CONTINUOUS-LENGTH SPREAD TOW +α /-β FABRICS

Transcription

1 1/7 CONTINUOUS-LENGTH SPREAD TOW +α /-β FABRICS Fredrik Ohlsson, Product Manager - Materials Dr. Nandan Khokar, R&D Manager Oxeon AB, Borås, Sweden ABSTRACT Fabrics with +α/-β orientation of spread tows is a recent development. They are composed of two sets of Spread Tow Tapes arranged at an angle, either equal or unequal, relative to the fabric s length direction, such as +45/-45, +30/-60, +50/-25 etc. These +α/-β fabrics are advantageously produced in continuous-lengths and are innovative solutions for effectively complementing the existing 0/90 Spread Tow woven Fabrics to realize easily and quickly an optimized multidirectional lightweight reinforcement by plying them directly. This paper explains how the indicated +α/-β fabrics directly help replace the conventional cross-plied UD structures with the added advantages of improved delamination resistance and inclusion of virtually crimp-less fibers oriented in two angular directions. As a consequence, production of lightweight multidirectional reinforcements becomes at once labour-time-cost saving and efficient while according the benefits of obtaining relatively high performance, thin, easy-to-handle and well draping reinforcement material. With use of continuous-length +α/-β fabrics the thickness variation associated with overlapping joints/splices is eliminated besides wastage of material reduced. Key words: Spread Tow Fabrics (STF), +α/-β STF, Lightweight carbon reinforcements INTRODUCTION Carbon fiber Spread Tow Fabric (STF) in 0/90 fiber angles has rapidly grown in popularity among composite producers with high demand on weight savings, improved mechanical performance and surface smoothness. Its usage has spread throughout a variety of application areas for example within sports and leisure industries. Products such as ice hockey sticks, golf shafts, rowing boats, wind and kite surfing boards, skis and snowboards, as also Formula 1 cars, luxury yachts and ultralight aircraft are some products where STF is being used currently. At the same time STF materials are being

2 2/7 evaluated in other industries such as aeronautical/aerospace, automotive and engineering industries. The reason for STF s success originates from its unique woven structure resulting by interlacing mm wide stabilized Spread Tow Tapes (STT) as warp and weft in plain and twill weave patterns. Use of STT brings a number of advantages which are discussed below. With a view to further improve composite materials, use of plied materials has been considered for long to obtain certain mechanical properties in different orientations. However, to ply a conventional woven material in different orientations requires cutting relatively smaller pieces from a larger sheet and placing them in relatively different orientations. This approach evidently creates discontinuities in the fiber structure, which reduces the reliability of the final composite material, besides creating uneven thickness due to overlapping. Therefore, as a natural step to complement the existing range of 0/90 Spread Tow Reinforcements, continuous-length Spread Tow +α/-β Fabrics with variable fiber angles such as +45/-45, +30/-60, +50/-25 etc. have been developed recently. This paper explains how the indicated +α/-β STF directly helps replace the conventional cross-plied UD and other plied fabric structures. As a first step to evaluate the +α/-β STF, the +45/-45 variant has been produced. +α/-β FABRIC FEATURES The need for including fibers in relatively different orientations has so far been realized by plying either UD sheets or woven fabric sheets. Whereas use of UD sheets ensures fiber continuity without any structural integrity, the use of woven fabric sheets, though having structural integrity, creates fiber discontinuity because they have to be cut angularly from a larger sheet and laid. Such patches of woven material have to be laid carefully to match with the edges of the previously laid patches. Often times the woven material patches are laid in certain sequences with some overlapping, which causes the problem of thickness variation at the joints. These fiber discontinuities and thickness variations adversely affect the performance of composite materials. In addition to the indicated performance related problems, use of woven patches also has an adverse impact on the economics of production relating to cutting, plying and wastage of material.

3 3/7 An approach to overcome some of the discussed problems has been through use of Non-Crimp-Fabrics (NCF) materials. However, use of NCF materials has certain drawbacks such as their non-symmetrical arrangement of UD plies, relatively higher thickness and areal weight of material, in-plane delamination, use of epoxy incompatible polyester stitching yarns with its corresponding knock-down factor due to stitching crimp. In an attempt to overcome the problems associated with the use of UDs, NCFs and conventional woven materials, the +α/-β STF has been developed. In Figure 1 are shown three different +α/-β constructions. As can be inferred, they represent the constituent STT arranged in (a) obtuse angle, (b) right angle and (c) acute angle. These constructions of course have the STT oriented in equal but opposite angles. Should there be a need for having an +α/-β construction wherein the STTs are incorporated unequally, then such an +α/-β STF can be also produced as shown in Figure 1(d). The fiber angle flexibility in the material provides, for example, the possibility to tailor produce an +α/-β STF with STT in about +55/-55 degrees to uniquely enable direct production of pressure vessels. Such a fabric could be wound over itself to as many layers as needed to meet the required performance demand. Figure 1. Different variants of +α/-β STF constructions: (a) obtuse angled, (b) right angled and (c) acute angled. While these variants have the STT in equal and opposite orientations, the variant (d) has unequal orientations of the STT.

4 4/7 At present +α/-β STF can be produced using STT of different width from high strength (HS), Intermediate modulus (IM) and high modulus (HM) carbon fiber yarns of high tow count (12k and above) in fabric areal weights starting at 80 gsm, 76 gsm and 130 gsm respectively. STTs of lower areal weight will most likely be available shortly hence, the areal weight of +α/-β STF can also be reduced. All these variants of +α/-β STF constructions are producible in continuous lengths, which benefits in eliminating cutting, laying and splicing. Further, such a continuous +α/-β STF material can be directly preimpregnated as the tapes are well integrated to resist fabric deformation in both length and width directions. An example of +α/-β STF produced in +45/-45 STT orientation is shown in Figure 2. Figure 2. An +α/-β STF with +45/-45 STT orientation

5 5/7 MECHANICAL PERFORMANCE Given the arrangement of STT, a composite made from an +α/-β STF is not optimal to take loads in its longitudinal and lateral directions without getting deformed just as a composite made from 0/90 STF will shear when loaded angularly. Notwithstanding this aspect, +α/-β STF is designed to be used along with conventional 0/90 STFs to meet the required laminate performance demands. Needless to state, individually +α/-β STFs is considered beneficial for torsional applications. The mechanical performance of +α/-β STF comes from virtually no in and out of plane fiber crimp (i.e. crimp angle), fewer interlacing points (i.e. crimp frequency) and relatively higher fiber volume fraction (i.e. cover factor). Further, because STT has its fibers well distributed, its wetting by the matrix can be achieved easily and quickly to ensure improved load transfer in a composite material. Because the constituent STT are integrated throughout the +α/-β STF the in-plane delamination risk is substantially reduced. In principle this implies that each +α/-β STF sheet comprising fibers in two equally opposite angles is symmetrically balanced. Therefore, use of such +α/-β STF enables to obtain a symmetrically balanced plied construction directly using relatively fewer sheets. Tables 1 and 2 below together with Figure 3 and Figure 4 illustrate how symmetrical quasi- and bi-directional balanced constructions are obtained using relatively fewer STF sheets compared to the existing NCFs and UDs. Table 1. Symmetrical Quasi-Isotropic construction Material Ply sequence No. of fiber layers to achieve symmetry UD [0/90/+45/-45/-45/+45/90/0] 8 NCF [NCF (0/90/+45/-45) /NCF (-45/+45/90/0) ] 8 STF [STF (0/90) /STF (+45/-45) /STF (0/90) ] 6

6 6/7 Figure 3. Symmetrical Quasi-Isotropic construction of UD/NCF (left) and +α/-β STF (right) Table 2. Symmetrical bi-directional construction Material Ply sequence No. of fiber layers to achieve symmetry UD [+45/-45/-45/+45] 4 NCF [NCF (+45/-45) /NCF (-45/+45) ] 4 STF [STF (+45/-45) ] 2 Figure 4. Symmetrical bi-directional construction of UD/NCF (left) and +α/-β STF (right)

7 7/7 METHOD OF PRODUCTION The method of producing +α/-β STF is completely new in that it is unlike the principles of weaving and braiding. The production is virtually endless as there are no STTs oriented in the fabric length direction. Accordingly, the fabric can be produced in any desired length that may be required. The angles of STT can be varied as desired on the same machine. CONCLUSIONS The new +α/-β STF offers new opportunities in composite material production both in terms of performance and economics. It can be produced using STT of different widths, areal weights and fiber types (HS, IM and HM). Its lowest areal weight at present is 76 gsm. The availability of +α/-β STF in different angular variants allows to ply them directly for obtaining multidirectional fiber orientations. The availability of such fabric in continues length enables waste reduction and splicing/joints whereby a composite of uniform thickness can be obtained. ACKNOWLEDGEMENTS Sincere thanks are expressed to Mr. Henrik Blycker, CEO, Oxeon AB, Sweden, for his encouragement and support at all times.