An efficient approach for damage quantification in quasi-isotropic composite laminates under low speed impact

An efficient method to determine the type, size, and location of damage in impacted quasi-isotropic composite laminates is presented. The method uses the peak force during impact obtained from energy balance, a Hertzian contact formulation and energy minimization to determine the complete state of stress in the laminate. Comparisons of the analytical predictions to limiting cases of infinite thickness plates or to detailed finite element models for finite thickness plates shows the predicted stresses to be in excellent agreement with other methods. The stresses are then modified to account for the creation of damage and used in out-of-plane and in-plane failure criteria to predict delamination sizes, matrix failure and fiber failure. The predicted damage states are then compared to published test results for two different materials, eight different stacking sequences, and a range of impact energies from 5 to 50 J. Very good agreement of the predicted damage sizes with the experimentally measured values is observed for a wide range of energy levels but, for two laminates, the discrepancies are significant. Possible improvements of the method are discussed briefly. This method is very promising and can be used in preliminary design allowing extensive trade studies and, eventually, layup optimization. It can also form the beginning of an efficient methodology to predict compression after impact strength of quasi-isotropic laminates.