PSI - Issue 14

T Sreekantha Reddy et al. / Procedia Structural Integrity 14 (2019) 265–272 Author name / Structural Integrity Procedia 00 (2018) 000–000

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1. Introduction Fibre-reinforced composite materials have remarkable potential for use in aerospace and defence applications where high strength, damage tolerance and weight reduction are important design aspects. They can also be used as valid alternative armour materials for replacing steel and light alloys due to their better energy absorption capability under impact loads. However, composites are sensitive to out-of-plane impact, which can initiate damage even at very low impact energies. A low-velocity impact is known to generate delaminations between the layers of composite laminates with no visible surface damage. These delaminations may grow with time during service. It will result in reduction of stiffness and ultimately affect the structural integrity of the system. Evaluation of composite laminates subjected to low energy projectile impacts has shown drop in residual tensile and compression strength of up to 50%, conditions resulting in a complex fracture process (Cantwell et al., 1983 & 1984). As a result, impact damage is one of the most significant damage types, since this can initiate delaminations which greatly reduce the compressive and fatigue strengths of a composite structures (O. Ley and V. Godinez, 2013). Hence, low-velocity impact of glass fiber-reinforced composite laminates has been the important subject of many experimental and analytical investigations (AE Bogdanovich et al. 1994, NK Naik et al. 1998, WQ Shen 1997, G Belingardi et al. 2002, JN Baucom et al. 2006, Reddy et al. 2016 & 2017). R. Karakuzu et al. (2010) have studied the impact behaviour of glass/epoxy laminates with [0/±θ/90] s fibre orientation numerically at equal energy, equal mass and equal velocity. They also investigated the effect of thickness of laminates numerically and found that higher thickness plates cause the higher contact force and lower deflection. Experimental results of low-energy impact on E-glass/polyester composites are presented by L.S. Sutherland et al. (1999). They concluded that no significant effect of incident energy on the impact response was seen in the impact energy of 10-180J, except for some possible differences in the failure mode at the highest incident energy. F.J. Yang et al. (2010) studied the effect of thickness ( t ) and temperature on critical force to initiate the damage (P crit ) on glass/epoxy composites and found that impact force required to initiate damage varies linearly with t 3/2 and influence of test temperature on damage initiation is complex. The main objective of present study is to evaluate the impact behaviour of E-glass/epoxy composite laminates of various thicknesses under low velocity impact. Peak force, maximum displacement and damage area were experimentally evaluated at three different impact velocities viz. 4.34, 6.08 and 7.51 m/s. Degree of damage was estimated by visual observation and compared with IR thermography technique. 2. Experimental 2.1. Materials Epoxy resin and hardener (LY556, HY5200) were procured from M/s. Huntsman chemicals. Commercially available E-glass woven roving was used as reinforcement. E-glass/epoxy prepregs were prepared by hand layup technique and the laminate was cured through compression moulding. Laminates of in four different thicknesses viz. 3, 5, 7 and 10 mm with a variation of ±0.1mm were prepared. Test samples were cut in to the dimensions of 150 x100 mm for low velocity impact experiments using diamond wheel cutting machine. 2.2. Low velocity impact tests Low velocity impact tests were carried by using instrumented drop weight impact tester (CEAST- 9350 model). It has got pneumatic clamping provision, with 76.2 mm dia opening to position the test specimen during impact test. There is also provision of an anti rebound mechanism to prevent the multiple hits. The impactor consists of striker and hemi spherical tup of 16mm diameter. The tup is non-deformable in nature which is made up of high strength steel (60-62 HRC hardness). The tup is attached to the striker at one end. The striker is also equipped with force transducer of 45kN capacity to measure force exerted by the test specimen on the impactor during the impact. Impactor mass was kept constant and required impact velocity was obtained by dropping the impactor from a pre calculated height. Data acquisition system was used to record the force–time history at the sampling rate of 500 kHz.