Impact damage prediction and failure analysis of heavily loaded, blade-stiffened composite wing panels

Within the framework of a European research programme to develop design methodology for the improvement of damage tolerance within composite materials, two heavily loaded, stiffened composite wing panels were designed, fabricated and tested. The panels were impacted at the vulnerable stiffener edges and the failure modes and mechanisms related to the infliction of impact damage and the subsequent compression after impact loading were determined. A capability to predict the occurrence of impact damage by finite element analysis was demonstrated and guidelines for the design of damage tolerant panels were established. The laminate composition of two panel skins was quasi-isotropic. The test results were compared with test results obtained earlier for two similar panels with soft skins, i.e., panel skins with a low axial stiffness. The latter panels were shown to be more damage tolerant, which is accredited to the much smaller number of 90 degrees plies present in the soft skins. The failure mode was found to be a three stage phenomenon: a load eccentricity is present from the start causing local bending near the damage area, impact delaminated sublaminates then buckle out of plane and eventually propagate leading to global bending and to overall instability and collapse. Delamination growth occurred mainly in the lateral direction along 90 degrees ply interfaces, but remained within the C-scan damage area until the final unstable propagation. The stability of the damage configuration, and in particular of the sublaminates formed by the impact and the subsequent compression loading, seems to be the key with respect to the damage tolerance of heavily loaded, stiffened panels. (C) 1999 Elsevier Science Ltd. All rights reserved.