IMPACT BEHAVIOR AND IMPACT-FATIGUE TESTING OF POLYMER COMPOSITES

Damage mechanisms in continuous-fiber-reinforced resin composites subjected to repeated low-velocity impacts have been investigated. The drop weight and height of an instrumented falling-dart tester were varied to provide a spread of incident energies. A wide range of fiber/resin composites was tested and their failure mode versus loading-history relationships were examined. These include graphite-, aramid-, and glass-fiber/epoxy composites prepared from different preform styles and stacking sequences. The results indicate the existence of a critical incident energy, E(c), above which delamination will occur in a given composite at the first impact; both the stiffness and the strength of this composite are thereby impaired. The strength (maximum load) and stiffness (curve slope) values of this damaged composite upon subsequent repeated impacts can be measured as a function of number of cycles. These strength values, if normalized with respect to the strength of the original composite and plotted against the number of impact cycles on a log-log scale, usually exhibit a straight line of slope -b. When subjected to a sub-critical incident energy (E(in) < E(c)), no appreciable impairment to the composite integrity would be observed until a critical number of impact cycles, N(c), was reached. The magnitudes of E(c), b, and N(c) can be used as indices to measure the damage tolerance of a composite if a small variation of impactor mass and incident velocity is allowed. An elastic strain energy theory was developed to predict the critical incident energy values of various composites, and the prediction was found to be in good agreement with the experimental data in the case of low-velocity impact measurements.