Mechanisms Governing the Damage Resistance of Laminated Composites Subjected to Low-Velocity Impacts

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Abstract
The damage resistance and damage tolerance of laminated composites sub jected to low-velocity impacts are two major factors limiting their applications in load bearing structures. A current industry measure of damage resistance and damage tolerance is performed through Compression After Impact (CAI) testing. Although these tests are considered to provide a realistic evaluation of a composite's performance, translation of their results to actual performance in a structure is questionable since the test results are a combined outcome of the composite material properties, the specimen geometry, and the associated boundary conditions. In this paper, we present the results of an experimental in vestigation whose aim is to elucidate the mechanisms governing the damage resistance of composite laminates subjected to low-velocity impacts. Generic damage characteristics are reviewed and it is shown that they result from two different events during the impact pro cess. First, the laminate experiences a local failure which resembles a Hertzian failure process. This occurs once the impact load reaches a critical threshold level and results in a collection of oriented matrix micro-cracks resembling that of a Hertzian Cone shape together with pairs of intra-ply radial cracks. Next, as the laminate continues to carry ad ditional load, subsequent delamination occurs between the plies with the size of the delaminated area increasing with increasing load. It is demonstrated through these studies that the impact force is a critical parameter in defining the extent of damage in the laminate during impact as opposed to the impact energy in itself. Also, methods are introduced to

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