Methods for Improving the Impact Resistance of Composite Materials

Abstract

A three-phase program to improve the impact resistance of composite materials was undertaken. In the first phase, the Charpy impact test was investigated for its utility in characterizing impact performance. It was found that the results of the standard Charpy test can be misleading, but the addition of instrumentation to provide load and energy histories can make this test a useful one. The importance of differentiating between initiation energy and propagation energy is shown, and a new parameter (the “Ductility Index”) is proposed, along with maximum stress and total energy, as a useful measure for comparing impact behavior of materials. The instrumented Charpy test was used to investigate impact behavior of unidirectional, epoxy-matrix composites reinforced with fibers of E-glass, Kevlar 49, and graphite. In addition, two unidirectional hybrid composite materials consisting of graphite and Kevlar 49 fibers in epoxy were investigated. Comparisons were made with quasi-static three-point bend tests. It was found that the hybrid materials had significantly higher impact energies, ductility indices, and maximum stresses than the all-graphite fiber material. Impact behavior of [±45] graphite/epoxy and hybrid Kevlar 49-graphite/epoxy panels, representative of aircraft fuselage shear panels, was studied using a ball-drop test. Results indicate a significant improvement in impact resistance with hybrid construction.