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Fire damage involving mechanically failed composite aircraft structures can dramatically alter their exposed surface characteristics in ways that inhibit fire forensic analyses. In this work, the effects of fire exposure on mechanically failed Cytec T40- 800/Cycom® 5215 graphite/epoxy composites were examined. Coupon level vertical fire tests were performed on mechanically failed unnotched compression and in-plane shear graphite/epoxy specimens. The fire damage was characterized by visual inspection and scanning electron microscopy. The fire damage development in the specimens involved a concurrent and sequential interaction between multiple physical, chemical, and thermal processes. This damage included melt dripping, matrix decomposition, char, soot, matrix cracking, delamination, and residual thickness increases due to explosive outgassing. The composite thermal degradation due to heat conduction, combustion, and/or thermal deformation was significantly affected by the specimen layup, ply orientation relative to the heat source, and the fracture surface morphology. Plies burned with fibers oriented parallel to the flame axis conducted heat into the interior of the composite. This resulted in melt dripping, internal pockets of matrix decomposition, and surface char deposition that, in some cases, completely obscured pertinent aspects of fiber fracture surface morphology. In contrast, plies burned with fibers oriented perpendicular to the flame axis acted like a thermal protection layer that impeded (slowed) heat transfer to the specimen’s interior. Furthermore, the thermal damage development was influenced by the specimen layup and the total available free surface area created during mechanical failure. Specimens with more free surface area promoted better airflow and oxygen availability for combustion and sustained far more thermal degradation for given fire exposure. Key fractographic features in exposed fiber bundles were destroyed due to severe thermal oxidation and thinning. A thorough understanding of these coupon-level fire tests represents a critical first step in developing a coherent strategy for the Federal Aviation Authority post-crash forensic analysis of composite aircraft structures.

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.