Figure 1: Photograph of Drop Weight Test Machine

Transcription

1 Design, Development and Fabrication of a Fixture for Drop Weight Machine Mahesh S, Hemantha B S,Nandish M Hiremath, Chethan M Students, Department of Mechanical Engineering, Vemana Institute of Technology, Bangalore, India Abstract- Extensive research is happening around the world to develop a new material or to modify an existing one, so that it satisfies a particular purpose. Material property is a set of factors that are considered while developing or modifying a product. To determine material property, two types of testing are carried out Destructive testing and Non-Destructive testing. Drop weight testing is one of the destructive testing methods, where a required amount of weight is dropped on a specimen from a considerable height. Thus the amount of stresses absorbed by the specimen is determined which in turn is the source to determine all other material properties. Proposed work is to develop a fixture for testing flat specimens on the existing drop weight machine. INTRODUCTION Mechanical properties are useful to estimate how parts will behave when they are subjected to mechanical loads. In particular, our aim is to know the amount of energy absorbed by the material until it fails, under different dynamic loading conditions. Thus testing of materials becomes necessary. One such test method is Impact testing or Drop weight testing. Drop weight test machine is shown in figure 1 and its specification is given in table 1. It is a type of testing where impactor assembly drops on to a specimen from a certain predetermined height. Impactor assembly has a provision for varying the mass. As impactor assembly falls on the specimen placed on the load cell, it deforms, and the load cell records the strain in the specimen in terms of voltage. Impactor assembly is raised to the required height using a motorized rope drum by means of lever mechanism along the guideways. Generally thick solid specimens or hollow tubes are tested in drop weight testing machine and there is no provision to test flat specimens of thickness below 15mm. Hence a fixture is designed to test flat specimens of thicknesses ranging from 3mm to 15mm on drop weight machine. Guide ways Lever Mechanism Impactor Assembly Motorized Rope Drum Load cell Figure 1: Photograph of Drop Weight Test Machine IJIRAE , All Rights Reserved Page -131

2 The fixture is such that the indenter perforates the flat specimen and there is no damage caused to the load cell of the machine. The design was carried out using CAD software Solid Edge V19. TABLE 1: SPECIFICATIONS OF DROP WEIGHT MACHINE SL. NO. PARTICULARS DETAILS 1. MAX. DROP HEIGHT 5.8 M 2. MAX. VELOCITY M/S 3. DROP MASS KG 4. MAX. CAPACITY 6960 J 5. MAX. SIZE OF SPECIMEN 0.28*0.28 M 2 6. LOAD CELL SEMI-CONDUCTOR STRAIN GAUGE, 0 500KN METHODOLOGY Design of fixture was developed using CAD technology. This designed fixture is quite suitable for mounting flat specimen on to the drop weight machine. Selection of material for the fabrication of fixture is explained eventually. It has two major parts viz., Specimen Holder and Indenter Assembly. SPECIMEN HOLDER Specimen Holder is a device on to which the flat specimen of thickness 3mm to 15mm is mounted rigidly. Parts of specimen holder are shown in figure 2. Figure 2: Parts of specimen holder Main parts of specimen holder are listed below I. BASE PLATE SUPPORTING BUSH OR SPACERS CLAMPING PLATES II. III. I. BASE PLATE Base Plate is the foundation of specimen holder which is mounted rigidly on the load cell by means of threads. This acts as the base for rest of the parts of Specimen Holder. Figure 3 shows base plate. Figure 3: Schematic of base plate IJIRAE , All Rights Reserved Page -132

3 II. SUPPORTING BUSH OR SPACERS Spacers are used in order to raise the specimen to a certain height, so that indenter penetrates the specimen. It supports the two clamping plates between which our specimen is mounted. Four spacers are used along four bolts. Spacer is given in figure 4. Figure 4: Schematic of spacer III. CLAMPING PLATES These are annular shaped discs that support and hold the specimen rigidly. They decide diameter of the specimen, which is 150mm according to our design. This is shown in figure 5. Figure 5: Schematic of clamping plates These parts of the fixture are held together using bolts and nuts. Implicating certain precautionary measures like, nylon bush and rubber sheets, damage to the specimen holder is avoided. The specimen mounted on the Specimen Holder is shown in fig. 6. Specimen Figure 6: Specimen mounted on specimen holder INDENTER ASSEMBLY Indenter assembly is similar to impactor assembly shown in figure 1, but a small modification is to be done in order to hold the indenter i.e., indenter-chuck is to be incorporated. Parts of indenter assembly are shown in figure 7. IJIRAE , All Rights Reserved Page -133

4 Figure 7: Parts of Indenter Assembly MAIN PARTS OF INDENTER ASSEMBLY ARE LISTED BELOW- I. INDENTER INDENTER CHUCK HOLDER OR LIFTER II. III. I. INDENTER Indenter penetrates the specimen when it falls from a certain height. Different types of indenters used are flat face, hemispherical, conical and knife-edge indenters. Any type of indenter based on the test to be carried out is used. Hemispherical indenter is shown in figure 8. Figure 8: Schematic of hemispherical indenter II. INDENTER CHUCK Indenter is held in indenter chuck by means of nut and bolt. It is held between top and bottom plate along with variable weights clamped together by means of bolts and nuts as shown in figure 7. Indenter chuck is shown in figure 9. Figure 9: Schematic of Indenter Chuck III. HOLDER OR LIFTER Holder is the top most part of indenter assembly. Rope drive mechanism lifts the indenter assembly by means of lever mechanism which clamps the lifter. The geometry of holder is shown in figure 10. IJIRAE , All Rights Reserved Page -134

5 Top plate Bottom Plate Variable Mass Figure 10: Schematic of holder or lifter The weights are held between the top and bottom plates. Mass of entire assembly can be varied by increasing or decreasing the thickness of the disc. These parts are shown in figure 11. IJIRAE , All Rights Reserved Page -135

6 FABRICATION Fabrication of the fixture is done using EN8 steel. The machining was carried out in lathe, drilling machine, shaping machine and grinding machine. Properties of EN8 steel are - Ultimate Stress N/mm 2 Yield Stress 465 N/mm 2 0.2% Proof Stress 450 N/mm 2 Hardness 108 Rockwell The hardness of indenter and indenter chuck is enhanced by heat treatment process, they are heated to C for an hour, and then oil quenched. The Rockwell hardness number was found to be 100. ASSEMBLY The design assembly of fixture on drop weight machine is shown in figure12. Indenter Assembly Specimen Holder Load cell Figure12: Assembly of Fixture on Drop Weight Machine. The material selected for fabrication of the fixture must withstand forces acting on it. The bolts and nuts used for clamping of the fixture is TVS M14, which has an allowable load of 6.88MN and yield stress of 446 N/mm 2. As these bolts are subjected to bending on the whole length due to the hear force developed by clamping plates and specimen during impact, this shear force is determined to ensure safety of the entire fixture. Stress acting on the specimen is given by, τ = F A = F πdt Figure 11 We know yield stress of EN 8 steel is 465MPa; therefore the maximum shear stress developed by the material on the bolts is, τ = σ y /2 τ = 465/2 τ = 232.5MPa From the above calculation we can come to a conclusion that the design is safe when the test specimen used has its yield stress below 465MPa. ACKNOWLEDGEMENT The authors are thankful to Dr. T Yella Reddy and Prof. Mohan in design and creation of this manuscript. REFERENCES [1]. T.Y. Reddy, H.M. Wen, S. R. Reid, P. D. Soden. [2]. Penetration and Perforation of Composite Sandwich Panels by Hemispherical and Conical Projectiles.Journal of Pressure Vessel Technology (May 1998, vol. 120). [3]. Giovanni Belingardi, Roberto Vadori. Low velocity impact tests of laminate glass-fiber-epoxy matrix composite material plates. International Journal of Impact Engineering 27 (2002) [4]. Sunith Babu Loganathan, Hanumanahally Kambada Shivanand.Effect of Core Thickness and Core Density on Low Velocity Impact Behaviour of Sandwich Panels with PU Foam Core.Journal of Minerals and Materials Characterization and Engineering, [5]. S.H. Song, Y.S. Byun, T.W. Ku, W.J. Song, J. Kim and B.S. Kang.Experimental and Numerical Investigation on Impact Performance of Carbon Reinforced Aluminum Laminates. Journal of Material Scienceand Technology, [6]. GayatriVineela.M, GayathriTadepalli and A.Krishnaiah.Impactbehaviour of Fiber Reinforced composites with change in FiberOrientation.International Journal of Current Engineering and Technology. [7]. Artūras KERŠYS, Neringa KERŠIENĖ, Antanas ŽILIUKAS Experimental Research of the Impact Response of E- Glass/Epoxy and Carbon/Epoxy Composite Systems.Article in materials science January IJIRAE , All Rights Reserved Page -136