblies with prearranged fibre orientation, preshaped and often preimpregnated with matrix for composites formation. Since reinforcements play a major role in dominating the mechanical properties of composites, the continuity and integrity of the architecture of fiber preforms becomes a main concern in advanced composites [8]. In this research, different methods to manufacture textile preforms have been discussed and explored to produce near net shape structures. 17

19 1.4 Recognition of New Need Generally Continuous fibre composites are characterised by a two dimensional laminated structures in which fibres are aligned along the plane of the material. A distinguishable feature of 2D laminate is that no fibres are aligned in the throughthickness direction. The lack of through thickness reinforcing fibres can be a disadvantage in terms of cost, ease of processing, mechanical performance and impact of damage resistance. 3D textile processes are aimed at reducing performing costs associated with multi-ply preforming methods and also provide interlaminar strength with through thickness reinforcement. The key criteria for the selection of textile preforms for structural composites are 1) The capability for inplane multiaxial reinforcement. 2) Through-thickness reinforcement 3) The capability for formed shape and /or net shape manufacturing. [3] Textile processes which can effectively translate stiff, strong yarns into stiff, strong composites are taken in account to find potential technology to develop complex nearnet shape structures [2]. There are several patents to produce 3D fibre reinforcements for composite parts. Mohammed, Anahara, Fukuta etc invented techniques to produce 3D fabric with fibres oriented in multidirections [12,16,17,18]. However, each technique has certain limitations. One of the difficulties is to employ the fibres other than three directions. Bias direction fibres can be introduced in some methods, but then there is a limitation in getting near net shape preforms. One of the solutions is to get 3D fabric and then use braiding process for bias direction fibres. After reviewing several textile technologies for 3D reinforcement in composites, weaving and braiding were considered to develop new material for near net shape structure to replace current methods. The reason to choose these two processes is similarity between their structural geometry. If we consider 2D weaving, there is interlacement between warp and weft yarns at 0 and 90 direction. And if we consider braiding, there are two different structure geometries which are biaxial and triaxial. In biaxial braiding, yarns are interlaced between ±0 to ±90 direction, whereas in triaxial braiding, there is one more set of yarns going straight in the structure. Hence, in weaving and braiding sets of yarns are interlaced, although at same or different direction. Thus, these two processes and its products were studied and reviewed for current composite requirements. For weaving, the main aim was to produce a 3D 18

20 complex structure on 2D machinery and later on to develop a 3D weaving machine for near net shape structures with faster production. In addition, to get the fibres in more than 3 directions, braiding can be done to the 3D woven composites. In this way, better properties can be achieved in more than three directions. 1.5 Aim and objectives Aim: to explore current weaving and braiding technology to fulfil composite material requirements. Objectives: Review of weaving and braiding methods to find out their feasibility of producing composite parts which can replace current products Manufacturing of near-net shape structure in conventional weaving machinery Acquire the concept of the conventional weaving machine to the purpose built weaving machine to get fast production and versatility Design and develop 3D weaving machine Explore 2D braids to optimise the fibre properties on 3D woven fabric Establish a method of resin impregnation for proposed structure Preliminary mechanical testing of 3D woven and braided composites 19

21 Chapter: 2 3D Weaving 2.1 Introduction This chapter concerns literature survey on textile preform manufacturing methods for composites, mainly three-dimensional weaving and three-dimensional braiding, and understand the potential of near-net shape preforms. Textile preforms can be generally categorized as two or three dimensional based on the degree of reinforcement in different directions. Properties to get better performance can be achieved with continuous fibres rather than small particles or whiskers [2]. Currently these composite structures are produced from two dimensional laminated structures, however such 2D laminates have some disadvantages. These can be listed as the following: Expensive Not suitable for complex shapes Set-up problems in lamination process such as yarn slip-up and misalignment of fibres resulting in irregular orientation Low through-thickness mechanical properties Poor impact damage resistance Low post-impact mechanical properties It has been proven at many instances that it is easier to manufacture 3D preforms than 2D laminates particularly for complex shapes. A 3D fabric generally consists of fibres extending along a direction in the X, Y and Z directions. Fabric can also be regarded as 2D or 3D depending on their fibre orientation. Fibres are normally aligned in specific directions depending on fabric types such as woven, braided, knitted or stitched. It is very hard to define 2D or 3D fabrics in terms of their three dimensional form as they have definite thickness, width and length. 20

22 2D fabric: a single fabric system the constituent yarns of which in one plane as shown in fig 2. 1 e.g. common woven and knitted fabrics [1]. Fig 2.1 2D fabric structure 3D fabric: a single fabric system the constituent yarns of which are in a three (mutually perpendicular) plane relationship as shown in fig 2.2 [30], e.g. woven angle interlock fabrics, woven and knitted spacer fabrics [5]. Fig 2.2 3D fabric structures Development of 3D composites has been driven by the need to reduce fabrication cost, increase through-thickness mechanical properties and improve damage tolerance. Basic textile techniques such as weaving, knitting, braiding and stitching are utilised to produce 3D textile preforms [30, 31]. Weaving is the most common amongst these processes as it allows 3D composites to be fabricated at high production speeds and offers flexibility to produce a diverse range of 3D fibre structures. 21

23 Textile preforms can be classified with regard to different textile techniques used to manufacture them as shown in Fig 2.3. The main category includes four basic textile techniques, weaving, stitching/nonwoven, knitting and braiding. The subcategories of these methods are also listed [2]. The methods, along with their uses, are discussed below in fig 2.3. Fig 2.3 Classification of textile preforming 22

24 2.2 Weaving Weaving is the most widely used textile manufacturing technique and accounted for seventy percent of the two dimensional fabric produced in The process of weaving is suitable for making panels and woven fabric textiles and it has been used for many years in two-dimensional laminated composites. Some yarn crimp is involved with woven preforms unlike unidirectional tape laminate. However, weaving offers a relatively low cost for mass production. Fibres in two dimensional weave have zero and ninty degree (warp and weft) orientation. Orientation of fibres lead to poor impact resistance, low delamination strength and reduced in-plane shear properties in the woven fabric. Through-thethickness reinforcement is required to improve impact and interlaminar properties. 3D weaves such as angle-interlock and orthogonal can provide this reinforcement into the textile structure.. Three-dimensional woven preforms can be produced using almost any type of reinforcement yarn, and the proportions of the yarns in the plane and through-thickness directions can be controlled to tailor the properties of the composite for a specific application [6]. 2.3 Braiding Processes originally developed for the textile industry are being used in the manufacture of composite structural components. Braiding is one of these techniques, which has the capability to fabricate near-net-shape preforms of the desired structural part and offers required fiber reinforcement. It can be incorporated with resin infiltration process such as RTM to produce low-cost and high-quality components with improved damage tolerance and resistance. There is growing interest in design, manufacture and performance characterization of braided composites. 23

25 Three dimensional braiding is the most suited textile process for making highly complex, long and slender components that require high levels of conformability, torsional performance and structural integrity. However, three-dimensional braiding is only used for limited applications. In addition, there is limited understanding of the effect of braiding parameters on the mechanical properties of the resultant composite structures [6]. 2.4 Stitching/Nonwoven Stitching is the simplest way of fabricating three-dimensional textile preforms; however, stitching can cause significant in-plane fiber damage that may result in degradation of in-plane mechanical properties of the composite. One way of introducing through-thethickness reinforcement without causing significant in-plane fiber damage is by using non-woven manufacturing technique [6]. Stitching equipment currently used in the textile industry is capable of manufacturing preforms for aerospace-grade materials, and there are various high performance yarns such as aramid, glass, and carbon, that can be used as stitching threads. In addition, stitching process is relatively flexible in terms of producing near-net structures and it also improves the handling of standard two-dimensional fabric [7]. 2.5 Knitting Weft-knitting is ideal for the manufacturing components with complex shapes. A wide variety of complex net shape structures can be produced on current industrial knitting machines. Alternatively, flat-knitted fabrics (warp-knitted) can also be produced from glass and carbon fibre yarns. Such knitted fabrics are highly stretchable and thus can be formed into desired complex shapes and liquid moulded to produce composite components. Due to looped structure of the knitted preform, properties such as strength and stiffness are much lower than other textile preforms. However, their bearing strength and impact resistance and tolerance are superior to those of woven composites. The knitting process is particularly suitable for fairings or other similar lightly loaded structures for which bearing and impact are often the critical design cases [6]. 24

26 Until now, many efforts have been made to achieve multi-axis reinforcement in textile composites. Mainly weaving, braiding and stitching methods are explored to find a solution for the current problem which is multi-directional strength. Loop formations in knitted structures do not allow them to fit into the composite criteria, whereas there is slight or no crimp in other textile structures. Various methods have been developed for producing three or more dimensional textile performs with different textile techniques. By considering various factors such as the shape of the product and based on literature survey of 3D weaving with conventional and purpose built looms, 3D braiding has been performed D Weaving Weaving is the most widely used textile manufacturing technique for production of two dimensional fabrics. It allows efficient production of flat panels which is the main reason why woven fabric textiles have been used for many years in two-dimensional laminated composites However, as mentioned above, there are certain issues which need to be resolved for better performance of composites. One of the key solutions to enhance properties is by introducing third direction reinforcement. This can be achieved through modification of weave structures and looms, or alternatively with the help of custom built weaving machines. There are various types of 3D woven fabrics, such as [33]: 3D solid structures (orthogonal, angle interlock, multilayer weaves) 3D hollow structures (honeycomb, nest structure) 3D shells (curved, double curvature in the third direction) Amongst these structures, 3D solids can fulfil the requirements for the third direction reinforcement. In Angle-interlock fabrics, the warp yarns can enter one or more layer of weft yarns (layer to layer angle interlock), or go through all layers of weft yarns (through-thethickness angle interlock). A large number of 3D woven structures can be achieved by modifying weave pattern, number of layers, yarn type, and yarn count, etc., thus allowing large number of variables to predict structural and mechanical properties under applied loads [24]. 25

27 Fig 2.4 Angle interlock structure (a) with warp stuffer yarn (b) without warp stuffer When yarns are placed in three mutually orthogonal directions, they form an orthogonal woven fabric (as shown in Figure 2.5). The yarns are interlaced uniformly in all three directions to provide quasi-isotropic properties or alternatively the interlacements can be allowed to vary in each direction when anisotropic properties are required. Orthogonal structures provide a greater fibre volume fraction than angle interlock structures. Fig 2.5 Orthogonal weave The layer-to-layer is a multilayered woven fabric in which binding warps travel from one layer to the adjacent layer and back (As shown in Figure 2.6). A set of (non-interlacing) warp yarns is introduced to reinforce all the layers of the preform. In multilayer fabrics, binder yarn path may vary according to design. These weave structures can be modified accordingly; by altering the lifting plan it is possible to create different binding points and hence distinct near-net shape structures. 26

28 Fig 2.6 Layer to layer weave Weaving of multilayer reinforcements has developed into a complete process, utilising modified weave structures and lifting plans, with through-the-thickness yarns incorporated as part of the weave design [4]. Three dimensional multilayer woven composites have shown to be effective in improving translaminar properties of laminated composite structures with the introduction of reinforcing fibres in the thickness direction of the laminate. 3D weaving also has an added advantage of producing complex shapes in one piece. 27

29 D weaving by Conventional Weaving There are a number of weaving methods that can be used to produce multilayer preforms. Standard weaving machines, with minor modifications if required, can be used to weave high modulus carbon and glass fibres. Specific proportions of yarns can be arranged in the three mutually perpendicular directions referred to as warp (X), weft (Y), and through-the-thickness (Z) directions [24, 25, 26]. Yarns in the third direction which bind the structure can either belong to warp or weft yarn set. Secure selvedges and near-net shaped performs can be produced with relatively little waste by using a shuttle for weft insertion; however it is difficult to handle high modulus yarns using this technique. Alternatively, leno selvedges can be employed to assist in the production of near-net shapes. The basic difference between 2D fabric and 3D fabric produced on conventional loom is illustrated in the fig 2.7. Manufacturing 3D fabrics make use of a more complex shedding technique as compared to 2D fabrics. For a 2D fabric, the shed is created to insert one weft layer, whereas in a 3D fabric, wefts are laid one above the other, hence several sheds need to be created to accommodate wefts at different levels. The warp tension, which is an essential weaving parameter, varies because of multiple layers in the 3D fabric; multiple warp beams are used to maintain desired warp tension in the layers. 28

30 shedding warp beat-up cloth fell pick let-off shuttle take-up projectile rapier air/water jet Fig 2.7 2D and 3D weaving process 29

31 Conventional looms are traditionally set-up to produce fabrics comprising of one to three layers. Normally, simple rectangular or circular 3D woven preforms can be produced without much modification with 3D weaves. However preforms for composite applications sometimes are required to offer variation in thickness. In addition to this, warp or weft density is required to vary accordingly with near-net shapes. Weaving multilayer fabrics on conventional looms can become very complicated if a multilayer shaped reinforcement is required. These factors impose restrictions for using standard looms for near-net shape performs, but they can be overcome by carrying out some loom modifications. Another approach taken for fabrication of shaped reinforcements on conventional looms concerns weaving the shaped reinforcement in a flat multilayer form, and then folding or opening it to the desired shape after removal from the loom [24, 45]. One of the key advantages of using conventional looms is that 3D woven structures can be produced without incurring vast expense of specialised weaving machinery. Conventional looms can be used for the production of different near-net 3D structures such as T, I, Pie etc (fig 2.8). It is possible to weave different complex shapes on conventional weaving machines by varying the weave designs. Raymond et al claimed a method where I-beam can be produced by interlacing a number of warp and weft layers to form rib and flange according to the requirements. Through-the-thickness yarn is utilised in order to lock stuffer layers in the rib and the flange of the I-beam. Number of layers for the rib and flange can be changed according to the mechanical properties required [4,26]. Soden et al produced various complex shaped preforms such as T-sections, I-beams and tubular sections on a conventional loom in a flat form and were folded after removal from the loom. Several designs employed to form the complex near net shape on a conventional loom. The concept was to produce the complex shape at the same time with good conformability when folding. The design had a high crimp structure combined with low crimp structure which would enable the preform to fold easily. The design increased fibre reinforcement level in the joint region. Also a localised reinforcement was introduced at each flange which would enable rib and flange to be of equal thickness [4,26]. 30

32 Fig 2.8 Near net shape woven fabric There are several patents and methods to produce different 3D woven structures on conventional weaving machine, but not many to produce taper or varying thickness laminates. In aerospace and other composite industries, there is high demand for composites in repairs and maintenances, high cost of parts and complex shapes being the key reasons. Near-net-preforms are hence required. There are certain structures which can replace the damaged part and avoid replacement of the whole structure. Many complex structures also comprise of joints such as lap joint, scarf joint, double lap joint etc. These joints, either adhesive or mechanical, do have certain drawbacks. Such joints can be designed and manufactured in the weaving machine according to design requirements. In the work presented in this thesis, multiple step fabrics have been designed and produced on a conventional weaving machine. Different 3D weaves have been employed to get the desired end product with better mechanical properties in comparison to 2D laminates. A drawback of 3D woven composites is that the load bearing fibres may be damaged and distorted during weaving operations. Fibre damage appears to occur during weaving due to repetitive abrasion and bending of the yarns. This damage can be minimised by modifying some loom parts or with the purpose built machines. Proceeding with this concept a 3D weaving machine was designed and developed to fabricate a similar product, but with enhanced quality and higher production rate. 31

33 2.5.2 Purpose built weaving machine for 3D weaving 3D woven composites were first developed nearly 30 years ago in an attempt to replace expensive high-temperature metal alloys in aircraft brakes [8]. Although standard industry looms can be modified to produce 3D woven preforms, investigations into 3D weaving processes has led to the development of a number of specialised looms that are capable of weaving complex preforms [9]. Some of these processes are discussed below. One of the earliest weaving processes was developed at Avco Corporation to manufacture cylindrical carbon/carbon composites. This process utilises a skeleton of rods that allows carbon yarn to be placed in axial, circumferential and radial orientations. The rods are removed prior to the consolidation of the composite cylinder. Variations of this method have also been developed to allow the manufacture of rectangular billets of 3D preforms. To achieve high tensile strength in these directions in fabric, Greenwood [10] has developed an apparatus which can produce such kind of fabrics. These fabrics can be used as reinforcing structures in composite materials. Each ground warp and weft yarns have negligible crimp in the resulting fabric. Ground warps extend longitudinally in the structure; binder warp is a continuous thread lying between adjacent pairs of vertical columns of ground, going through the depth of the structure. Wefts are formed along the width of the structure from top to bottom in between the ground warps. Fig 2.9 is diagrammatic view of Greenwood s apparatus to produce resultant fabric (Fig 2.10). The resultant fabric is much thicker and rigid than conventional fabrics, thus it has to be withdrawn on a rectilinear path. Due to this factor, the length of fabric is limited. 32

34 Fig 2.9 A schematic view of weaving machine according to the invention Fig 2.10 A perspective view of a short length of fabric structure 33

35 In 1974, Fukuta et. al claimed an invention concerning a three-dimensional fabric comprising of warp, weft and vertical yarns, and a method and a loom for its production [11]. 3D fabric is formed by inserting doubled weft yarns into spaces between warp yarns. The weft yarns are looped with the assistance of an eyelet at the head of the weft insertion needles. The warp yarns are aligned parallel in vertical and horizontal directions to produce a number of layers and rows. Weft yarns are secured by inserting a selvage binder yarn into the weft loops. Vertical binding yarns are introduced into spaces between vertical rows of the warp yarns. Since the vertical yarn inserting device is curved at the fore feeding ends, it provides a shed through which the weft yarns can be introduced Fig 2.11 is a 3D view showing a loom embodying the invention. Fig 2.12 shows the fabric structure. Fig 2.11 A Perspective View Showing a Loom Embodying the Invention 34

36 Fig 2.12 A diagram showing structure formation according to invention Another method for making 3D fabric and apparatus was developed by Fukuta [12] in He provided a method and apparatus which enables a 3D woven fabric consisting of three component yarns, referred to as longitudinal, lateral, and vertical yarns, and secondly another 3D fabric, also consisting of three component yarns referred to as circumferential, radial, and longitudinal yarns. The three mutually orthogonal set of yarns are made to intersect with the assistance of yarn carriers. As shown in Fig 2.13, an array of bobbins is provided at the base which feeds one set of (vertical) yarns, whereas the second (longitudinal) and third (lateral) set of yarns are extracted from individual bobbins. The later two set of yarns are made to intersect with the first with the help of yarn carriers. These carriers are designed to manoeuvre in two axis hence can be utilised to create various yarn interlacement paths and hence 3D fabric performs of desired shape. 35

37 Fig 2.13 A schematic structural diagram of a weaving machine Krauland s [14] invention provides a method and device for making continuous 3D fabrics with different cross sections at high speeds, which have the ability to resist distortive forces. These fabrics comprise of a set of warp yarns arranged in a predetermined way, along with two sets of orthogonal weft yarns interconnecting with them. These sets are laid alternately in courses; each course is made to interloop with the previous course of the same set at opposite ends with the help of a needle. Hence this mechanism comprises of a secure selvedge. Figure 2.14 shows the perspective view of resultant fabric. Fig 2.14 A perspective view of resultant fabric 36

38 King s [13] invention concerns a three dimensional structure shown in fig The structure comprises of three sets of mutually perpendicular filaments. As shown in Figure 2.16, filaments lying in X and Y axis are introduced into the weaving frame already comprising of the third set of yarns pre-aligned (vertically) in line with Z axis. Weaving takes place as X and Y directional filaments are inserted in loop form between the Z filaments with the help of a feed tool. These loops are locked in place at the ends by inserting a rod which can easily be removed after the structure is secured following few insertions. Finally the frame is lowered to compress the interwoven X and Y layers. Hence, a structure comprising of a rectangular cross section can be formed. Fig 2.15 Resultant Structure Fig 2.16 Apparatus to Produce Fabric Mohamed [16] developed a 3D weaving method for producing 3D orthogonal fabrics having a variety of predetermined and variable cross-sections. This method allows the warp set of yarns maintained under tension to be aligned horizontally and vertically, hence the number of warp layers can be defined in order to yield the predetermined cross-section. A plurality of selected weft yarns connected by a loop at the respective fore-ends is inserted into spaces between warp layers. The weft yarns are inserted at a predetermined and non-uniform horizontal distance from at least one side of warp yarn threading binder or selvage yarn through the loops at the fore ends of weft yarns bringing reed into contact with the fell of fabric being formed. Vertical yarns are inserted perpendicular to both warp and weft yarns. Figure 2.17 shows a schematic side view of the process at the beginning of the fabric formation cycle. 37

39 Fig 2.17 A schematic view of process of fabric formation cycle Mohamed [17] claimed a multi-axial, 3D fabric formed from five yarn systems. The yarn systems include warp yarns arranged longitudinally, two coupled sets of bias yarns on either surface (of the plurality of warp yarns), comprising a plurality of continuous bias yarn layers arranged so that each layer is inclined symmetrically with regard to each other and the warp set. Vertical yarns are arranged along fabric thickness, perpendicularly intersecting with the warp yarns. Weft yarns are arranged width-wise and also intersect perpendicularly with the warp yarns. The end result, as shown in Figure 2.18, is a multiaxial three-dimensional fabric with enhanced resistance to inplane shear; introduction of bias yarns improves in-plane shear and modulus. 38

40 Fig 2.18 A schematic view of three-dimensional resultant fabric Anahara s [18] claimed a formation of 3D fabric where vertical yarns along the fabric thickness intersect with parallel arranged warp yarns and inclined arranged bias yarns along the fabric length.. A method of producing the fabric is also provided. The subject of the invention resides in provision of a 3D fabric having an arrangement of threads of such bias directions and a fabrication process by which such three dimensional fabrics can be produced efficiently. The bias directional yarn layer is composed continuous yarns which are arranged in such a way that they incline in two directions symmetrically with regard to each other. Each pair of bias thread layer makes a set. Figure 2,19 shows (a) the concerned machine design, (b) the fabric structure, and (c) a modified feature. 39

41 Fig 2.19 (a) a schematic diagrams of weaving machine and screw shaft mechanism (b) a resultant three dimensional fabric (c) non-uniform bias arrangement from screw-shaft weaving mechanism 40

42 This 3D fabric process was developed by Nandan Khokar and discussed by Khokar and Peterson [19]. The produced fabric constitutes of a multilayer set of warp yarns which occur in accordance with the cross sectional profile of the fabric and are interconnected by interlacement through two orthogonal sets of weft yarns. This interlacement is achieved through a dual directional shedding system which alternately forms multiple column-wise and row-wise sheds. Such complex shedding is made possible through the employment of specifically designed heald wires. These two sets are arranged in a mutually perpendicular orientation. Alternate insertion of corresponding number of weft yarns, through the formed sheds in each of the two mutually perpendicular directions, results in interlacement of the multilayer warp and the two orthogonal sets of weft yarns. Cross-sectional profiles of various sizes can be produced by employing the same shedding system. Tension in warp yarns is greatly reduced as the system creates small shed heights. Figure 2.20 shows the formation of row wise and column wise sheds. Figure 2.21 shows a cross-section of the resultant fabric. Figure 2.22 shows a perspective view of three dimensional fabric. Khokar has also developed a method which he named as Noobing process-uniaxial type. The principle of this non-woven 3D fabric-forming process can be described through the following simple arrangement. The axial Z-directional yarns are disposed in columns and rows and the binding X- and Y- directional yarns are simply laid between these columns and rows alternately. Fabric integration can be achieved either directly or indirectly. Binding yarn spools are made to traverse in a closed loop path about the corresponding column and row of axial yarns. The reversals in the direction of the spool's traversal cause the outermost yarns of the respective column and row to be bound or tied directly. Basically, the binding yarns tie the opposite exterior layers of the axial yarns and the three mutually perpendicular sets of yarns X, Y and Z are placed linearly with or without any interlacement with regard to method described above. According to Khokar, the employment of healds and knitting needles does not categorise this process as weaving and knitting. 41

43 Fig 2.20 A schematic diagram of the formation of row wise and column wise sheds Fig 2.21 A side view of the resultant fabric Fig 2.22 A perspective view of three dimensional fabric 42

44 Chiu and Cheng [20] developed a weaving method for fabricating a spiral woven fabric as shown in Fig Spiral woven fabrics are fabricated by modifying the basic weaving processes including shedding, picking, beat-up, let-off and take-up mechanisms. Individual warp beam and tensioning systems are used to feed various warp lengths and reduce warp friction. Four harnesses are used to lift or lower the warp yarns in a specific sequence to form the shed. Weft insertion is achieved via air/fluid based mechanism or with the help of carriers. A pressure roller is used to keep the shed straight and open. Beat-up is achieved using a reed and the weft yarns tend to align radially since the take-up rollers are conical in shape. A slanted reed can be used to achieve enhanced radial arrangement. For take-up spiral rollers were used. The picking motion was performed in a FFHH cycle for reducing the difference in weft density between the inner and outer dimensions of the spiral fabrics. Fig 2.23 Spiral woven fabric Another method to produce 3D textiles has been developed by the Shape3 Innovative Textiltechnik GmbH as shown in Fig 2.24 [50]. The main principle of this process is the integration of different long warp and weft yarns. The textile expands into the third direction through these surplus yarn lengths. Although, it is not the through thickness reinforcement, but because of its near net shape and specialised machine, it is added to the Specialised 3D weaving machine. Fig 2.25 demonstrates few examples of near net shaped preform manufactured on this machine. 43

45 Fig 2.24 Shape 3D weaving machine Fig 2.25 Shaped woven preforms 44

46 2.6 Critical analysis of existing methods Significant proportion of the aerospace composites is currently produced using prepregs, cured in autoclaves. Dry textile preforms in conjunction with liquid molding techniques can lead to significant reductions in material costs, manufacturing costs and cycle times. Dry fibers are converted into woven, stitched (NCF) broadcloth or braided sleeves using conventional textile techniques. A number of textile layers are draped over a mold surface in preferred orientations, and subsequently bonded or stitched together for ease of handling. It is widely recognized that the preform assembly is the most time-consuming, expensive and labor intensive process. The assembled preform is then transferred to an RTM or an autoclave. As a result, composites industry has been looking at alternate methods of automating the preform manufacture. 3D weaving can be explored in order to get near net shape performs to satisfy structure design as well as properties. Despite the current methods and advances, 3D woven fabric is not been used too much in the composite industry. Two of the main drawbacks with current technologies are near net shape preform manufacturing and lack of fibres in direction other than 0 and 90 degree. There are certain specialised looms which can produce the fabric with the bias yarns, but overall cost is very high. The aim of this research is to produce the 3D fabrics as multistep, taper structures. It can be produced by selecting weave style and with some modification on the conventional loom. However, the final product is inferior in quality than other laminates. This is mainly due to the fibre damage during the weaving operations. To get better properties, one of the solutions is to design and develop specialised weaving machine which can produce similar product. As discussed in this chapter, there are several patents and methods for a specialised 3D weaving machine. But there are no such methods which can produce 3D fabric with taper or variable plies. To produce these kinds of structures, there is a need of designing and developing a specific machine. Several patents and methods were considered for the designing of such machine. Lot of considerations has been made through analysing these patents and methods in order to achieve the required structure configuration. 45

47 High-performance fibres such as carbon, glass have superior properties when they are straight in their path direction. It is observed that some of the structures made with discussed patents and methods have crimp involved in fibres. To get optimum composite property, it is advised to have crimpless fibres in the structure. Furthermore, shedding is done excessively in this method which creates fibre damage. As the aim of the project is to produce near net shape structures for composite industry, these processes cannot be considered. To fulfil demand for the composite industry, process should be economical and efficient in production of preforms. Several discussed patents have limitations as fixed length and width in terms of production. Machine set-up is also time consuming. This could be drawback in order to achieve mass production and flexible product sizes. This is expensive and labour intensive process which is not suitable for mass production. Therefore, there is a need for flexible and efficient machine set-up. One of the critical problems in getting the isotropic properties in composite structures is lack of fibres in the bias direction. Anahara and Mohamed developed methods to employ the yarns in the bias direction, but they are complicated and need a complex mechanism. This mechanism can slow down the production rate and limit structure flexibility. It can only produce certain configurations, as bias fibre mechanism follows a certain pattern. 46

48 2.7 Discussion After exploring and analysing these methods, the need for the new machine has appeared to overcome existing limitations. In order to achieve goals of this research, it is required to design and develop machine which can produce near net shape preform with fibres in 0, 90 and bias direction with through thickness fibres. This machine needs to be economical, efficient and flexible in terms of structure configurations. In order to achieve fibres in bias direction, two textile technologies can be combined to get end results. 3D weaving can provide us fibres in 0 and 90 direction with through thickness reinforcement, whereas braiding can satisfy the requirement of bias direction fibres. This would be a much simpler process compare to process by Anahara and Mohammed. This can also be taken forward as future design concept which will incorporate weaving and braiding in one process. It is proposed to manufacture 3D woven structures with purpose built machine after which braiding machine will be utilised to place bias direction fibres. Although, braiding will not provide structure integrity to 3D woven structure, it will place bias fibres in any configuration as it shapes according to mandrel. Thus, the braiding process is reviewed and developed to get multidirectional fibres lay up on the woven fabric. 47

49 Chapter: 3 Near net shape manufacturing on conventional weaving machine 3.1 Background As discussed previously in chapter-2, a key challenge for aerospace and other composite industries is the maintenance and repair cost of composite parts. Manufactures either have to replace the damaged composite part or repair the tailored laminates with adhesive or mechanical joints. Sometimes these laminates are not strong enough to avoid crack propagation and show poor fatigue properties because they are normally a stack of, or 2D-woven, unidirectional laminates [30, 34]. Such issues can be overcome by preventing crack growth in the laminates or joints by designing them appropriately. 2D laminates cannot be easily moulded into complex shapes, as a result of which some of the composite components need to be assembled from a large number of separate parts that are joined by co-curing, adhesive bonding or mechanical fastening. Further problems can be associated with such processes in the composite industry. The wing structure of an aircraft comprises of complex shaped parts. One of these parts is in multiple step form. This research concerns fabrication of a multiple step form glass fabric, characterising of a 3D orthogonal structure, in a single process. Such a structure offers through thickness and is easy to manufacture at low costs. The basic weaving process includes three major mechanisms, shedding, picking and beat-up, and two secondary motions, let-off and take-up. Conventional weaving machines are capable of producing 3D woven fabrics after undergoing some modifications. The project also aims to develop an approach to overcome the problem of weaving 3D preforms on a conventional weaving machine. Along with the near net shape of stepped lap joints, the weaving machine can also be used to produce double lap step joints and taper 3D structures. Research efforts on modifying traditional weaving methods to introduce through the thickness or z direction yarns for the production of such multilayer woven preforms are discussed. 48

50 3.2 Introduction The use of fibre reinforced composites has come to a point where design engineers have to control the fibre direction and the properties more precisely. Developments in the weaving process for advanced materials are required to fulfil the need for various combinations and placement of fibres in woven composites. Woven fabrics, formed on a conventional loom by interlacing two or more sets of yarns, are two dimensional constructions. The longitudinal and the widthwise yarns are referred to as the warp and the weft respectively. Fig 3.1 shows the typical woven fabric patterns. Fig D woven fabrics Woven fabrics provide more balanced properties in the fabric plane than unidirectional laminate. The ease of handling and low fabrication cost has made weaving suitable for production of structural application fabrics. Weaving produces both single-layer and broad-cloth fabric as a reinforcement. A few automated and semi-automated systems have been created or are under development to reduce cost. With minimal modification, standard textile machines can manufacture flat, multilayer fabrics in a wide variety of architecture with improved impact performance [32]. These multilayer fabrics can be woven using almost any type of yarn, including carbon, glass, aramid and ceramic fibre or various combinations. The proportions of the yarns in the X, Y and Z directions can also be controlled to tailor the properties of the composite for a specific application. The basic principle of producing 2D and 3D woven fabric has been discussed in the previous chapter. 49

51 The process of weaving is suitable for producing flat panels and woven fabrics which are used for two dimensional laminated composites. However, these composites exhibited poor impact resistance and delamination strength, and reduced in-plane properties due to two dimensional weaves which comprise of fibres in the zero (warp) and ninety (weft) degree directions. To improve the impact and interlaminar properties, through-the-thickness reinforcement is required. This can be achieved by using multilayer fabrics charecterised by angle interlock, orthogonal or layer-to-layer weave structures [7]. These weave structures have been analysed in this research to manufacture different near-net shapes, primarily step lap joints. The main objective of the research is to weave a multilayer fabric with near net shape on a conventional weaving machine and take this concept to design and develop the purpose built weaving machine for near net shaped preforms. Prior to building the new machine, existing conventional weaving machine were selected. The criteria to choose the machines were mainly the primary and secondary motions. Minor modifications were required to achieve these motions to weave the multilayer fabric. 50

52 3.3 Design considerations As this work primarily concerns fabrication of a 3D woven perform with multiple steps in the weft or warp direction, it demands emphasis on weave design. Two weaving machines (Jumberca rapier and Northrop shuttle) were used for the production of these 3D woven preforms. Three-step 3D woven fabric with weft-directional steps was produced and analysed; this concept was further developed to produce a six step 3D woven fabric in warp direction Selection of the Weaving machine Several types of weaving machines such as rapier, projectile, shuttle, air jet and water jet are available to manufacture woven fabrics. These machines have their own advantages and disadvantages but amongst these, both rapier and shuttle weaving machines are used to produce multilayer fabrics in this research. Rapier loom offers more control of weft yarn and allows weaving with any type of yarn. Due to unavailability of rapier weaving machine with a jacquard shedding mechanism, a shuttle loom comprising of this shedding mechanism is also used for the production of step preforms. The jacquard shedding mechanism allows full control of all warp yarns and hence, serves the benefit of fabricating complex structures. Another advantage that a shuttle loom offers is the formation of a closed loop weft which produces selfselvedge on both sides of the fabric along its length. On the contrary, the available rapier loom provides with a leno selvedge which forms an open loop resulting in relatively less secure edge. The creation of a leno selvedge is complex and requires different tension in the yarns. 51

53 3.3.2 Selection of the structure Orthogonal, angle interlock, layer-to-layer and orthogonal-hybrid structures are the 3D weave patterns selected for production. Yarn path for all these structures are shown in fig 3.2. Fig. 3.2 (a) Angle interlock (b) Layer to Layer (c) Orthogonal Hybrid (d) Orthogonal From the above mentioned, the orthogonal structure is used in the production of the step preform. Steps are created along the weft direction due to limitations in the dobby shedding mechanism of the rapier weaving machine. To achieve flexibility in warp-wise step formation the shuttle loom with a jacquard shedding mechanism is used. 52

54 The jacquard shedding mechanism allows formation of different structures such as angle interlock, layer-to-layer, orthogonal-hybrid, along with orthogonal structures. The orthogonal structure is best suited for a step preform fabricated in the conventional weaving machine whereas several problems can be associated with other structures. The step structure requires variation in depth across warp or weft direction which causes layer binding problems along the step in the angle-interlock and layer-to-layer structures. Variation in tension between the binding yarns is relatively higher in angleinterlock and layer-to-layer weaves. Depending upon the total number of layers used and the weft density, the orthogonal warp binding architecture provides a rigid reinforcement with inherently poor drapability characteristics. Orthogonal structure is also preferred for their uncrimped yarn paths, ordered construction, and high stiffness properties [31]. They are bound together by yarns penetrating the total fabric thickness from surface to surface interspersed with groups of stuffed warps. In order to produce structures with superior mechanical properties, the design and production of multi layer woven composites are emphasised in which the principal warp and weft threads should remain straight and uniformly distributed to minimise resin rich areas. These threads are held together by relatively thin binders inserted in either the warp or weft direction Selection of yarns The final fabric structure of the woven composite is dependent upon both, the initial fibre geometry adopted during weaving, and the effects of manufacturing process. Yarn selection is thus very crucial to achieve good results. Glass yarn (1200 tex) is selected to get the desired composite properties with its high stiffness and strength. Glass fibres have been used for many years to produce fabrics and reinforcement material for composites because of their traction and tearing resistance, their high modulus and dimensional stability. Cotton yarns, relatively finer in comparison to the warp and weft (glass) are used in weft-wise step performs whereas fine glass yarns are used for warp-wise step formation. 53

55 3.4 Design and development of 3D step woven preforms The basic design criteria for the 3D fabric are step formation along weft and warp directions. (1) The first design consideration is manufacturing the steps in weft direction as shown in fig 3.3. Fig. 3.3 A schematic view of 3D fabric with the steps in weft direction In this design, the length of the step is limited due to the working width of the loom, whereas the width can be higher. (2) The second design consideration is manufacturing steps in the warp direction. Fig. 3.4 A schematic view of 3D fabric with steps in warp direction In this case, the maximum width of the step is restricted to the working width of the loom, whereas the length can be higher. 54

56 3.4.1 Design of the 3D fabric with three steps in weft direction Orthogonal weave is considered for the 3D fabric with the steps in weft direction. Orthogonal structure binds all the layers from top to the bottom. This attribute helps the stability and integration of the structure. In addition, it also helps to keep the warp and weft yarn straight to a great extent. Another advantage is the yarn path for binding yarns as the binding yarn tension can be easily maintained with the help of only one beam. The basic concept of weave design for all layers along the step is same for multi-step fabrics as shown in Fig 3.5. The binding yarns and leno yarns are arranged in such a way that they hold the edges of each section of the fabric after cutting off the extra weft yarn lengths. Binding yarns are introduced after an interval of two straight warp yarns to achieve high strength in z-direction. Double picks are inserted through rapier mechanism to achieve good fabric reinforcement. The step edges are locked using leno weave hence there is no need for resin application. Leno yarns need to be arranged at edge of each step formation to avoid warp yarn slippage after cutting extra weft yarn lengths The draft of the fabric is shown in Fig First three heald frames are for body warp yarns and another six and four heald frames are for binder yarns and for leno yarns repectively. The binding weave is plain so it needs six heald frames to bind three sections. The weave design for the fabric the single layer binding weave and leno yarns for single layer are the same, so only four heald shafts are required to arrange the four leno yarns to lock the two layers and three layer arrangements. The leno yarns can be set in single layer binding yarn heald shafts Fig. 3.6 represents three dimensional view of multi step fabric after cutting the extra wefts. However, manufactured preform slightly differs from schematic as there are two warp stuffers in between binder yarns. 55

57 Fig: 3.5 Weave Design, Draft And Peg Plan for the Three Step Fabric 56

58 Fig. 3.6 step formation in weft direction Design of the 3D fabric with steps in warp direction The next step after designing the 3D fabrics with the steps in the weft direction is to design the fabric with the steps in the warp direction, and jacquard shedding mechanism is utilised for this purpose. Jacquard shedding mechanism gives the flexibility and versatility for production of the 3D fabric. Shuttle loom with jacquard shedding mechanism is explored for the production of the 3D fabric with steps in warp direction. This weaving machine demonstrates good potential of the weaving process for the production of 3D fibre reinforcements as, along with weaving flexibility, it provides with a secure selvedge without any assistance of a selvedge creating mechanism. Machine parameters are considered to make assumptions regarding production of maximum number of steps in the 3D fabric. Although the jacquard shedding mechanism is capable of making a number of steps in the fabric due to its versatility, certain aspects also need to be acknowledged. Shed size is an important parameter that requires consideration; increasing the number of layers reduces the shed size reduces, and this can lead to yarn damage and interfere with weft insertion. As for the warp let-off motion, a separate beam needs to be provided for each warp layer in order to maintain uniform tension in all warp yarns. Practically, for the given loom setup, six 57

59 warp beams and hence six warp layers qualify as optimum. Thus the final design includes six warp layers and seven weft layers which are bound by binder yarns. Orthogonal structure still remains the ideal weave for the design of this fabric as it binds the warp and weft very well. Also, one of the reasons to choose this structure is the need of step formation in warp direction. The fabricated structure is such that it starts with the six warp layers which are gradually reduced one layer. It is thus very important to choose the binding yarn path around the step formations. As shown in Fig 3.7, binding yarns follow an orthogonal yarn path from the topmost to the bottommost warp layer thus enhancing structural integrity at the step formations. Fig D view of the multi step fabric The smooth transition stage of the binding yarns at the step formations clearly illustrates the stability of the structure. This binding yarn path also helps prevent weft yarn slippage at the end of the steps, and thus there is no need of leno yarns, as in the first structure design, to avoid the yarn slippage. The next stage after weave design selection is neat drafting and denting of all the warp yarns. In order to avoid any yarn entanglements and minimise fibre damage, warp yarns are drawn from each beam, starting from the bottommost to the topmost beam in ascending order. The structure design of the resultant fabric is as illustrated below in Fig 3.8 which clearly reveals the step formations. Extra warp yarn lengths are cut down manually after completing fabrication. 58

60 Fig.3.8 3D view of the multistep 3D fabric Fig 3.9 Production of Preform on loom After successfully designing and manufacturing the fabric with multistep in warp direction, other weave structures such as layer-to-layer and orthogonal-hybrid are implemented to achieve one-step preforms. These fabrics are mainly fabricated for testing and analysis purposes, thus a one-step fabric is designed with six warp layers stepped down to three warp layers. The design and structure of these fabrics are 59

61 shown in fig These fabrics are laminated and then tested to compare structure properties. Fig 3.10 one step 3D fabric (orthogonal and orthogonal hybrid) 60

62 3.5 Manufacturing Process In order to produce desired 3d woven structure, Different multilayer design structures have been selected as described earlier in the design consideration. For this research two looms are used for the production of different 3D woven structures. The manufacturing process on shuttle loom and rapier loom are explained. Weaving process consists of primary and secondary motions which are performed in specific sequence. The normal sequences to produce the weave structure are Warp beam preparation Warp let off and tensioning Shedding Weft insertion Beating The diagram which shows this operation of the weaving process is shown in fig Fig: 3.11 Weaving process 61

63 3.5.1 Warp Beam Preparation. Before weaving it is necessary to wind the warp yarns onto the warp beams of the loom. The purpose of warp preparation is to transfer yarn from the small package to a weaver s beam which can be placed on a loom. Geometry of the structures consists of several layers of warp, so the process requires several beams of warps. Some 3d weaving processes generally use the warp creel to maintain the tension in warp yarns, but different beams for the warp layers can also be employed to solve this problem. The warp beams hold the source of the warp yarns for the weaving. Design of the fabric necessitates two sets of beams, one for the straight yarns and one for the binder yarns. In the rapier loom, leno yarns are supplied from individual bobbin arrangement. Three beams for straight warp and three for binding yarns are made for the production of step (the weft direction) woven fabrics. Leno yarns are supplied from self tensioned bobbins. For the step (warp direction) woven fabric, six warp beams were used for the straight yarns and one beam for the binding yarns. Single end sizing machine was used for beam preparation. After winding the yarns on the big wheel, it has to be unwound on the warp beams. These different sets of yarns are unwound to the special beams. There is a special arrangement to mount these beams on the loom. The arrangement of the beams on the rapier weaving loom and shuttle weaving loom are shown below in Fig 3.12 and

64 Fig 3.12 Arrangement of Beam in Rapier weaving machine Fig 3.13 Arrangement of beams in shuttle weaving machine 63

65 The binding yarns for these structures are very fine. The reason to use this kind of yarn is basically to avoid creating resin rich areas in between the warp layers. This provides rigidity and integrity to the structure. The beams for binding yarns are wooden cylinders for the rapier weaving machine. As the binding yarn does not require too much unwinding tension and they are very less in numbers, the binding yarns are wound on the beams manually. On the top of these beams, there is a circular rod which assists yarn path to the heald shafts as shown in Fig Leno yarns are coming from different bobbins with self tensioning by the spring arrangements in set up. For the shuttle weaving machine, only one binding yarn beam is used because of limited availability of beam mounting space. Fig: 3.14 Binding Yarn Beams 64

66 3.5.2 Warp let off Warp let off mechanism is very important in the weaving process. This is the motion that delivers warp to the weaving area at the required rate and at a suitable constant tension by unwinding it from a flanged tube known as the warp beam. The warps are fed in equal amount for conventional weaving looms in which a warp beam is used to perform the let-off motion. However, warp shedding lengths are different; hence the separate beams are used for each layer. The let-off for warp yarns must be controlled separately to fit the shedding motion. As external (not mounted on the loom) beams on the machine, negative let-off mechanism is used. Here, depends on the requirement of the unwinding of the yarns, weights are attached with individual beams. By these weights, the warp yarns are tensioned. The take-up force gives the pulling force to the sheet of warp yarns. So by this mean, the let off is achieved in this project Shedding For the rapier weaving loom, warp yarns pass through the heald eyes and reed dent. Heald wires are arranged in the heald shafts. Heald shaft are raised or lowered according to the design. So the vertical movement of the heald shaft controls the weaving design. Here, dobby mechanism is used for the shedding mechanism. The number of heald frames limits the thickness of the fabric as thickness depends on the number of layers. To avoid this, shuttle weaving loom is used with jacquard mechanism. In this mechanism, each heald wire is individually operated by jacquard which gives more versatility in the design of the structure. Each end can be raised or lowered according to the need of the structure to create shed as shown in fig Fig Shed Creation in shuttle weaving machine 65

67 3.5.4 Weft insertion In rapier weaving machine, the weft insertion is done by rapier. The machine has the rigid rapier. This machine is modified by the technical staff to do some special jobs. Originally it was automatic, but now it is manual. It has one side (left) rapier which pass the weft through the warp shed. Rapier is used to pull glass yarns through the shed formed between the warp yarns. The other side rapier (taker) is removed. Because of the very high stiffness of the glass yarns used for this fabric, rapier is the best to get the desired quality. The advantage of using rapier is due to the positive insertion of weft. In the rapier mechanism, rapier carries yarn through the shade, so there is no extra force on the yarn. So it avoids damaging of the yarn and handles the yarn very carefully. Due to the size of the rapier, the opening of the shed is not required very big like shuttle loom. As per the requirement double pick is inserted into the shed. Hence on the other side of the shed, grabbing the yarn (form the loop not from the tip) achieved the double pick insertion. As there is no automatic cutter on any side, yarn has to be caught after each pick cycle. The diagram of the weft insertion is shown below (fig 3.16). 66

68 Fig: 3.16 Weft Insertion by Rapier in Jumberca Loom For the shuttle weaving machine, weft insertion is done by the shuttles. Because of the weft yarn stiffness, the weft insertion was done manually. In addition, because of the narrow working width of the fabric, manually weft insertion was done to reduce the amount of yarn wastage. The weft yarn was wound on the handloom shuttles in the small length. By this kind of weft insertion, natural selvedge can be formed to avoid the slippage of warp yarns. 67

69 3.5.5 Beating In the rapier weaving machine, the beat-up is done manually. As the machine is rotated by hand, the beat-up force is not too firm. As glass yarn is used, so manual beat up creates less damage to the yarn. After picking, the weft yarns were closely beaten up against the fabric fell line in the shuttle loom. Beat-up was performed by the reed which is positioned on the sley mechanism that derives it s to and fro motion from the crank shaft. The reed simply pushes the newly inserted weft into the fell of the cloth Take up mechanism After beating, a take-up mechanism pulls the 3D woven fabric along the loom so that the weft insertion and beating stages can be repeated. It is in the take-up stage that the woven fabric is complete, and ready to be processed into 3D woven composites [32]. In the Jumberca machine, the rotation of the pick cycle is manual, so it is very hard to maintain the take-up speed. This process in this machine is automatic but it is not precise. After rotating the wheel per each pick cycle, take-up has to be done by rotating the take-up roller. This is done by the take-up button on the machine. Take-up is performed after insertions of certain number of pick insertion. This process defines the weft density in the structure. To keep the weft density uniform, one has to be very careful in operating this mechanism. In the shuttle loom, take up is performed by sets of rollers which are driven by gears. The ratio of these gears changes the pick density of the fabric. Some modifications have been done on the take up roller to avoid slipping of the fabric during winding due to glass yarn characteristics. To explore the weaving effect on the 3D woven structure, these structures were impregnated with resin to do analysis. The main reason to examine this structure is to discover any fibre damage during the weaving process for this kind of structures and to find out the basic yarn geometry in the structure. 68

70 3.6 Observations and Analysis After manufacturing the fabric, some observations and analysis have been made to find out the problem and their likely solution. 3D woven fabric with step in weft direction The extra weft yarn lengths i.e. wefts which are not interlaced with warp or binding yarns need to be cut. But this cutting was done manually which results in unwanted length of fringes at each step. At each step creation, there is set of leno yarn which stops the warp slipping off. In these fabrics, there is no natural selvedge. At the edges, there is set of leno yarns which help in holding the structure at that position. This set of leno yarn is capable of holding the structure, but there is a varying tension in the leno yarns. This problem can be sorted out by using individual leno yarn bobbins with tensioning device. The resultant fabric has almost no crimp in the warp and weft. Binder yarn joints the structure. The figure of this structure is shown below. Heald shafts were arranged so that excessive opening of sheds is avoided which results in low fibre damage. The positive insertion of weft was done by rapier mechanism. The manual take-up helped to minimise damage in the fabric. Although much care was taken during the warping and weaving, there was a filamentation problem in the warp yarns which is not desirable in the final product The initial filamentation can become more and more due to weaving cycle and yarn properties, if it is not removed at an early stage. There are few limitations for this design and the used weaving machine. The step length can be limited as per the working width of the weaving machine. Although the width of the step is not restricted and can be made up to the requirements. As this weaving machine has dobby shedding mechanism, there is a limitation in the terms of the maximum numbers of the step it can make. In addition, other multilayer fabric weaves cannot be implemented as they need more numbers of heald frames because of their designs. 69

71 Also special care was taken in different stages during the weaving. Like for warping, separate beams were used for each warp layer and for each binding warp yarns as the warp shedding lengths are different for each layers and binding yarns. 3D fabric with steps in warp direction To increase the flexibility of the weave design and more yarn layers, a shuttle weaving machine with electronic jacquard was used for the production of this 3D fabric. Jacquard gives flexibility in selecting weave design and opportunity to increase the warp layers. Also the limitations on number of steps depend upon the maximum warp beams accommodated on the loom As the shuttle weaving machine was used, there was no need of leno yarn to bind the yarns at the selvedge. Shuttle cannot be used as it was not possible to wind the glass yarn on the pirn due to its properties. Hence, manual winding of yarn was done on handloom shuttle. There was a problem in the shedding due to multiple warp layers and heavy yarns. The shed size was not enough sometime due to stack of multiple layers in the bottom shed layer. Filamentation problem was greater than the previous case as it was very dense structure. Jacquard was used as shedding mechanism which carries individual yarn. But the heald wire was creating abrasion with the subsequent yarns creating the filamentaion. Although beat up and take up were done automatically, but there was not proper control on ratio between the cycles. This also resulted into the weft yarn damage and varying thickness in the fabric. From the cross sections, it can be seen that the warp and weft yarns remain straight and the binder yarn binding all the layers. The weft cross section was varied across the fabric due to the different take up rate as seen in Fig 3.22 which resulted in slight taper. Although, the machine produced the fabric with multiple steps in the uniform shape, the production rate is very slow. 70

72 Fig: 3.17 Step Glass 3D Fabric Fig Six step fabric on loom 71

73 3.7 Prospects of other 3D fabric designs The concept of weaving step joints can be taken forward to produce taper laminates. If several warp beams are used, thick structure can be produced. The thickness of the structure can be reduced gradually by removing warp layer at regular intervals. These kind of taper structures are in demand in composite applications. 3D woven structure can be a good replacement for unidirectional laminates in these structures to get through thickness reinforcement. Three weave configurations have been manufactured in order to do preliminary testing. These structures are also produced with one step formation in order to explore prospects of joint structures. Fig 3.19 demonstrates these three weave configurations. Fig 3.19 (a) Orthogonal Hybrid (b) Orthogonal (c) Layer to layer Multi-step tapered preform has been produced in the warp direction as shown in Fig Each warp is supplied from a separate beam so that the dropped warp layers can float with correct tension. These floats need trimming after the preform is taken out of the loom. Alternately, each dropped ply can be cut using an automatic cutter positioned near the warps this will eliminate wastage and minimize post-trimming operations. 72

74 Fig Weave design for the taper section For double lap joint, two three dimensional structure will be produced to get the optimum joint properties and then joint by vacuum infusion technique. These woven structures will be produced on conventional weaving machine with some modifications. Now, the next stage is to develop 3D structure for scarf joints. These structures can be joined by consolidation method as shown in Fig Fig Double lap joint weave design 73

75 Multi-stepped preforms are suitable for lap joints however steps are too coarse for scarf joints and also for applications that require gradual taper. Here, we have developed an improved weaving process by controlling the degree of compaction applied to the weft tow. We can achieve gradual taper without dropping individual warp layers as shown in Fig However, the degree of taper is limited with this process. The idea is to incorporate this tapering technique along with the multi-step preforming method (section B) in order to obtain gentle transition at each step. Fig 3.22 Taper 3D fabric cross section 74

76 3.8 Conclusion Production of these fabrics has revealed that the weaving machine can be used for preform production. The weaving process can be used for the production of preforms with yarns of different compositions. It is possible to manufacture variety of shapes required to meet the specific requirements. However, there are few problems which need to be addressed. One of the problems is the extra yarn length because of the shape of the preform. This extra yarn length results in unwanted waste which makes this process uneconomic. Among its other limitations, multilayer fabrics contain fibres orientated in three directions. Some fibre damage is expected in manufacturing. This damage is dependent on mainly weave design, machine parameters, yarn paths and fibre types. As fibre moduli increases, there is more chance of damage while manufacturing it. These can be minimised with some modifications of the process and weaving machine details. In addition, the production of the fabric is low compared to other composite manufacturing processes. Short beam test was carried out on produced structures along with step joints. Result and analysis is discussed in chapter: 8. Microscopy was also performed on these structures in order to understand tow behaviour and damage during processing which is described in chapter: 8. 75

77 Chapter: 4 Design of the purpose built 3D weaving machine 4.1 Introduction As discussed in the previous chapter, step and taper 3D preforms have been manufactured successfully on the conventional weaving machine. However, there are some issues related to the weaving process while manufacturing. Main issue is the production of fabric, where one weft is inserted at one time. This way it follows all the weaving motions (primary and secondary) to insert one weft. In addition, conventional weaving machine is used for 2d fabrics; in which shedding is done by two warp layers. For 3D fabrics, multiple shed is created for several warp layers at the same point to insert weft one over each other. Such arrangement makes the shed size smaller compared to normal cycle. This creates abrasion between the warp layers to the weft insertion mechanism. There is also a problem with shedding and abrasion between the heald eye and warp yarns. Another issue with 3D fabrics is that the beat up where reed takes the newly inserted weft into the cloth fell position. However, due to several weft yarns being aligned in one vertical line, reed has to take those weft yarns into that one position. During such operation, the contact between weft yarns and reed is more than one time and that results into the damage of the yarns. These issues lead to the need for a purpose built weaving machine. The proposed machine can solve the problems stated in the above. There have been so many proposed and new methods from different authors from all over the globe as discussed in chapter-2. In the present work we have made an attempt to make a 3D weaving machine based on work described by several patents and methods. The drawbacks of the existing methods are already mentioned earlier. The primary object of this research is to develop a machine which can produce the 3D structure to replace the current 2D laminate and its deficiency. We have considered different mechanisms involved in weaving to overcome the problems related to 2D fabrics in order to create near net shaped preform. 76

78 4.2 Basic Design The main criterion to develop the new machine is the fast production of near net shaped preforms. In the proposed machine we have tried to overcome the problems associated with the conventional weaving machine and other processes to produce 3D preforms. The key criteria for the design are stated below. Multiple weft yarn insertion Permanent shed creation Uniform warp yarn tension Flexible binder yarn arrangement Linear take-up Open needle beat-up Knitted selvedge formation Automatic cutting system for taper creation The schematic diagram of the proposed machine is shown in Fig 4.1. Multiple weft insertion is done by rapiers which take the yarns in loop form. These rapiers go through the shed formed by several warp layers. These warp layers create shed with the help of separating rods. Once the rapier reach to the other side of the shed formation, the needle bed arrangement holds the weft yarns into the selvedge needle. After retraction of the rapiers, the needle bed takes the newly inserted weft yarns into the cloth fell position. Linear take-up is designed to maintain the fabric architecture. Binder yarns bind warp and weft yarns in the structure. 77

79 Fig 4.1 A schematic view of proposed machine After drawing outline of the principle, different factors and process methods were evaluated according to the requirement. The geometry of yarns at different stages were assumed for the dimension and working of the machine. Different factors such as yarn geometry, yarn property, weave design were considered for the frame and machine part dimensions. 78

80 4.3 Design parameters for weaving processes The basic principle of the proposed machine remains same as the conventional weaving machine. To produce the structures with superior composite properties, the principal warp and weft yarns should remain straight (non-crimp) and uniformly distributed which should be the main design consideration for developing the 3D weaving machine. In order to reduce the fibre damage, processing of the yarn at each weaving motion cycle has to be very smooth and cautious. An attempt has been made to modify the primary and secondary motions to achieve the objective of this research. Once the weaving mechanisms were designed, electric and pneumatic drives were chosen to perform the machine operations Yarn feeding device In order to get the thick 3D structure, several warp and weft yarns were fed into the weaving zone. It is very difficult create dense zone using these yarns due to their properties and behaviour. As discussed in the basic design consideration, the stacks of warp yarns were arranged horizontally and vertically both as shown in Fig 4.2. Such arrangement can be set with either the use of several beams or individual yarn end creel. Both of these arrangements have their own advantage and disadvantages. Fig 4.2 Vertical and horizontal stack of warp yarns 79

81 In the yarn creel arrangement, each warp yarn is fed from individual bobbin. In order to feed into the weaving zone, each yarn comes to the reed from passing through several guides and tensioning roller. The advantage of this set up is proper tension control of the yarns. This gives flexibility of the different individual yarn paths which can result into the different multilayer weave designs. In addition, the yarn is stored in the bobbin, so it gives more length of the yarn to work with. This helps to produce longer length of fabric on the machine. There are also certain drawbacks in this setup. First of all, it needs large area to accommodate many warp layers as it requires so many bobbins. The set up for all the yarns into the creel and then into the weaving zone is very time consuming as each yarn has to pass through several guides and tensioning devices. In addition, it is difficult to handle the work space for these yarns. If some warp yarn breaks somewhere in the middle of the region, it would be very difficult to amend that yarn set up. Fig 4.3 shows the yarn creel arrangement in which if we consider yarn breakages at the middle region (black mark), it would be difficult to amend that yarn. Fig 4.3 Yarn creel arrangement 80

82 In the warp beam arrangement, several warp yarns are wound on beam with the help of warping machine. Normally in the conventional weaving machine, there is one warp beam which carries all the warp yarn to the weaving zone. As the produced fabric is two dimensional, the feed rate of all warp yarns is similar. However, in the multilayer fabric, warp yarn feed rate is different due to their yarn path pattern. There would also be yarn tension issue, if only one beam is used for all the layers. To avoid this, several warp beams can be used depending on the yarn path pattern and number of yarn layers. The advantage of such set up is that the control of the yarns and its passage from beams to the weaving zone is easier. In addition, each warp beam can carry several warp yarns which permits an opportunity to increase the size of the fabric in terms of thickness and width. Each beam is individually under the tension control mechanism which controls the tension during the let off operation. This system has also some drawbacks such as limitations in the length of the warp yarn on the beam. In addition, there are also some chances of the difference in the warp yarn tension and this can create problems during the weaving process. After reviewing both warp yarn setup, warp beam arrangement was found to be more appropriate for the proposed weaving machine. One of the reasons to choose the set up was the ease of handling warp yarn passage. The warp yarns from each beam remain straight, in the designs for the 3D fabric. Hence, there is less chance of tension variation in the warp yarns. In addition, length of the warp yarn accommodated on the warp beams was not a critical issue due to small length of the final fabric. Figure 4.4 shows the schematic diagram of the warp beam set up. 81

83 Fig 4.4 Warp beam arrangement As mentioned earlier, these binder yarns can be from either warp or weft yarns. Along with the warp stuffer yarns, there are binder yarns in the design of the structure in the warp direction. These binder yarns bind the warp and weft yarns together and make structure. Hence, the binder yarn is very important in terms of integrity of the structure. The binder yarn path is normally dependent upon the weave structure such as orthogonal, angle interlock and layer to layer. Depending on the structure, there is a different yarn path for each binder yarn. This results into the tension variation between the binder yarns. To maintain the tension in the binder yarns, separate beams are made for different path binder yarns. Binder yarns pass through heald eyes before entering into the weaving zone. These healds are lifted and lowered according to design requirements. The movement of the healds take the binder yarns to their position to bind the warp and weft layers. These healds are joined with springs at one end and strings with other end. The spring end is fixed on one plate, whereas the strings are attached with the roller on the bottom. Alternate healds are attached on both side of roller, so when one set of healds are going up other is going down. This mechanism demonstrates negative type of shedding motion, where raising of healds are with the help of springs. 82

84 Warp yarns are drawn into the reed from the warp beams. The main function of the reed in this development is the separation of the warp yarns in individual warp layer. Reed height is kept more than normal reed to accommodate all the warp layers. Before passing through the reed, warp yarn layers go over or under the metal rods as shown in Figure 4.5. The position of these rods is adjustable according to the requirements. The main function of these rods is to separate each warp layer from the other warp layer such that it creates shed between the warp layers. Hence, the shed size is determined with the help of adjustable rods. Reed size is kept larger compare to the conventional reed to accommodate all the sheds. Basically, each warp layers are separated with the help of rods and each warp yarn in the layers are separated with the help of reed. The metal rod position is arranged such that it creates multiple shed between the warp layers. Fig 4.5 Adjustable rod arrangement for warp yarns 83

85 The shed size of the individual shed is such that the weft insertion mechanism can pass through the shed. The geometry of the shed has to be very precise for easy weft insertion and less fibre damage. Normally, the shed size depends on the distance between the warp layers and distance between reed to cloth fell position. The metal rods were arranged after considering these facts to optimise the shed size. Fig 4.6 demonstrates multiple shed geometry with required sizes. Fig 4.6 Final shed size Geometry Weft yarn feeding device The weft yarn set up is not as much complex as the warp yarn set up. Warp yarns remains in the weaving area all the time, whereas weft yarns are introduced into the weaving zone during each weaving cycle. In conventional weaving machine, wefts are inserted into the shed by means of different weft insertion mechanism such as shuttle, rapier, projectile etc. These weft insertion mechanisms only supply one weft yarn during weaving cycle. Here, the proposed weaving machine illustrates multiple weft yarns during each cycle by means of rapiers. These weft yarns can be drawn into the rapiers from the packages. These packages could be either cone or cheese yarn packages. 84

86 Cone packages are normally adds the twist into the yarn while unwinding as it is open end. Yarn twist was not desirable in the proposed fabric structure. Hence cheese package was used for the weft yarns. These weft yarns pass through series of guides and tensioning device and then into the rapiers. The schematic view of the passage is shown in Figure 4.7. It can be seen that the yarns pass through a roller which is between the two guide series. This roller compensates the extra weft yarn length during the weaving cycle. There is also one freely rotating bearing guide roller near the rapier base to assist the weft yarns feed. Fig 4.7 A schematic view of weft passage The weft insertion principle is based on rapier mechanism. The main reason to choose the rapier mechanism was their positive insertion of weft yarns. Rapier can handle any yarns and it does not add any unwanted properties such as false twist or yarn stretch. Several rapiers were used to insert the multiple weft yarns in the loop form. Each rapier takes double weft yarn into the shed created by multiple warp layers. 85

87 These rapiers were designed from the basic principle of needles. Figure 4.8 shows the rapier design. It can be seen that the series of rod is tapered into the arrow shape at one end and fixed onto the plate at other end. There is a ceramic guide in the front end for smooth passage of the weft yarn. To give flexibility in the design of the fabrics, this rapier length can be shortened or increased as per the requirements. Size and distance between the respective rapiers were decided upon the shed geometry in front of the reed. As discussed earlier, the shed geometry is dependent on the distance between the warp layers and their respective position from the cloth fell. The rapiers were designed in order to pass through the respective sheds very smoothly without disturbing any warp yarns. Fig 4.8 Rapier design 86

88 4.3.3 Selvedge yarn feeding device In this proposed machine, there are selvedge needles which hold the weft yarns on both side. Once the rapier inserts the weft into the warp layers, the selvedge needle enters into the loop created by the double weft insertion. The basic concept is to have several needles on the needle bed and then insert those needles into the layers of warp and weft. After vertical moment of the needles, the next movement for the needles is the horizontal. This causes the needles to take the newly inserted weft yarns into the cloth fell position. This concept is like the finger reed beat up where open wires go into the shed and do the beat-up. Figure 4.9 shows how the needles insert into the shed area, when the weft yarn is inserted, The rapiers were designed such that it inserts double weft yarn, thus needles has to take double yarn into the cloth fell position. At the same time, needles have to lock the weft yarns at the other end for proper weft insertion. To perform this, needles have to get engaged with both yarns. Fig 4.9 Needle insertion into warp shed 87

89 Hence, design of the needles bed is such that only last needle is kept off axis then other needles. The last needle goes in between the weft yarn loops, and other needles are in position behind the double weft yarns as shown in figure So the last needle locks the weft yarns and other needles provide support to the weft yarns and takes it to the cloth fell position. The bobbins for the selvedge yarns are mounted on a machine frame according to the requirements for near net shaped preforms. The selvedge needles are around 2 mm in diameter for the easy penetration in to the shedding area. Fig 4.10 Weft loop catching with needles One of the reason to do the needle beat-up is to reduce the extra movements of the reed for beat-up. There must be some sort of mechanism which can bind the weft yarns at several points to get near net shape structure. In this concept, this mechanism is utilised for dual purpose, first to lock the weft yarns and then secondly to take the weft yarns into the cloth fell position. Some of these needles carry the selvedge thread along with them. These threads bind the weft yarns at the respective positions. These selvedge threads transfer to the knitting needle from the selvedge needle. The knitting needle takes the selvedge yarn and movement of the knitting needle causes the loop formation in selvedge yarn. This cycle results into the loop selvedge formation at the respective position. 88

90 The selvedge yarns are supplied from the bobbins which are equipped with tensioned brakes. As the selvedge yarns are very fine, yarn pulling force should be very low for easy operation. Special care has to be taken when handling the yarns. To minimise the pulling force, the bobbin rotation was made friction free with the aid of lubricant and bearing roller. The yarn from the bobbin passes through ceramic guides before entering into the selvedge needle. These needles have to travel horizontal first near to the weft insertion mechanism. After the weft insertion, these needles go into the shed making vertical moment. This follows with the horizontal movement of needles taking the weft yarns into the cloth fell position. After shed change over, these needles go into their original position with the vertical motion. This whole cycle covers a great distance. During this travel, selvedge yarn also draws along the needle. Due to these travel, the friction between the needle and yarn has to be as low as possible with the moderate yarn angle between needle hole and guide. These factors are considered while designing this mechanism. The guides are kept near to the base of needle to such that it makes enough space to enter the knitting needle between the yarn and the needle as shown in Fig It also helps smooth passage of the yarn during this operation. But at the same time, the yarn angle should not be very large which can distort the weft yarns while entering the needles into the shed area. 89

91 Fig 4.11 Knitting needle mechanism Another important criterion is the yarn compensation during this operation. The selvedge yarns travel with the selvedge needle, but their consumption in the fabric is very small length. Hence, the extra drawn yarn from the bobbin needs to be compensated. If this extra yarn is not compensated, then it can result into the slackness of selvedge yarn. When knitting needles go into the yarns, selvedge yarns should be under tension for smooth transition of the yarns. To compensate the yarns, there is a need for the yarn compensator. Figure 4.12 shows how the yarn compensator was attached with the bobbin. This compensator is like a flat spring with guide on the top. As mentioned earlier, the yarn bobbins are equipped with tension brakes. Here, the flat springs are attached with these brakes for proper selvedge yarn flow. The yarn tension increases upon movement of needle. This tension pulls the flat spring which results into the release of the brakes. Hence, the yarn is unravelled easily from the bobbin. When the needle is retracting, the yarn compensator goes down taking the extra selvedge yarns and applies brakes onto the bobbin to stop the further yarn feeding. 90

92 Fig 4.12 Yarn compensator mechanism Knitting needle mechanism The needles are designed for lock the weft yarns and then take it to the cloth fell position. But there is a need for some mechanism to hold the weft at its position on both ends. To fulfil this purpose, some needles (depending on the preform near net shape) take the selvedge yarn along with them to lock the weft yarn at the respective position. But some locking mechanisms are needed to lock these yarns at top end to create the selvedge. Hence, knitting needles are used in order to catch the selvedge yarn and form selvedge. Once these yarns are locked, selvedge needles can move down. This results into the selvedge formation at the respective position. The positions of knitting needles are designed such that it takes the selvedge yarns from the needles and create knitting loop. These knitting needles have to be fine for easy transferring of the yarns. 91

93 4.3.5 Take-up mechanism One of the disadvantages to produce 3D woven structure on conventional loom is their take-up. Normally, take-up in conventional loom follows the passing of the fabric under several rollers to maintain the fabric grip and tension. In this sequence, the fabric creates several curvatures during the take-up which is suitable for 2D fabric. But, this kind of take-up is not good for 3D fabrics, as it changes the yarn geometry within the preform. Normally, 3D preforms are stiff which limits its draping, so this kind of take up can create the yarn damage as well. To avoid this, linear take-up is designed to maintain structure integrity and to reduce the fibre damage. After the fabric formation at the cloth fell position, fabric passes through the set of rollers under the tension. These rollers are tensioned with springs as shown in Figure Due to these rollers, the fabric maintains its position with reference to the take-up mechanism. Generally, if the fabric is pulled from one side, then it has tendency to drop downwards. This behaviour can cause the change of the shedding geometry which can lead to problems in the weft insertion. It is important to grip the fabric after the fabric formation. Hence, rollers are kept as near as possible to the cloth fell position. These rollers also assist the forward movement of the fabric which is done by the take-up grips. 92

94 Fig 4.13 Tension roller arrangement After drawing and denting, all the yarns pass through the rollers and are taken between the two plates. These two plates grip the yarns very firmly without damaging the yarn properties. The surfaces of these plates are covered with emery cloth material to increase the friction between the yarns and the surface. This leads to firm grip of the yarns such that take-up plates can take the fabric without slippage of yarns. Above considerations were discussed and finalised to built proposed weaving machine. Machine parts and drives were reviewed from available sources for final selection. Some machine components have to be very precise in dimensions due to their precise role in the processing cycle. In addition, the machine frame was customised to accommodate all the parts and drives. Selection of the drives and controls were crucial in this process. This was done by taking account of process parameters, cost and easy to use features. 93

95 4.4 Single yarn multiple insertion It is understood from the initial design concept that a double yarn insertion system has certain limitations such as weave design flexibility. In order to produce 1:1:1 (warp: weft: binder) structure, the previous concept cannot be utilised as it inserts yarn in loop form which makes it double weft insertion. Weft insertion and beat-up mechanisms have been modified in a new design to generate 1:1:1 structure which is illustrated in Fig 4.14 whereas let-off and take-up mechanisms are the same as previous design.. Fig 4.14 Single yarn multi insertion system 94

96 4.4.1 Weft Insertion In this system, the main criterion for the weft insertion system was transfer of weft yarns from one end to another end rather than locking the yarn onto other end. This ensures less stress on weft yarns during weft insertion. Weft yarn tail is provided at the end of tube (rapier) in order to grip yarns accordingly. Weft insertion rapier head is modified from double yarn insertion where multiple tubes are installed with pneumatic brakes. In addition to rapier head, three gripper systems are installed to assist weft yarns gripping during weft transfer from one end to other end. Figure 4.15 illustrates the single weft insertion process in sequence with multiple insertion system. (A) Starting position of weft insertion system where yarn tail is provided at the end of rapier with pneumatic brake on inside rapier housing (B) Rapier transfers weft from one end to other end with pneumatic brakes on to ensure there is no slippage of yarns during the motion (C) Gripper system-3 is activated at other end and weft yarn tail is gripped (D) Pneumatic brake is deactivated and rapier is retracted to original position (E) Gripper system 2 and 3 are activated as well as pneumatic brakes in rapier housing to ensure yarn is gripped thoroughly prior to cutting process (F) Cutter is activated and weft yarn is cut in between gripper 2 and 3. This ensures required yarn tail is left at the end of rapier for next process cycle. 95

97 Fig 4.15 Weft insertion system 96

98 4.4.2 Beat-up Beat-up is done with insertion of needles into shed and then needles take weft yarns into cloth fell position in first concept. There is a chance of fibre damage during needle insertion into shedding zone. To improve structure performance, movable reed is designed to replace beat-up process. Reed is placed on platform which takes weft into cloth fell position once weft yarns are transferred into warp shed. After weft insertion, all gripper systems are open to release newly inserted weft yarn. Reed takes these yarns into cloth fell position and comes back into original position. Shedding and takeup process cycle remains same as previous design concept. Fig 4.16 Beat-up position 97

99 4.5 Selection of Drives and controls The selection of drives and controls for the machine processes are crucial for success of weaving. Different drives such as stepper motor, servo motor, pneumatic cylinders were reviewed on the basis of their specifications and compatibility. To simplify the process automation, all the drives and controls were selected from one company. FESTO automation offers wide range of electric and pneumatic drives to satisfy requirements for processing cycle. As this machine is designed and built on basis of concept, drives and controls need to be flexible to accommodate minor changes if required in the design while production. Electric and pneumatic drives have their own advantages and disadvantages. Hence, selection of drives for particular process is based on the requirement of process and merit of the drives. All the processes per one cycle are shown below with the movement specifications Shedding Mechanism As discussed in the design of the machine, the warp yarns are fed straight into the shed from the beams. Warp let off is kept negative. Hence, there is no need for any drives or controls for that. Warp beams are attached with springs, which controls let off depending on the warp yarn tension. Still we need some drive to control the shedding of binder yarns according to requirement as binder heald wire goes up and down to bind the warp and weft stuffer yarns. The binding heald wires are attached with grooved roller at bottom. Arrangement is such that if the roller rotates in one direction, one set of heald wires will go up and other set will come down and vice versa if the roller rotates in other direction as shown in Fig Hence, there is a need of electric drive to rotate the roller in both directions. 98

100 Fig 4.17 Roller for binding yarn movement Rotary motions can be done with the help of either stepper or servo motor. Both drives have their advantage and disadvantages. If we look at the requirements for this motion, drive should rotate in both directions with maximum accuracy. Stepper motors are relatively inexpensive, and provide the same or greater accuracy as servo motors if it has sufficient power to drive mechanism without losing steps. As this mechanism does not drive heavy weight, stepper motor is ideal for this application. It can also be controlled digitally with precise angular increments. The smallest incremental path on a positioning axis is determined by the motor s step angle and positioning axis parameters. Even if stepper motor is very simple to use, it needs a control system to receive electrical pulses which corresponds to the same rotational angle. Smart electomotor controller stepper motor has been designed for use in control cabinets for controlling stepper motors. In addition, this can also be programmed in PLC to control the stepper motor. 99

101 4.5.2 Weft Insertion mechanism Weft insertion is the other mechanism where it needs drive to insert the multiple wefts. In the design, multiple rapiers are designed and attached on one plate to insert weft yarns. Hence, there is a need for the drive to do linear motion which is forward and backward. In addition, the position of these rapiers can be controlled such that it stops anywhere in its limit zone. Stepper motor can be used for this application, but motion needs to be converted from rotational to linear. It can be done with the help of extra components which is expensive and complicated. Hence, pneumatic cylinders are the right selection to satisfy the motion requirements. If we look at the standard pneumatic cylinders, they have got only two positions which are forward most and backward most. Here, the position of the rapiers should remain flexible. Hence, pneumatic cylinders with encoders from FESTO automation are utilised. The speciality of this drive is inbuilt displacement encoder which detects the position of the cylinder and according to that controller can stop or move the axis. The design of this drive eliminates external attachments and cables. In addition, it comes with wide range of maximum strokes which can be customised according to requirement. Single yarn carrier system has need for additional pneumatic cylinders which can operate at low pressure range just enough to grip and hold weft yarns. Although DNCI has inbuilt encoder, it requires axis interface for positioning technology. Axis interface is three way technology where it reads cylinder position from encoder, then gets signal to move particular position accordingly and activates proportional directional valve to send the axis to that position. 100

102 4.5.3 Take-up mechanism After weft insertion, the next mechanism which requires automation is take-up process. Take-up process is designed in such a way that fabric is pulled linearly unlike curved path in the conventional weaving process. This is to maintain integrity of the structure and avoid damage in modulus of the fibres. As this machine is prototype, the maximum produced length of the fabric is kept around 3 meters to reduce the space requirements. Hence, the take-up should be such that takes fabric linearly up to 3 metres as per the design requirements. To simplify the mechanism, two plates are designed to grip the produced fabric, which pulls fabric according to the take-up rate. Therefore, these plates need to be driven according to the requirements. Belt drives are normally ideal for this type of motion. In addition, plates should be easily mounted on the drive. A DGE drive from FESTO has all the specifications which required for this motion. It is very compact with high load capacity and high mechanical torque. This drive consists of toothed belt drive which is driven by normally stepper motor. It has got wide range of options to mount plates and motor. It can also be customised in length according to the requirements. The specification of the stepper motor to run this drive is kept same as stepper motor for the shedding mechanism. 101

103 4.5.4 Beat up mechanism In conventional weaving process, beat-up is normally done with the help of reed movement. Here, open finger beat-up is done with the help of needle bed. As discussed earlier in the design of the process, needles enter into the shed zone, lock the weft yarns and then take weft yarns into the cloth fell. Drive motions are required to process this mechanism as shown in Fig To fulfil these requirements, two separate drives or one drive which can fulfil both motions are needed for horizontal and vertical movement. Fig 4.18 Beat-up process cycle movements As the movement of this process doesn t have to be flexible, standard cylinders are used. In addition, the displacement of horizontal and vertical direction is different. Hence, different stroke standard cylinders are used. As horizontal and vertical movements have to be synchronised, cylinder for vertical movement is attached with the cylinder for horizontal movement. Standard cylinders have two positions, which can be controlled with digital signals (onoff). Hence, they don t require individual controller like other drives as it can be operated from PLC directly with digital signals. This digital signal activates or deactivates solenoid valve which is used for pneumatic supply. 102

104 4.5.5 Selvedge knitting needle mechanism As the formation of the selvedge is with the help of knitting needle, knitting needle has to move forward and backward to form the loop formation. Due to lack of time, knitting needle was initially not installed in the machine building as selvedge formation can also be done with the help of fixed rod which can collect the selvedge yarn. However, selection of the drive was done for future work. As the displacement of knitting needle is less compare to other motions, pneumatic muscle is selected for forward and backward movement of the needle. 4.6 Control of drives Once all the drives are selected for the respective mechanism, the next step is to control them according to the process cycle. Hence, proper controller needs to be selected for drives to run the operation smoothly. The selected drives are stepper motor, Pneumatic cylinder with encoder, and standard pneumatic cylinder. All drives need control to send and receive signals to perform the function. Hence, we require controller with multiple input and output to control all the drives. A PLC (programmable logical controller) is normally used for motion and position control with inbuilt input and outputs. Initial step to build PLC based control system is to identify type and number of inputs and outputs. This is dependent on type and number of drives. This information will decide the type of PLC and I/O module to be purchased. Normally I/O ports are advised to keep more that required for future changes. PLC normally processes digital and analog signals to control the system. Digital signals are binary switches, acting as a on or off signal. Analog signals are like volume controls, with a range of values from zero to full scale. All drives have their own controller apart from standard cylinders. However, all drives have to be synchronised to run the smooth operating cycle. To do this, we require PLC which can be connected with individual controller and programmed accordingly. FESTO has a wide range of PLCs to run these drives in conjunction. However, it needs to be compatible with individual controllers and can accommodate all drives. SPC200 and FC640 have been used to run all the drives. Because of the drive and their control selection, SPC 200 was needed to run the stepper motor and DNCI pneumatic drive. SPC200 can be programmed for normally 1 to 4 axes depending on system requirements. However, there are five axes which need to be operated in this process. 103

105 In addition, future work needs to be considered as well for additional drive configuration. Hence, FEC640 was selected for other two standard cylinders and it can also act as a master controller for SPC200. I/O module from SPC 200 can be connected to FC640 to run all the drives from controller. Thus, we can synchronise all the drives accordingly in FC640. Winpisa and FST4.2 softwares are used to programme SPC200 and FC640 respectively. Winpisa is programming software for drives associated with SPC200. FC640 is used for programming the standard pneumatic cylinders along with executing winpisa programme for other drives. Programming and processing of drives and controller is discussed in next chapter. 104

106 Chapter: 5 Manufacturing Process 5.1 Introduction The concept for 3D weaving machine was designed and developed, next step was to utilise this machine to produce the 3D preform. This chapter explains the manufacturing process of 3D woven near net shape preform and difficulties associated with the process. Design and development of individual processes have already been discussed in the previous chapter. However, some of the design concept was not carried forward due to some hurdles. Final structure of 3D weaving machine is as seen in Fig 5.1. Selvedge needle mechanism was not employed; hence natural selvedge has been created in the form of weft loop. Fig 5.1 Machine set-up 105

107 Set up for the machine is done for the manufacturing of 3D preforms where mechanical and electronic parts were utilised to control the machine. Machine frame is built based on machine concept with the help of aluminium struts and angle brackets. Necessary attachments are machined in order to attach relative drives to machine frame. Flexibility is considered while building the machine frame in order to do changes into drive locations if required. Once machine is programmed for running the required cycles for drives, the next procedure is to set-up yarns and relative mechanisms. As already mentioned in the previous chapter, warp yarns are feed from the small warp beams and weft yarns were feed from the cheese packages. In addition, warp yarn layers are separated in such a manner that it creates multiple shed to insert the weft yarns. Programming and control of the drives are done after reviewing software and process cycle. 5.2 Software and Electrical Installation Different drives such as stepper motor, pneumatic cylinder and pneumatic cylinder with encoder are used for various motions of the process cycle. These drives can be run by supplying digital or analogue signals to their interface. However, there is a need for PLC which can run and synchronise all the drives with the help of software. Selection of the PLC and software was done on the basis of compatibility with all drives and interfaces. In addition, factors such as future modification into the machine and user friendly are also considered. PLC and interface to the drives are selected such that it synchronises all the motion with required speed and direction. An interface program is written for SPC200 which can be integrated into FST software. FST software is utilised to write programs for FC 640. Figure 5.2 illustrates schematic diagram of controllers and drives connection. 106

108 (1) Computer (2) FC 640 (3) SPC 200 (4) Pneumatic axis with encoder (5) Stepper motor interface (6) Stepper motor (7) Standard Pneumatic cylinder Fig 5.2 Schematics for drives control 107

109 5.2.1 Electrical Installation As discussed in the previous chapter, two PLC FC 640 and SPC200 are selected to run all drives. FC640 can run and synchronise other drives with the help of SPC200. Two standard cylinders are connected to FC640. In addition, Slave controller SPC200 is also connected to FC640. Two stepper motor and DNCI cylinder are connected to SPC200 with the help of interface module and valve. Health and safety regulations are considered while wiring and cable installation for controllers and drives. Figure 5.3 illustrates control cabinet with controller to drive connection. Fig 5.3 Control Cabinet for machine process cycle 108

110 5.2.2 Programming of SPC200 SPC 200 is a positioning controller which can control 1 to 4 pneumatic axis or stepper motor axis. It can be used with the help of Winpisa software to run and control various axes [37]. It consists of 4 or 6 slots to fit required modules such as stepper motor, input/output, power etc. According to requirement, there are 5 modules introduced in SPC200 as seen in fig 5.4. Fig 5.4 SPC 200 with 5 modules SPC200 is utilised to run two stepper motor and one pneumatic cylinder with encoder. SPC200 can not directly give signals to drives, therefore interface are needed for stepper motor and pneumatic axis. Required interfaces are mentioned in fig 5.2 which shows connection from SPC 200 to drives. Connections to different interfaces and from interfaces to drives are done with the help of manual provided. Winpisa permits various procedures to create and manage projects for SPC 200. After installing and configuring winpisa, project needs to be created for running drives in synchronised manner. When SPC 200 is connected with computer, Winpisa will detect controller and its interfaces. It will ask step by step to configure individual interface and drives. Individual drive parameters can be set here depending on process requirement and safety regulations. Projects can be uploaded or downloaded to SPC200 subsequent to configuration and creation of projects into Winpisa [38]. Program can be written with the help of command manual for Winpisa. Thorough study was done before writing program for required process cycle. Normally, SPC 200 can be controlled with two types of modes [38]. 109

111 (1) Run Commands can be used to send drives to move relative or absolute which can be synchronised and executed for process cycle (2) Teach- Positions of respective axes are taught which can be used in program FC 640 is utilised here as a master controller which requires SPC 200 to run in teach mode. According to process cycle, individual drive positions were taught and implemented in Winpisa Programming. Fig 5.5 demonstrates user interface of WInpisa software. Fig 5.5 User interface of Winpisa software Fig 5.5 illustrates position list and program for proposed process cycle. Position list shows target positions of each drives at respective index number. This index number is notified in program to move axis at respective positions. As this program is done in teach mode, each line can be represented in FC 640 with binary positions. Once this program is created and uploaded, FC 640 can be programmed to run all axes. 110

112 5.2.3 Programming of FC 640 FC 640 is utilized here in order to accommodate extra drives which cannot be controlled with SPC 200 [39]. Furthermore, FC 640 can act as a master controller which can control SPC 200 with additional pneumatic drives (needed for machine process cycle). FST 4.1 software is installed and utilised to run FC 640. Fig 5.6 illustrates FC640 with all connections. Fig 5.6 FEC640 PLC with connection Based upon steps in Winpisa programming (Teach mode), Digital input and output pins are connected with SPC I/O interface. This connection works on binary principle e.g. If three input and output pins are connected, 9 steps of Winpisa program can be executed with FST programming. Standard pneumatic drives can be connected directly to digital inputs on FC 640. Fig 5.7 represents user interface of FST 4.1 software. 111

113 Fig 5.7 User Interface of FST 4.1 First of all, project needs to be created following software installation. FC 640 is connected to computer with Ethernet port which provides faster communication. SPC200 is detected and uploaded into FST program as it is already connected. Once configuration and installation is done, program is written and uploaded in to FC 640. FC640 allows user to store upto 16 programs which can be run without computer [40]. FST programming is responsible to run machine in required process cycle. Therefore, much care is needed in writing each movement. Assigned input and output are recorded according to their application e.g. IO1 is assigned as weft insertion. Programming step has been created to execute movement of each axis. Appendix represents FST program to run weaving process cycle in steps discussed later in this chapter (5.5 process cycle). 112

114 5.3 Warp supply Warp yarns are supplied in the machine from the multiple beams. These beams are actually cheese spools i.e packages which are used for single end yarn. The winding of warp yarns are done on these beams with the help of winding machine as seen in Fig 5.8. Multiple warp yarns are drawn after passing through the tension roller into the reed from the supply packages. These warp yarns then taken forward to the warp beam. The tension rollers help to maintain tension between individual warp yarns. Reed helps to maintain the distance between warp yarns on the beams. Once all the warp yarns are arranged on the warp beam, winding of warp yarns slowly starts on the beam. According to the structure requirements, eight beams were used for the straight warp yarns and two beams for the binder yarns. Fig 5.8 Winding of warp beam 113

115 5.4 Weft supply Weft yarns are feed into the rapier from the cheese packages as seen in Fig 5.9. Supply package for the weft yarns were wound normally on the cheese packages unlike the warp beams. These packages were arranged horizontally on the creel and several guides and roller were kept in the path of the weft yarn to reduce the friction and maintain tension of the yarns. As the number of straight warp yarn layers is eight, nine weft yarns would be supplied due to the orthogonal weave structure. (a) (b) Fig 5.9 (a) Weft yarn supply (b) weft yarn rapier 114

116 5.5 Process cycle The focus of this research is able to produce the 3D near net shape fabric in purpose built weaving machine. Once the mechanism and controls for the machine is developed and built into the machine frame, the next move is to synchronise them to run the weaving process cycle. It is very important to control each movement according to the system requirements. Design and set-up for the respective motion have already been discussed in the previous chapter. Normal sequence to produce the structure is as follows assuming warp shed open. Weft insertion by the rapier The weft insertion mechanism inserts weft into the shed in a loop form. Pneumatic cylinder drives this mechanism to the respective position as seen in Fig The distinctive thing about this concept is to insert several wefts at the same time. Tension in the weft yarns is controlled with the help of guides and roller. It is very important to maintain the tension in the weft yarn for the accomplishment of the next process and better fabric properties. Weft insertion needs to be performed very carefully, as shed size is very limited due to multiplicity. There is more chance of fibre damage or deflection in the warp yarns from their relative layers due to the penetration of the rapier. 115

117 (a) (b) (c) Fig 5.10 (a) Overview of weft insertion mechanism (b) Rapier shed entrance (c) Rapier shed exit In a single yarn rapier system (Fig 5.11), weft yarns are guided to rapier head same as previous system. Rapier housing has pneumatic cylinder gripper which is activated while transfer of weft yarns to other end and released while coming back in order to maintain yarn tension accordingly. Weft yarns are gripped on other end with gripper system which ensures weft yarn transfer. Careful attention is needed while transferring weft yarns as yarn tail should be maintained in order to grip yarns. 116

118 Fig 5.11 Overview of single yarn rapier system Movement1 of needle bed for beat up Once the weft is inserted into the warp shed, the next thing is to take the weft yarns at cloth fell position with the help of needle bed. Set of Standard pneumatic cylinders are used for this motion. Needle bed is attached on vertical pneumatic cylinder which is fixed on horizontal cylinder as seen in Fig Initially, needle bed moves in horizontal direction underneath the weft insertion mechanism. Then, needles go into the shed to catch several inserted weft yarns. The crucial criterion in this motion is the penetration of the needles in-between the warp yarns. As the warp yarns are drawn into the reed, there is limited space between the warp yarns. To minimise the fibre damage, needles need to align properly to the reed wires. Hence, needle can penetrate without too much intervention of the fibres. 117

119 (a) (b) Fig 5.12 (a) Needle bed insertion to warp shed (b) pneumatic cylinders for beat-up process 118

120 Retraction of the weft insertion mechanism Once the needles lock the weft yarns into the weaving zone, weft insertion mechanism needs to retract onto its initial position for beat-up. Hence, weft yarns are now in between the needles as a loop form. Movement2 of the selvedge needle bed After the retraction of the rapiers, the needle bed does the horizontal movement to cloth fell position. Weft yarns are locked in between the needles, hence taken forward to the cloth fell position. Weft yarns should not be slack, otherwise it can result into the fault in the fabric. Needle bed then goes into its initial position for the next cycle. Fig 5.13 Needle beat-up process In single rapier system, movable reed is installed on top of horizontal cylinder which does the beat-up. Once, the yarn is clamped and cut, reed takes all weft yarns into cloth fell position. 119

121 Binding yarn shed change over Binding yarn shed changes when weft yarns are placed into the cloth fell position. This motion creates new shed to insert next set of weft yarns as well as lock the inserted weft yarns into its position. Binding yarns are carried on the set of heald wires which is controlled by stepper motor. Due to the weave design of the fabric, the distance to move is substantially more compared to the conventional weaving process. Thus, extra long heald wires with springs were used for the binder yarns. Fig 5.14 Binder yarn change over process 120

122 Take-up All the warp yarns are tied in between the metal grips. These grips are driven by the stepper motor. After changing the shed and locking the newly inserted weft yarns, takeup is done in small increment according to the density requirements. To maintain the architecture of the structure and its integrity, linear take-up was designed. Thus, the fabric is not wound on the cloth roll as conventional fabric. Fig 5.15 Take-up process 121

123 5.6 Produced Preform Orthogonal weave was selected to produce on developed machine. These performs were then laminated with the help of resin infusion method. Three 3D woven performs were produced with different specifications in terms of width and ply orientation. Two of them were identical in structure with width variation. Third structure was produced with dropping selected ply at certain interval. Fig 5.16 represents produced perform. There are some faults visible on perform which can be avoided with certain modification in machine and process cycle. Fig 5.16 (a) 3D glass preform (b) 3D carbon Preform (c) Multistep Preform (d) 3D narrow carbon preform 122

124 5.7 Problems and Solutions 3D near net shape fabric with orthogonal structure was successfully produced on the purpose built machine. However, there were few difficulties in the production of these fabrics. Primary reason for these is the initial stage of machine development. Although machine can produce the 3D woven structure, there are several things which need to be resolved by modification in the machine. Difficulties which occurred during the process cycle are analysed and discussed in order to do amendments in the machine parts and process cycle. Weaving process and machine automation is already discussed and explained. In order to understand difficulties associated with processing cycle and machine parts, problems are stated here in the same order as the weaving process explained earlier ( warping, shedding, weft insertion, take-up) Warping Warp beams for the weaving process were prepared on the cheese spool (normally used for single end package). The warping process is already explained earlier where 50 ends were wound on to the cheese spool with the help of winding machine. However, there were few problems associated with the warping process as it was customised according to requirement. The main difficulty in producing these beams was to maintain uniform tension among warp yarns. In addition, the passage of warp yarns to the warp beam is not appropriate to produce uniform tension. However the passage of warp yarns can be modified according to requirement. Warp beams are mounted on the creel in the form of multiple end packages. Warp yarns enter into the shed area after passing through rods (which separates each warp layer). The problem associated with these set up is unwinding of warp yarns from the package under tension. As the warp beam is held on the creel package holder, tension is only applied with the help of spring attached to package holder. This makes unstable unwinding of warp yarns in longer run of process. This problem can be minimised with the help of better tension control. In addition, this set up is customised according to available sources in university lab. To enhance performance of let off mechanism, warp beams can be customized and automated according to requirements. 123

125 5.7.2 Shedding There are two types of shed creation in this process. One of these is created by the main warp layers with help of adjustable rods and another is shedding between binding yarn with help of stepper motor and spring arrangement. Shed created between straight warp yarns is fixed, whereas binding yarn arrangement moves up and down to lock the entire weft layers for structure integrity. There were few problems associated with shedding while running weaving operation. Adjustable rods were arranged such that it supports respective warp layer from bottom or top to determine the position of warp layers into the shed zone. As, warp layers were supported from only one side, sometimes there was a problem to keep all warp yarns in one line to create clear shed as shown in Fig 5.17 Due to uneven tension among warp yarns, it was very difficult to achieve clear shed for weft insertion. Fig 5.17 Uneven warp tension 124

126 Shedding for binding yarns was done with the help of stepper motor. As explained earlier, grooved roller was attached with the stepper motor and heald wire with the strings were fixed onto these grooves. This setup is satisfactory for processing cycle, however it is not reliable for longer run. The main problem is selection of heald wire which ideally needs to be customised according to requirement. Furthermore, springs attached with these heald wires are not strong enough to operate heavy yarns and large shedding. In addition, sometimes string comes out from the grooves which could be entangled with other strings. The possible solution for straight warp yarn shedding is the new arrangement. This set up consists of two supporting rods rather than one supporting rod in current set up as shown in Fig This will help to maintain warp yarn in one line to maintain tension and create clear shed. Fig 5.18 Warp support rod system 125

127 Binding yarn set-up requires customised heald wires which should be long enough to avoid any damage to yarns. In addition, spring attached to heald wires should be strong enough to carry high performance yarns. Grooves depth can be higher to keep strings into the grooves Weft insertion Weft insertion was done with the help of multiple rapiers. Weft passage from package to the rapier is explained earlier which is designed according to available source. The main drawback of this set up was to maintain weft tension which can only be done with extra tension roller or disc set up. In addition, tension was only applied on weft during withdrawal from the package. There was no applied tension after that point in the weft passage. This leads to slackness in weft yarn. Rapier design is such that it inserts weft into the loop form, however weft is passed through ceramic guide which makes weft pulling difficult during weft insertion. To solve the problem of maintaining tension in weft yarn, several tension rollers needs to be implemented on the weft passage. This could solve the problem of slackness in weft yarns as well as better weft insertion. Rapier design can be modified for better weft insertion control. Guide rollers can be designed in place of guide hole for better handling of weft yarn during weft insertion as shown in Fig These modifications could solve the difficulties and enhance process cycle. 126

128 Fig 5.19 Proposed rapier head Beat up Beat up is done with the help of needle bed which is driven by two standard pneumatic cylinders. Needles enter into the shedding area to lock the weft and take weft yarns into the cloth fell position. Timing and movement is explained earlier in the manufacturing section. Although the beat up process is very simple, there are few drawbacks associated with the process parameter and parts. Design of needle bed needs to be considered for modification as these needles are not ideal for this motion. Needles need to be very fine and strong that it can carry weft yarns along with them to the cloth fell position. Needles which create selvedge also need to be stronger as it holds weft yarns during the weft insertion process. One of the challenges is to keep needle size optimum as large needle can cause damage in yarns and smaller size needles may bend during weft insertion. Position of needles on needle bed is also very crucial factor as any misalignments in needles can cause major damage in machine parts as well as yarn system. These positions should be such that needles could enter into the shed zone without damaging warp or weft yarns. 127

129 5.7.5 Take-up Two metal plates are occupied to grip and pull produced fabric. To maintain structure of the manufactured fabric, linear take-up was designed which is driven with the help of stepper motor. Movement and process timing of this set-up is better compare to other motions. Only problem lies during initial setup of the machine when warp yarns need to be gripped with plates. Due to higher number of warp yarns, it is very difficult to grip all yarns under tension. However this problem can be easily resolved with the help of adhesive plates which can hold warp yarns more firmly. 5.8 Conclusion This chapter describes the manufacturing process and various stages in the process cycle. Various motions were illustrated in order to understand production of 3D woven fabric. In addition, problems arose during manufacturing are also demonstrated and possible solutions are discussed in order to reduce difficulties related to process cycle. 3D woven fabrics were produced on the machine with different near net shape. Laminates were prepared from these samples and tested to analyse properties. 128

130 Chapter: 6 Braid Weaving 6.1 Introduction In this research, main focus was to produce structure with fibres in multidirection to improve mechanical properties. To achieve this goal, 3D weaving machine has been designed and developed. However, fibres are orientated only in three directions. For better properties, bias direction fibres are also needed in the structure. It was very complex to introduce the bias yarns in the 3D weaving machine. Keeping that in consideration, one of the methods to introduce the bias fibres is braiding. The idea is to braid the fibres on the 3D woven fabric on the braiding machine. Further concept can be explored where braiding machine head is arranged before the take-up process to do braiding over the 3D fabric. However, research and experiments are required to establish the braiding method on 3D woven product. 6.2 Braiding process Processes originally developed for the textile industry are being used in the manufacture of composite structural components. One of these techniques is twodimensional braiding, which has the capability of fabricate near-net-shape preforms of the desired structural part while providing the required fibre reinforcement [7]. Braiding is a system of three or more yarns intertwined in such a way that no two yarns are twisted around one another [41]. Braided fibre architecture resembles a hybrid of filament winding and weaving, like filament winding, tubular braid features seamless fibre continuity from end to end of a part. The braiding machine places two or three sets of yarns simultaneously; therefore, in contrast to a series of unidirectional laminate, a braided composite is compared of a series of multi-directional plies having identical properties. Braided structures are widely used in aerospace, automotive, biomedical and recreational sports applications [42]. A 2D braided fabric structure resembles a hybrid of weaving and filament winding. In this project, braiding is introduced to wrap cylinders which are done with filament winding at present. 129

131 6.3 Why Braiding Braid is currently the reinforcement of choice in components that serve a wide variety of market applications like aerospace applications (aircraft engine containment, aircraft propeller blades, missile nose cones and bodies, self lubricating bearings, aircraft ductings), industrial applications (automobile airbags, industrial rollers, lamp and utility poles, shipping containers and boat hulls), medical (prosthetic limbs, surgical devices, implantable devices), recreational (wind surfing mass, snow boards, water skis, snow skis, wake boards, golf shafts, baseball bats, tennis racquets) [43]. Braiding process has several fibres at different angles varying from 0 to 90. Braid is cost competitive and least expensive compared to other reinforcements. The quality makes braid the ideal preform since it takes on the exact shape of the part that it is reinforcing so there is no need for stitching, cutting or manipulation of fibre placement is not needed. Each of these factors contributes to the cost effectiveness of braid [44]. Because all the fibres within a braided structure are continuous and mechanically locked, braid has a natural mechanism that evenly distributes load throughout the structure. Braid s efficient distribution of loads also makes braided structures very impact resistant. Braid structures are also excellent with regard to fatigue. Like a filament wound structure, braided fibres are coiled into a helix just like wire in a spring. But cracks can propogate through the matrix of filament wound whereas crack can be arrested at the intersections of the reinforcing yarns in braided structure. Braid also provides efficient reinforcement for parts that are subjected to torsional loads [42]. Any fibre with a reasonable degree of flexibility and surface lubricity can be braided. Fibers like carbon, glass, ceramic, aramid, natural fibres, and synthetic fibres can be braided. There are some disadvantages of the braiding as well. The packages on the braiding machine needs to be changed frequently as per the requirements. This can be done only by manually. This is time consuming process. 130

132 6.4 Braiding on complex shapes In order to braid over the near net shape 3D woven preform, significant research needs to be done over methods to braid on complex shapes. In order to fit in with industrial requirements, there is a need to provide braid structures in a complex form, that is to say in a form with a cross-section other than that of a simple rectangle or tube, or a moderate variation there from. Attempts to form such cross-sections in braiding apparatus have previously not been particularly successful since, at any area where there is a re-entrant portion, the yarns of the braid tend to span the entrance and hence defeat the form being sought after [44]. The braid may be formed over a mandrel and this may be of a cross-section other than circular to a limited degree. Multilayer braided structures have been proposed where radial yarns project from a mandrel and the package carriers of yarn weave their yarn around the radial yarns. Such structures have been difficult to manufacture. Three-dimensional braiding was developed in the late 1960s and was the first textile process used to manufacture a three-dimensional preform for a composite aircraft engine component. There has been significant development of the three-dimensional braiding process and currently there are number of processes to produce threedimensional braided preforms [41]. A unique feature of the three dimensional braids is their ability to provide through the thickness reinforcement composites as well as their ready adaptability to the fabrication of a wide range of complex shapes ranging from solida rods to I beams to thick walled rocket nozzles. 131

133 6.5 Braid Parameters In this research, two dimensional braiding is been explored on circular tubes to establish the optimum parameters for better structure properties. After initial findings, optimum braiding was done on 3D woven preform to get fibres in bias direction. An attempt has been made to establish the relation between tow geometry and braid angle. To do the braiding on the cylinder following parameter needs to look through to get the required properties. Take-up speed Braiding speed (carrier speed) Braiding starting point Tension in packages Braid angle 6.6 Machine specifications In order to achieve goal of this research, braiding machine is used with specifications as follows. Cobra Braiding machine * Type of Braider: Tubular Braiding machine * Take-up System: Linear take-up * Speed: braiding rotary speed 9.78 r.p.m Take-up speed m/min * No. of Biaxaial yarns (max): 48 * No. of Triaxial yarns (max): 24 Fig 6.1 and 6.2 respectively represent braiding machine and creel for triaxial packages. 132

134 Fig: 6.1 Biaxial Yarn packages Fig 6.2 Triaxial Yarn Packages After mounting all the packages, threading was done very carefully to reduce the abrasion of the carbon fibres with machine components. As carbon fibres can generate static problem with electrical parts. 133

135 6.7 Braid Geometry It is very important to get the right braid geometry on 3D fabric for better structure properties. To achieve this, right combination of braid angle and tow dimension is required. However, it is very difficult to braid 3D woven fabric (from purpose built machine) due to small size of braiding machine. Hence, small width of the 3D woven fabric was used in order to get better surface cover with braiding. Several trials are required to get right braid angle. Thus, tubes are initially used with the same surface area to find out the braid speed and take-up speed. As the braiding also depends on the tension in each yarn packages, tension in yarn can be also one factor which can improve the surface cover property. So tension settings were changed to find out the result. Lower tension settings were tried taking assumptions of low yarn tension will help smooth laying of yarns on cylinder surface. The tension settings were done with the help of spring balance. Fig. 6.3 shows the tension scale on the individual yarn packages which can be changed by rotating the square nut. Fig. 6.3 Tension setting of package 134

136 Graph below shows the actual and set value of the tension in individual packages. Graph shows that before adjusting tension, tension was varying in packages which can be factor affecting the good surface cover on the 3D woven laminate. Tension in Packages gms Actual Set Fig. 6.4 Actual and set tension value in packages Fig 6.4 Actual and set tension value in packages Several braiding trials on tubes (considering same geometry as 3D woven coupan) have been carried out to get the optimum braid angle and tow geometry in order to achieve good isotropic properties. These process parameters are as below. Linear Take up speed Rotation speed 1.5 m/min 9 rotation/min 135

137 6.8 Braiding on 3D near net shape woven preforms Braiding was carried out on 3D woven laminate after establishing the process parameters. As discussed previously, it would be very difficult to get better braiding on wide 3D fabric. Hence, it is suggested to cut the fabric in the right size to do the braiding. However, it is very difficult to handle the 3D fabric during and after cutting. The main reason is its dry fibre form. When we try to cut the fabric, there is more likely chance of yarn slipping off. There are three possible solutions for this problem. Larger braiding machine to get better surface cover without cutting the fabric Lock the warp yarns with binder yarns at certain places Impregnate the fabric and then cut according to the size requirements The first option is more suitable where there will be no disturbance to the structure geometry and can get better braiding done on the fabric. This also results into better structure integrity between the bias fibres and fibres in 3D woven fabric. However, it is not feasible at initial stage until more recognition to the process. The second option is not as good as the first one, but still it is more efficient than the third one. However, there is a compromise in the design of the 3D woven fabric. It would require more binder yarns to lock the fabric at cutting points which can lead to more resin rich areas in the fabric. Even after employing extra binder yarns, there will be problem of cutting the fabric due to its thickness. The third option is least suitable in terms of structure properties. However, this is more suitable option at this stage of the research due to complexity of other options. Braiding is done on the 3D fabric which is already impregnated and then another impregnation to bond the braided fibres to the sample. Structural integrity of this final part is not very good due to weak bonding of braided fibres to fibres in 3D woven fabric. However, this is the right option to prove the concept and then take it to other options in further work. 136

138 Resin impregnation was done on the 3D fabric (discussed in next chapter) and then cut into the right size for the braiding process. The two configurations which are designed on 3D weaving machine are used. One of them is orthogonal 3D fabric and other is multiple step joint. The multiple step joints are firstly joined as scarf joint as seen in fig 6.5 and then cut according to the size. Orthogonal fabric was cut in both warp and weft direction and then those samples were braided. Fig 6.5 Multiple step joint Process parameters are established with previous braiding trials on tubes. Triaxial yarns were not used as there is a requirement of only bias direction yarns. The braiding of laminated samples is shown in fig 6.6. The braiding process is much easier on the solid part which plays role of mandrel. After braiding, resin impregnation is required again to make the final laminated part. Fig 6.6 Braided narrow 3D laminate 137

139 6.9 Discussion In this work, successful trials were carried out to do braiding of the 3D woven fabric. Process parameters were optimized to get better structure properties in final composite part. During the braiding process, fibre damage is done while passing through the guides. This fibre damage can be minimized with the help of ceramic guides and proper tension control. It is very important to maintain the process parameters to get uniform braid angle. Some factors which might affect the braiding process is already looked through like tension in biaxial yarn packages, guide to the yarn lay-up where braiding is done, take-up and braiding speed, etc. The concept to employ the bias direction fibres is very good and effective, but there is still some work requires modifying few things. 138

140 Chapter: 7 Consolidation Process 7.1 Introduction The manufacturing of 3D woven and braided fabric described in a previous chapter is only first stage in the production of 3D composite material. Normally, textile reinforcements for composites show good tensile properties but have poor stiffness and compression properties. This necessitates the use of matrix to rich the fibres, thus improving the damage tolerance and enhancing the composite properties. To achieve uniformity in properties, the resin must completely fill the interstices within the fabric and also, significantly the spaces between the filaments making up the tows. So after production of 3D fabrics, we need to select the best method to do consolidation. The classification of different consolidation methods is described as below. 7.2 Classification of Consolidation methods Hand impregnation Hand impregnation is traditional method of consolidation process which involves the use of brushes and rollers to physically work the resin into the fibre preform. This method can cause damage to fibre architecture in the fabric and also it is not capable of removing all entrapped air from the fabric due to the process performed at atmospheric pressure [72]. This would result in low quality 3D composites Preimpregnation Instead of relying on the air pressure flow to force resin, dip coating and lick roll technology are used to apply a controlled and uniform amount of uncured resin to the reinforcement. Then, roll of prepregs are wrapped and stored under refrigerated conditions for a period of time. This method is better for unidirectional laminates. This method saves the time and cost for the resin impregnation into samples. But in this method, the properties of the prepreg should be very precise. Sometimes wrong temperature and pressure in later process may differ the final properties of the sample [72]. 139

141 7.2.3 Vacuum bagging Dry fabrics and prepregs can be cut according to requirement and placed in precise order and orientation on a tool surface. A layer of release film is laid on top of the ply lay up to prevent the resinous stack of plies from adhering to the fibrous breather cloth. Breather cloth is used for absorbing any excess resin and to help in maintaining the pressure. The complete assembly is covered with vacuum bagging material sealed around the assembly. This whole assembly can kept under vacuum for curing in next process. But, as it is low pressure profile, it is not sufficient to produce a high performance component with high fibre volume fraction and low void content [71] Autoclave Autoclave process helps to achieve high fibre volume and low void contents which are essential for better composite performance. Autoclave process follows the stages outline in the following manner. Prepreg stored at -18 C Gerber Cutting of Plies Plies hand-laid into tool Final Assembly Trimming of excess Material Consolidation in autoclave Fig 7.1 Autoclave process out-line In this process, Caul Plates can be used to produce a good surface finish on both sides of the component. While autoclave processing provides high performance composites, the operation of autoclave has associated high running costs. So sometimes, it is not ideal for low cost part [68,72]. 140

142 7.2.5 Liquid moulding There are many different variations in liquid moulding. These are as follows. RTM (Resin Transfer Moulding) VARTM (Vacuum Assisted Resin Transfer Moulding) SRIM (Structure Reaction Injection Moulding) Vacuum Injection Moulding Injection moulding RRIM (Reinforced Reaction Injection Moulding) The first four processes are mainly used for manufacturing of continuous fibre reinforced composites whereas, remaining are mainly used for short fibre reinforced composites [72]. RTM is the most commonly used in all, particularly for production of aerospace components. In RTM process; resin is driven into the preform by the pressure of a pump. The maximum injection length of the resin into the preform is therefore limited by the in-plane preform permeability, the resin viscosity and the pressure. The tooling used for RTM is mostly a closed mould system which has two main tools enclosing the preform as shown in fig 7.2. This helps in excellent surface finishes and high fibre volume fraction. The drawback of these process is expensive tooling and not suitable for larger components [1]. 141

143 Fig 7.2 Tooling for RTM VARTM is the same as RTM process except that vacuum is applied to preform assembly. This helps in removal of voids and speeds up the resin impregnation. SRIM is same as RTM. The only difference is high injection pressure which helps in fast resin flow as resin systems are generally fast curing systems in this process. In RFI process, resin is initially present in between preform as a film. So when heat and pressure is applied film melts, and it goes into the preform architecture. Here, the flow of resin is not in-plane but it is in thickness direction. In this process, resin film is placed against the tool surface and preform is placed on top of it. This set up is then sealed by vacuum bagging or similar arrangements. Then, it can be cured by heating. An advantage of this process is easy tooling and can be used for complex and larger components. The drawback of this process is production of resin film and also the handling of the resin film prior to process [1]. 7.3 Selection of resin impregnation method In this research, 3D complex shape structures were produced which need to be laminated with the help of resin impregnation methods. Classification of different methods is already explained. Each method has its own benefits and drawbacks. These methods were considered for its potential in resin impregnation of 3D structures. According to our requirement, 3D fabric needs to be impregnated and due to its structure, hand impregnation is not suitable for impregnation as resin will not penetrate in the fabric uniformly and leave air voids. In this research, dry fibres were used for production of the final structure as it is very difficult to weave preimpregnated fibres. So prepregs can not be used in the weaving machine and in addition, prepregs are expensive as well. One of the suitable methods to do resin impregnation in 3D structures is vacuum bagging. In this method, dry fibres or prepregs can be laid on tool surface and vaccum is achieved with the help of bagging and vacuum pump. For dry fibres, resin inlet is provided which allows resin to flow into the structure with the help of vacuum. Vacuum bagging technique is being a fairly common for the manufacturing of bigger and thicker composite parts. This method can be used as resin impregnation for 3D structures. However, there are few difficulties attached with this process. One of them is to 142

144 achieve uniform resin impregnation for whole structure as sometimes vacuum does not have enough pulling force to impregnate a larger and thicker part [65]. Another problem is the compaction force on dry fabric which is applied by vacuum bagging during impregnation. This may prevent easy flow of resin through 3D structure and may damage fibre orientation during resin s flow to its path. Another method to do impregnation is by using RTM (resin transfer moulding). RTM process consists of dry fibre reinforcement layers inside a rigid mould where liquid resin is injected into the fibres under the pressure to do resin impregnation. Although, this process is ideally suited for thicker and complex shape parts, the cost of set up is very high compare to other methods. In this process, temperature of the moulds can be controlled, thus controlling the viscosity of resin and improving the resin flow rate. VARTM is modified approach of RTM where vacuum is used for resin flow assistance. However the set-up cost is not suitable for small parts [68,69]. Another method is to use Quickstep (latest Australian technology) [66]. It can be used in conjunction with vacuum bagging to cure the resin. Its principle is same as RTM, however the basis difference is the use of flexible membrane rather than metal plates for moulds. Heated fluid controls the temperature of membrane in order to achieve resin flow and curing temperature. This can be used to impregnate 3D structures with the help of vacuum bagging, so some of the difficulties associated with vacuum bagging can be minimised such as resin temperature for easy flow path and curing. After reviewing several resin impregnation methods, vacuum bagging infusion was selected for resin impregnation as it is suitable considering economical and structure aspects. In addition, modifications can be done to improve resin flow and curing cycle. 143

145 7.4 Resin Infusion Process Vacuum bagging method was selected for resin infusion of produced structures. As Braiding cannot be done on 3D dry woven preform due to geometry and machine specifications, method was to laminate 3D woven first and do braiding. After braiding, same vacuum bagging procedures were followed for secondary resin infusion. In vacuum bagging method, a layer of release film is laid on top of the preform to prevent the final part from adhering to the fibrous breather cloth. Breather cloth is used for absorbing any excess resin and to help in maintaining the pressure. The complete assembly is covered with vacuum bagging material sealed around the assembly [69, 70, 71]. This whole assembly can kept under vacuum for curing in next process. But, as it is low pressure profile, it is not sufficient to produce a high performance component with high fibre volume fraction and low void content. But as it keeps the resin impregnation to required quantity, it is suitable method [7]. Vacuum bagging arrangement is shown in Fig 7.3. Fig 7.3 Vacuum bag arrangement 144

146 Flat aluminium tool is selected for infusion process. Material lay-up and vacuum bag procedure is followed in these steps Brown release film is laid on metal tool which will ensure easy removal of laminated part after infusion from tool Preform is placed on top of the release film with high care to limit fabric damage and contamination Nylon6 peel fabric is placed on top of the preform to assist in resin infusion and easy removal of part after consolidation Knitted Infusion mess is placed on top to assist resin flow, It is difficult to flow resin in to thick preforms without such infusion mess (open structure helps to flow resin onto surface of preform first and then enter into thickness direction) Breather material was kept on edges of preform where vacuum outlet is connected. Breather material acts as a bridge between preform and vacuum connection. It is also utilised to absorb resin and slow down flow of resin near to vacuum connection Spiral tube is kept at one end of preform for resin inlet and other end of preform is connected with vacuum port Vacuum bag is then placed on top of this lay-up and sealed on tool with sealant tape Fig 7.4 demonstrates steps of vacuum bagging procedure. After preparing vacuum bag, inlet tube is blocked and vacuum pump is connected to vacuum port. Vacuum should be achieved within the bag and check for any leaks in the set-up. If there is no vacuum leak, then set-up is ready for resin infusion process. Resin mixture is prepared and infused through inlet tube. Once, resin is travelled to other end of preform, inlet is clamped to prevent extra resin into laminate. Vacuum port is still kept open until resin starts to gel. This is to minimise void quantity into laminate. 145

147 Fig 7.4 Vacuum bag lay-up for resin infusion Epoxy resin is used here as its properties will help to understand mechanical properties of final laminate. However, this resin is no high performance as main aim of this research is to understand and manufacture 3D textile preform. Resin and hardener specifications are as follows. Resin Araldite LY Hardener Aradur 5052 Mixing Ratio 100gms: 38gms (Resin to Hardener) 146

148 Laminate is removed from tool after cure cycle is finished. Visual inspection is carried out to find out any damage on laminate. This sample is then cut in required size, braided (discussed in chapter-6) and infused again for mechanical testing. Fig 7.5 Braided 3D woven laminate The main question that arises in this process is structural integrity of braided fibres to woven fibres. The fibres in the woven fabric are already impregnated and then braiding is done. Hence, the bonding between the braided fibres and woven fibres is not good. In addition, this process creates a resin rich area between the braided fibres and woven fabric particularly at the edges as seen in Fig 7.5. This is due to the compaction of the braided fibres while resin impregnation. If laminate properties improve significantly due to braiding process, then this concept can be optimized with some modifications. 147

149 7.5 Conclusion It is very important to achieve uniform resin to fibre ratio in the structure. Therefore, it is crucial to select suitable resin impregnation method to achieve right results as well as it should be cost effective. Vacuum bagging method is selected on base of these criteria. However, produced structure is thick and complicated in structure which creates few issues with resin impregnation. Resin flow in this kind of structure can be unpredictable and could leave nonuniform fibre to resin volume content. To improve resin impregnation after initial trials, infusion mess was kept on both side of the preform to ensure uniform resin impregnation. One of the difficulties is to minimize void quantity. It is difficult to control void as preform is thick and it is difficult to remove trapped air from structure during infusion due to complex structure. It is also observed that resin pockets have been created on sides of 3D braided woven laminate. This is primarily due to two step infusion process where braiding is done over consolidated 3D woven sample and then infused. 148

150 Chapter: 8 Mechanical and Physical Testing 8.1 Introduction In order to establish manufactured material, significant mechanical and physical testing is required. Current test methods to evaluate composite materials are based on unidirectional prepregs or 2D fabrics. These test methods can be implemented on 3D structures bearing further investigation into test methods at later stage [31]. Considerable effort and experiments are needed to find out how materials will behave under tensile, compression, shear, cyclic load or other environment conditions. These data can be useful in order to establish end application for 3D woven structures. Test results represent material behaviour as well as production and other processing route. Although properties predominantly are from distribution of fibres, it also varies due to processing route (perform to laminate). Normally, testing is done in order to serve establish several criteria as follows [51]. Quality control Quality assurance Comparisons between materials and selection Design calculations Predictions of performance under conditions other than those of the test Indicators in materials development programmes Starting points in the formulation of theories. It is important to understand that proposed test for these structures are preliminary test in order to understand material and structure behaviour. At this point, testing is done in order to indicate material development programme and first order property indication. As this research is concentrated on designing and developing machine to produce 149

151 proposed structure, there was a limited time to explore material properties. Although some physical and mechanical testing is done for this structure, the best practice to evaluate structural property should be over a range of test methods. It is also important to notify that test standards utilised are for fibre reinforced plastics and it normally represents UD and 2D composite laminates. There are not specific mechanical test standards for 3D materials yet. Thus, in order to establish properties with existing materials these methods were chosen. Mechanical properties depend on several factors [51]. Properties of the fibre Surface character of the fibre Properties of the matrix material Properties of any other phase Volume fraction Nature of interface Primary reason for testing in this research is to understand tow behaviour in the structure and damage loss during manufacturing and processing route. Some of the work reported 3D woven reinforcement has mechanical advantage in terms of tensile strength, flexural strength and interlaminar strength compare to 2D laminates [60, 61, 62, 67, 68,]. Experimental work to evaluate structure composition and properties were carried out with standard test methods such as tensile, flexural, interlaminar and microscopy. 150

152 8.2 3D glass woven preform Preliminary testing was commenced on 3D glass woven preforms manufactured on conventional weaving machine. This was initial step towards understanding 3D weave architecture and properties between various 3D preforms. Image analysis was performed to evaluate fibre configuration within structure and damage done while processing. Flexural test was performed to recognise fibre properties within three weave architectures (Orthogonal, Orthogonal hybrid and Layer to layer) Image analysis One of the significant tools to analyze weave architecture, tow behaviour and material ration is imaging techniques such as Microscopy (Optical and SEM) and tomography. Cross section of 3D woven laminates was analysed in order to understand effect of weaving process parameters and consolidation methods on final geometry. Different 3D architecture were utilised to produce woven preforms on conventional weaving machine. Orthogonal structure was evaluated with SEM (scanning electron Microscopy) for flat preforms and step joint preforms. SEM Samples were moulded, grinded and polished to represent warp and weft cross section. Fig 8.1 represents cross section of 3D woven preforms for warp, weft and binder yarns. These images demonstrate equal fibre distribution and structure integrity. Binder path shows the effect of binder tension on top and bottom weft layers. Weft yarns in top and bottom layer are prone to change cross section behaviour due to binder yarn interlacement. These changes can be minimised by presenting binder yarns at low tension. Damage can also be observed in binder yarns due to acute angle changes in their path. Warp and weft density looks uniform along the structure which represents ideal structure for composite application. Observations have also been made for warp and weft tow path where negligible or no crimp is apparent. 151

153 (a) (b) (c) Fig 8.1 Cross section (a) Binder yarn (b) Warp yarn (c) Weft yarn One of the key structures produced on conventional weaving machine was taper preform where several steps were created. Two of these structures were laid up such that it created uniform thickness after consolidation process. Aim was to compare this structure with other woven preforms with different architecture and understand behaviour of step joints. Fig 8.2 represents warp and weft cross sections along laminate. Even though care has been taken prior to lamination, there is a slight overlap of step joint area. This affects thickness uniformity at some extent. Furthermore, it creates deviation in warp and weft stuffer yarn which can degrade final composite properties. However, it was good first effort in order to produce structure and compare it with other woven structure. 152

154 (a) (b) Fig 8.2 Step joint Cross section (a) warp (b) weft 153

155 8.2.2 Flexural test Flexural test was selected for preliminary test in order to evaluate effect of binder paths on properties. There have been many experiments done to understand tensile and compression properties for through thickness reinforcement. However, effects of through thickness reinforcement on flexural property are not investigated thoroughly. It is also important to assess step joint structure with ones without any joint in the structure. Flexural strength and modulus can be lower in 3D structures compare to 2D and UD laminates because of out of plane waviness and fibre volume fraction. Test Procedure Flexural test was performed according to test standard BS EN 2746 [52]. Fig 8.3 represents sample dimension and three-point loading conditions for proposed test. Fig 8.3 Loading conditions for flexural test Instron machine was set-up to perform this test according to standard. Fig 8.4 represents machine set-up with flexural sample. Procedure was set-up in machine according to required parameters. Machine parameters and sample dimensions are as below. Cross head speed: 1mm/min Loading nose radius : 5mm and Support radius: 3mm Sample dimensions: (Width: 11mm, Span length: 40mm, Thickness: 3mm) 154

156 Fig 8.4 Machine set-up for flexural test Results and discussion Specimens from each type have been tested under same machine configuration and load vs. extension graph has been plotted. Fig 8.5 represents load vs. extension of each weave configuration. Flexural stress has been calculated according to Eq. 8.1 for individual configuration and represented in table Where: = Flexural stress (MPa) F L b h = Force (N) = Span (mm) = Width (mm) = Thickness (mm) 155

157 Fig 8.5 Load (kn) vs Displacement (mm) graph for flexural test 156

158 Each configuration has been tested and analysed from results and graphs obtained. It has been observed that orthogonal hybrid has maximmum load and flexural stress compare to other structures. Layer to layer and orthogonal structures have similar flexural strength, however layer to orthogonal structure has a large load drop while breaking compare to layer to layer structure which shows layer to layer structure is better to prevent damage propogation. Step laminates were also tested with each configuration and analysed. These structures have lower values in terms of maximum load and stress as expected. However, different trend has been seen in the graphs where load doesn t drop rapidly. These can be perceived differently as a way to reduce sudden failures of the structure. At the same time, it is also necessary to do further analysis and improve structure integrity in step formation in order to get higher properties. Table: 8.1 Maximum Flexural Strength (Mpa) values Layer to Layer Step layer to layer Orthogonal Hybrid Step Orthogonal Hybrid Orthogonal Step Orthogonal Mean S D Table:8.2 Maximum Load (kn) values Layer to Layer Step layer to layer Orthogonal Hybrid Step Orthogonal Hybrid Orthogonal Step Orthogonal Mean S D

159 Fig 8.6 illustrates comparison between maximum flexural strength for all configurations. Orthogonal hybrid is superior to other two structures. This could be because of binder path in this structure (Fig 3.18) where its pattern lies in between orthogonal and Angle interlock structure. However, detail study is required in order to conclude establish these structures. Although orthogonal hybrid structure is better in flexural properties, orthogonal structure is selected for production on proposed 3D weaving machine. This is primarily due to machine design limitation and weave design simplicity. Also, more work is needed for establishing step formation in 3D woven laminates to improve properties. This could help to develop product which can be utilised in patch repair industry replacing expensive current method. Fig 8.6 Comparison of Flexural strength (Mpa)values 158

160 8.3 3D carbon woven preform 3d woven preform was manufactured in purpose built machine and consolidated with the help of vacuum bag method. Braiding process was followed to get bias direction fibres on laminates. Optical microscopy and tomography were done in order to so structural analysis. SEM process is very efficient to get cross sectional images. However, carbon fibre laminates are difficult to analyse under SEM X-ray Tomography X-ray imaging is one of the efficient and reliable techniques in order to understand material composition and structure. Microscopy is tedious and labour intensive process to analyse whole structure configuration where you have to prepare sample for each interested area. Furthermore, it is likely to miss observation of some important area. X- ray tomography can provide full 3D image of structure and it can also differentiate material composition. 3D woven braided laminate was tested under X-ray to evaluate structure and weave architecture. Fig 8.7 and 8.8 represents various cross section and 3D images of braided woven laminate. Fig 8.7 3D X-ray image of braided woven laminate 159

161 (a) (b) (c) Fig 8.8 X ray images of braided woven laminate cross section (a) weft cross section (b) weft/warp cross section (c) warp section 160

162 It is observed that warp and weft stuffer yarns have negligible or no crimp. Binder yarns follow smooth path from bottom to top with less impact on weft stuffer yarns due to very low tension. However, there is a significant gap between weft stuffer yarns due to twisted binder yarns and low binder tension. One of the critical finding is resin rich area between braided layer and 3D woven preform on both sides. This is due to two stage consolidation process where Final consolidation is after braiding over 3D woven laminate. This can be improved by one stage consolidation process. This can be achieved by braiding 3D woven preform at dry fibre stage. Preform specification is also measured with help of tomography (Table 8.3). Table: 8.3 3D Woven Preform specifications No. of Layers No. of yarns/cm Warp 7 14 Weft 8 32 Binder 1 1 Braid Fibre Volume Fraction A good understanding of composite material is essential in order to predict structure performance. Composite material is polymeric matrix system which can behave differently under different physical testing. It is also important to have better understanding of proportion of each constituent which can help to analyse mechanical property of the structure. Volume fraction, Glass transition temperature and thermal conductivity are generally performed in order to establish physical properties of the material. As in this research main aim is to establish preliminary product properties and understand structure behaviour, volume fraction test was carried out to find out Fibre and resin volume fraction. Acid digestion method is performed on 3D woven and braided preform to evaluate fibre volume fraction. Procedure of this method is described below. 161

163 Random samples were cut from panel with weight range of gms Samples weree thoroughly cleaned and dried in oven at 100 c for 24 hrs Density machine was utilised to get laminate density for each sample where samples are weighed in air and then in water in order to get laminate density These samples were then kept in container with 10ml of Nitric acid These container fit in specific vessel which sits inside microwave oven Microwave oven has temperature probe which monitors temperature inside container Specific temperature cycle then run to digest resin from sample After digestion, fibres from container are filtered on small glass beaker Glass beakers are then kept in oven at 100 C for 24 hrs for drying and then weighed where V f = volume fraction of fibers W f = weight of fibers W l = weight of laminate ρ f = density of fibers ρ l = density of laminate 5.2 Table 8.4 and 8.5 represents fibre volume fraction for 3D woven and 3D braided woven specimens respectively. Fibre volume fraction value is lower in 3D braided woven sample compare to 3D woven sample. This is due to two phase impregnation method where braiding is secondary process which is done over 3D woven laminate. This process produces resin rich area around the edge of the sample and on bonded surface in between braided and woven sample. 162

164 Table: 8.4 Fibre volume fraction for 3D woven laminate Areal Weight (gms) Laminate Density (g/cc3) Fibre weight (gms) Fibre Volume Fraction (%) Mean Table: 8.5 Fibre volume fraction for 3D braided woven laminate Areal Weight (gms) Laminate Density (g/cc3) Fibre weight (gms) Fibre Volume Fraction (%) Mean

165 8.3.3 Short Beam test One of the important properties for composite laminates is an interlaminar shear strength which relates to the amount of shear stress a specific material will handle before individual plies fail in shear [55]. It is very important to look at interlaminar properties as majority of structures are prone to fail at joints between plies. Furthermore, it is established that through thickness reinforcement improves interlaminar property of the laminates. Therefore, it is very critical to look at interlaminar shear strength of 3D woven structure according to standard BS EN ISO14130:1998 [53]. Test Procedure Instron machine was utilised to do short beam test. Machine was configured according to specified parameters in the standard. Fig 8.9 demonstrates loading conditions and dimensions for the test specimen. Due to thickness of the sample, it was not possible to use standard sample dimensions. However, thickness to width and span length ratio was considered to select sample dimensions. 164

166 Fig 8.9 Loading conditions and Sample dimensions 1mm/min loading rate was applied to centre of the sample after placing it carefully on two 3mm loading nose with parallel alignment. Load was applied till sample fractured and interlaminar shear strength was calculated based on that. Load vs displacement graph was plotted with cross head movement. Maximum load was taken to measure apparent shear strength of each specimen. Three specimens were tested for each configuration. Interlaminar shear strength was calculated with the help of Eq 5.3. Fsbs= 0.75 * Pm / (b*h) 5.3 where: Fsbs = short-beam strength, MPa Pm = maximum load observed during the test, N b = measured specimen width, mm h = measured specimen thickness, mm 165

167 Fig 8.10 Load (kn) vs Cross head (mm) graphs for short beam test 166

168 Fig 8.10 illustrates load vs. cross head trend for all configurations. It has been seen that braided woven samples are superior compare to woven samples for all configuration in terms of interlaminar properties. Interlaminar properties are slightly better in weft direction than warp direction which is due to higher weft density compare to warp density. One step joint structure was also tested here with and without braiding process. It can be seen from the graph that step provides gradual failure in the structure rather than sudden failure. At the same time, Braiding helps to hold the structure while performing the test and provides extra strength which could be important to design damage tolerant structure. Table: 8.6 Maximum Load (kn) values Warp Weft Step Joint Braided Warp Braided Weft Braided Step Joint Mean SD Table: 8.7 Interlaminar shear strength (MPa) values Warp Weft Step Joint Braided Warp Braided Weft Braided Step Joint Mean SD

169 Fig 8.11 and 8.12 represents comparison values of maximum load and Interlaminar shear strength respectively. One of the interesting aspects of this test was improvement of interlaminar properties with braiding structure over 3D woven structure. This behaviour could be because of integrity braiding provides to 3D woven structure. In addition, Bias fibres also help to gain better interlaminar shear strength. It is very difficult to conclude performance of each configuration with one set of tests. Further work is needed where detailed study can be carried out considering weave design and type of test. Fig 8.11 Comparison of maximum load value (kn) for short beam test Fig 8.12 Comparison of Interlaminar shear strength (MPa) values 168

170 8.3.4 Tensile Test An understanding of composite failure mechanics can help to establish individual structure performance and thus help to design improved structures based on that [56]. Limited work has been done on 3D structures in terms of tensile properties where it is superior to 2D laminates in some studies [57, 58, 59]. In addition, tensile test provides fruitful data which can be used to evaluate in plane properties in 3D are prone to damage occurred while weaving process or not. Conventional 2D structures normally are less affected with weaving process compare to 3D structures due to complexity of process cycle [27]. It is said that warp yarns in 3D structures are six times prone to damage in reed dent than conventional 2D structures. Tensile test will help to establish failure mechanism of 3D structures and understand effect of bias fibres on test results. Tensile test was performed in accordance with standard BS EN ISO 527-5:1997 [54] Test procedure Instron machine was configured in accordance with tensile standard. Machine set up is shown in Fig 8.13 machine parameters and sample dimensions are as below. Loading rate: 1mm/min Gauge length: 150 mm Sample dimensions Total length: 250 mm Width: 25mm Thickness: 5-7 mm Fig 8.13 Machine set up 169