A non-linear shear damage model to reproduce permanent indentation caused by impacts in composite laminates

For aeronautical composite structures impact damage is an issue of great concern. In service damage detection capability is related to the indentation left by impacts, thus it is crucial to understand the physical phenomena which control indentation. Recent studies suggest that indentation is greatly affected by the out-of-plane shear properties of laminates, nevertheless the simulation of such behavior is still an open issue. A non-linear material model, including both in-plane and out-of-plane shear, has been developed and implemented in an existing continuum-damage-mechanics-based UMAT routine for the ABAQUS code. The enhanced code can simulate the indentation caused by impacts and it has been used to simulate tests according to ASTM D7136. The comparison of simulation results with those obtained by means of the original UMAT routine, implementing a simplified shear model, allows the assessment of the importance of out-of-plane shear in the simulation of impact events. A comparison between simulated time histories, of both contact force and absorbed energy, and the in-house experimentally measured ones shows a remarkable agreement, allowing the code to be validated. The non-linear out-of-plane shear model also allows permanent indentations at the end of impact simulations be obtained, which are in good agreement with the experiments.