Assessment of impact damage in toughened resin composites

Abstract

It is well known that carbon/epoxy composites, because of their brittle nature, are highly susceptible to impact damage. While high and medium levels of impact energy cause surface damage that may be detected visually, low-energy impact can produce internal delaminations with little visible surface damage. This “barely visible impact damage” or BVID may only be detected using sophisticated non-destructive inspection (NDI) techniques such as ultrasonic C-scan. BVID is of major concern in the design of composite aircraft structures from a damage tolerance viewpoint because it causes a significant reduction in compressive strength and it reduces fatigue strength as a result of delamination growth under compression cyclic loading. The Structures and Materials Laboratory initiated a program to investigate the impact resistance of several improved epoxy-resin composites. An instrumented drop-weight impact procedure was used to produce impact damage in three commercially-available high strain/toughened epoxy composites as well as in T300/5208 which was tested as a baseline material system. The load-time history obtained during an impact test was used to indicate the energy required to produce the onset of damage in the monolithic specimens. The impact damage was assessed nondestructively using ultrasonic C-scan techniques and destructively using a sectioning procedure. The residual strength properties were determined by a compression-after-impact test under laboratory temperature and humidity conditions. In this paper, experimental results as well as a discussion on failure modes are presented.