Post-Crash Fire Forensic Analysis of Aerospace Composites
Abstract
The primary goal of this thesis is to investigate the effects of fire exposure on thermal damage development in mechanically-failed graphite-epoxy composites. Vertical and horizontal fire tests were performed on mechanically-failed unnotched compression, short beam strength, and in-plane shear Cytec Cycom 5215 T40-800 graphite-epoxy specimens. In addition, a single cone calorimetry test was performed on a compression-after-impact specimen. Fire damage included melt dripping, matrix decomposition, char, soot, matrix cracking, delamination, and residual thickness increases due to explosive outgassing. Visual inspection and scanning electron microscopy of burned specimens showed that the specimen lay-up, specimen orientation relative to the heat source, and fracture surface morphology all had a significant influence on composite thermal degradation.
Thermal damage due to heat conduction, combustion, and/or thermal deformation was highly dependent on the ply orientation relative to the flame. Plies with fibers oriented parallel to the heat-exposed surface acted like a thermal protection layer that impeded (slowed) heat transfer to the interior of the specimen and promoted convection of hot gasses that bypassed the specimen. In contrast, plies with fibers oriented perpendicular to the heat-exposed surface (i.e., burned parallel to the fibers) conducted heat into the interior of the composite, resulting in melt dripping, internal pockets of matrix decomposition, and surface char deposition that, in some cases, completely obscured salient aspects of fiber fracture surface morphology. Thermal damage development in mechanically-failed laminates can be compounded by the presence of different ply groupings in a given stack-up, as well as the total available free surface area. Burned specimens with more free surface area sustained far more thermal degradation for a given fire exposure. Exposed fiber bundles were susceptible to severe thinning and thermal oxidation which destroyed key fractographic features.
To the author’s knowledge, this research is the first to investigate i) the effects of fire exposure on mechanically-failed continuous graphite fiber-epoxy laminates, and ii) the influence of specimen lay-up, orientation, and fracture surface morphology on different thermal degradation mechanisms in aerospace composites. This research represents an important first step in the development of a coherent strategy for Federal Aviation Administration post-crash forensic analysis of composite aircraft structures.
Citation
Ouidadi, Hasnaa (2020). Post-Crash Fire Forensic Analysis of Aerospace Composites. Master's thesis, Texas A&M University. Available electronically from https : / /hdl .handle .net /1969 .1 /192856.