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 met hodology for the improvement of damage tolerance within composite materials , two heavily loaded, stiffened composite wing panels were designed, fabric ated and tested. The panels were impacted at the vulnerable stiffener edges and the failure modes and mechanisms related to the infliction of impact d amage and the subsequent compression after impact loading were determined. A capability to predict the occurrence of impact damage by finite element a nalysis was demonstrated and guidelines for the design of damage tolerant p anels were established. The laminate composition of two panel skins was qua si-isotropic. The test results were compared with test results obtained ear lier for two similar panels with soft skins, i.e., panel skins with a low a xial stiffness. The latter panels were shown to be more damage tolerant, wh ich 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 t he damage area, impact delaminated sublaminates then buckle out of plane an d 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 are a until the final unstable propagation. The stability of the damage configu ration, and in particular of the sublaminates formed by the impact and the subsequent compression loading, seems to be the key with respect to the dam age tolerance of heavily loaded, stiffened panels. (C) 1999 Elsevier Scienc e Ltd. All rights reserved.