Impact Resistance of Graphite and Hybrid Configurations

Abstract

This paper discusses the effect of several configuration variables on the impact resistance of graphite epoxy laminates. Geometrical effects were evaluated using monolithic panels from 0.04 to 0.18 inch thick or sandwich panels with face sheets from 0.02 to 0.5 inch thick. Separate series of tests were also conducted on 0.5 inch thick monolithic

panels that are typical of aircraft wing structures. Several alternate materials were also investigated to determine their effect on improving the impact resistance of baseline panels. Plies of more ductile materials were added to the basic graphite/epoxy tape panel, and in other configurations, all or part of the graphite/epoxy tape plies were replaced with woven graphite/epoxy.

Impact tests were conducted using a falling weight with both blunt and sharp impactors, which were strain-gaged to provide force versus time histories during the impact event, from which the absorbed energy versus time histories were calculated. Acoustic NDI scans and photomicrographs of cross sections through the damaged areas were made to investigate the extent of internal damage and failure modes.

Internal damage was shown to occur for impacts having little or no damage visible on the surface. The sizes of damage shown on the C-scans and cross-section photomicrographs were in close agreement. Increases in the impact energy required to cause specific damage were noted for alternate materials. Thin laminates have more back surface damage, and thick laminates have more front surface damage for damage near the visible threshold.