B. Z. Jang

Impact resistance and energy absorption mechanisms in hybrid composites

Abstract The response of hybrid composites to low-velocity impact loading has been investigated. The energy absorbing mechanisms of laminates containing various fibers were studied primarily by means of the instrumented falling dart impact testing technique. Static indentation tests and scanning electron microscopy (SEM) were also employed to assist in the identification of failure mechanisms. The composites containing polyethylene (PE) fibers, which were of high strength and high ductility, were found to be effective in both dissipating impact energy and resisting through penetration. Polyester (PET) fiber reinforced epoxy also exhibited superior impact characteristics even though the PET fabric layers without epoxy did not have good modulus or ductility. Good energy absorbing capability was also observed in epoxy reinforced with woven fabrics made of high-performance Nylon fibers. Nylon, PE and PET fibers were found to enhance the impact resistance of graphite fiber composites. Upon impact loading, the composites containing either PE or PET fibers in general exhibited a great degree of flexural plastic deformation and some level of delamination, thereby dissipating a significant amount of strain energy. Hybrids containing Nylon fabric showed analogous behavior, but to a lesser degree. The stacking sequence in hybrid laminates was found to play a critical role in controlling plastic deformation and delamination. This implies that the stacking sequence is a major factor governing the overall energy sorbing capability of the hybrid structure. The penetration resistance of hybrid composites appeared to be dictated by the toughness (strength plus ductility) of their constituent fibers. The fiber toughness must be measured under high strain rate conditions.

Impact behavior and impact-fatigue testing of polymer composites

Abstract 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/expoxy composites prepared from different preform styles and stacking sequences. The results indicate the existence of a critical incident energy, E c , above which delamanation 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 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 use 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.