Repeated Impact Failure of Continuous Fiber Reinforced Thermoplastic and Thermoset Composites

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Abstract
An effort was made to study the damage mechanisms in both thermoplastic and thermoset composites in response to repeated low-velocity impacts. An instrumented falling-dart impact tester was used that provided the load-deflection and energy-deflection curves plus other numerical data on material properties. Both poly(phenylene sulfide) (PPS) and epoxy composites prepared from different preform styles and stacking sequences were tested. For any composite with given dimensions a threshold incident impact energy (Ec,) could be identified above which delamination would occur at the first impact. As a consequence, both the stiffness and the strength of this composite were reduced and these properties could be measured using the same impact tester as a function of impact cycles. If instead a subcritical incident energy (Ein < Ec) was imposed on the material, no appreciable property loss would be observed until a critical number of impact cycles (Nc) was reached. In this subcritical stage, matrix cracking preceded delamination as impacts were repeated. Both the number and size of delamination cracks were then found to increase as the number of repeated impacts was increased. An elastic strain energy approach was developed to predict the threshold incident energy values of various composites and the prediction was found to be in good agreement with the experimental data. Distinct failure mechanisms were found to exist between epoxy and PPS composites.