Effects of ballistic impact damage on thin and thick composites
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Use of composite materials is growing rapidly because they offer weight reduction, durability and better fatigue life as compared to metals. Despite of their advantages, there are some problems associated with the use of composites, and impact resistance is one of the weak areas in composites. Low speed impact damage has been the focus of attention of researchers for quite some time but studies of high speed impact damage is still in its preliminary stages. In this study, effects of ballistic impact on thin and thick composite panels were investigated. Damage due to high speed impact in composites is a complex phenomenon and it is difficult to analytically model all the events taking place during the impact;An experimental approach was used to study the damage from high speed impact and the Finite Element analysis was performed to understand the damage sequence. Thin symmetric quasi-isotropic and cross-ply laminates consisting of up to 16 layers were studied using experimental vibration and Finite Element analysis. It was found that frequency response of damaged plates is dependent on the stacking sequence of the laminate. Results also show that the natural frequencies of the damaged plates decrease for few initial modes and increase for some of the higher modes. It was observed that cross-ply laminates exhibit very little effect on the natural frequencies due to damage;Thick symmetric quasi-isotropic and cross-ply laminates consisting of 56 plies were also studied. Results demonstrate that damage size is dependent more on the shape and size of the impactor, rather than the impact energy. Faster bullets cause less damage as compared to slower bullets. Damage was found to be dependent on stacking sequence of the laminate. Cross-ply laminates suffer more damage than the quasi-isotropic laminates. Damage is dependent on the thickness of the laminate and is more in thicker laminates. Damage is always more towards the exit side than the entry surface;A model of bullet penetration into a composite laminate is presented and the failure due to inter-laminar shear stresses was explained through this model. The model was verified using a quasi-static Finite Element analysis. It was demonstrated that outer-most ply of the laminate fails first and maximum inter-laminar shear stresses occur between two outer-most plies, causing delamination. It was also demonstrated that inter-laminar shear stresses increase progressively, as the number of effective plies in the laminate reduce due to failure and it is strongly dependent on the stacking sequence.