United States Department of Transportation
i
Post-Small Flame Forensic Analysis of Aerospace Composites
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2024-01-01
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[PDF-12.04 MB]
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Alternative Title:Post Small-Flame Forensic Analysis of Aerospace Composites
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Edition:Final Report
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Abstract:Fire can dramatically alter the exposed surfaces of composite aircraft structures in ways that inhibit forensic analysis. The char, soot, and several other fire by-products often mask relevant aspects of the structural damage morphology, impeding identification of the underlying failure mechanisms of composite materials. In this report, the effects of small-flame direct fire exposure and efficacy of char removal techniques on mechanically failed Cytec T40-800/Cycom® 5215 graphite/epoxy and pristine Hexcel® SGP370-8H/8552 carbon/epoxy specimens were examined. Small-flame vertical and horizontal Bunsen burner fire tests were performed on mechanically failed unnotched compression, short beam strength, and in-plane shear graphite/epoxy specimens. Vertical burn tests were conducted on pristine carbon/epoxy specimens for initial char removal assessment. Both visual inspection and scanning electron microscopy (SEM) were used to characterize the fracture surface morphology of the mechanically failed specimens, investigate the induced thermal damage due to fire exposure, and assess the effectiveness of the char removal techniques. The fire damage consisted of matrix decomposition, melt dripping, char and soot deposition, 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. Char removal approaches, including ultrasonication, chemical solvent soaking, liquid nitrogen dipping, and a combination of thermal cycling and ultrasonication, were explored, resulting in partial char removal. This research is intended to be related work to the Composite Failure Analysis Handbook, DOT/FAA/AR-91/23 | WL-TI-91-4032 published through a joint research effort by the U.S. Air Force at Wright-Patterson AFB and the FAA. It should be noted these tests were conducted using a small methane flame, rather than a large pool fire, and therefore conclusions pertaining to aircraft accidents involving fire may not be directly applicable. The next phase for this project will address the fuel type and flame intensity to more accurately represent realistic accident conditions.
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