Theses and Dissertations
ORCID
https://orcid.org/0009-0004-5877-5789
Advisor
Priddy,Matthew W.
Committee Member
Kundu, Santanu
Committee Member
Pittman Jr., Charles
Committee Member
Lacy Jr., Thomas
Committee Member
Smith, Dennis
Date of Degree
5-16-2025
Original embargo terms
Embargo 2 years
Document Type
Dissertation - Open Access
Major
Engineering (Mechanical Engineering)
Degree Name
Doctor of Philosophy (Ph.D.)
College
James Worth Bagley College of Engineering
Department
Michael W. Hall School of Mechanical Engineering
Abstract
Post-crash fires often impede accident reconstruction since the telltale fractographic evidence is obscured by combustion residues or the fires incinerate them. The aim of this work is to develop a treatment protocol for post-fire char removal while preserving the pre-existing mechanical failure features of carbon/epoxy composites. Preliminary tests explored ultrasonication in commonly used organic solvents and HNO3/H2SO4 solutions (in ambient conditions). Mechanically undamaged carbon/epoxy specimens were burned to produce charring. Since heated HNO3/H2SO4 solutions were more capable of removing char through oxidation, a direct heating process was adopted in succeeding experiments. In addition, char removal was attempted on mechanically failed graphite/epoxy composites after small flame burning. These following process parameters were varied: HNO3/H2SO4/H2O content, solution temperature, and immersion duration. Scanning electron microscopy was used for fire damage characterization and to examine composite surfaces after char removal. Irrespective of the high H2SO4 (~ 95.26 wt.%) content, the fiber surfaces remain obscured due to incomplete oxidation at low immersion times (t ≤ 1 min) in acid solutions with low HNO3 content (≤ 0.88 wt.%) when the temperature was maintained at ≤ 100 °C. In contrast, fracture surface degradation occurred when temperature and immersion time were high (T ≥170 °C and t ≥10 min) especially for higher HNO3 content (≥ 9.16 wt.%) solutions with reduced (but still high) H2SO4 content (~ 84 wt.%). However, optimal results were achieved with solutions of 3.12–8.07 wt.% HNO3 at 125–135 °C. The directly attributable failure features (DAFFs) for compression and flexure related failures were recovered minimal degradation from burned specimens before 5 min of immersion. The elevated temperatures of 125-135 °C combined with low water activity and high H2SO4 (≥ 85.21 wt.%) result in these exceptionally powerful oxidations of carbonaceous residues with limited fiber degradation. Structural scale experiments on PRSEUS open-hole specimens that were failed in tension established that HNO3/H2SO4 oxidations can eliminate combustion residues while preserving the underlying critical fiber-related fracture surfaces. The effectiveness of this surface treatment protocol in removing combustion residues represents a pivotal advancement in forensic methodologies for post-crash investigations of composite aircraft structures exposed to fuel-fed fires.
Sponsorship (Optional)
Federal Aviation Administration
Recommended Citation
Madabhushi, Abhijith, "Forensic analysis of fire-damaged aerospace carbon and graphite/epoxy composites" (2025). Theses and Dissertations. 6530.
https://scholarsjunction.msstate.edu/td/6530
