Fire impact on mechanically failed graphite/epoxy composites

Aircraft crashes often initiate or accompany fire incidents. Post-crash fires, in particular, can mask or destroy critical fractographic failure features necessary for accident reconstruction. As a first step in developing a coherent strategy for composite aircraft post-crash forensic analysis, a series of vertical flame tests were performed on mechanically failed Cytec T40-800/Cycom® 5215 graphite/epoxy unnotched compression, short beam strength, and in-plane shear specimens. Visual inspection and scanning electron microscopy were used to examine the specimens fracture surfaces before and after fire testing. The fire-induced damage included matrix decomposition, melt dripping, char, soot, delamination, and residual thickness increases. The composite thermal degradation was significantly influenced by the specimen stacking sequence, the fracture surface morphology, and the total available free surface area. In addition, microscopic inspection showed that char layers impeded heat conduction and oxygen transfer to the interior of the specimens resulting in less thermal damage to the underlying plies. Moreover, burned specimens with more available free surface area sustained more thermal degradation and fire combustion damage for a given fire exposure duration. Exposed fiber bundles from the fracture surfaces were susceptible to severe thinning and thermal oxidation, which destroyed critical fractographic failure features necessary for accident reconstruction.