Impact damage development in thick composite laminates

Abstract Impact damage tolerance is evaluated for three different thick graphite-epoxy composite material systems which were fabricated as 92-ply composite plates. The systems are designated as brittle, moderately tough and interleaved (very high impact resistance). Square specimens (63 mm × 63 mm) were tested in an instrumented drop-weight impact tower or in a high-velocity impact device (gas gun). Impact damage was inspected and comparisons made between the two different types of tests, and between the three different material systems. Residual strengths were measured on coupons cut from the impacted square plates and evaluated in interlaminar shear (three-point bending) and compression. Both tests show that for all three material systems there exists an impact energy threshold above which the residual strength drops dramatically. The interleaved system showed superior impact resistance than the tough and brittle systems. Attention is paid to the questions of energy distribution during impact, the relation between impact resistance and quasi-static energy storage capability, the structural response involved in the material comparisons and the dependence of impact resistance on velocity.