Investigation of Material Response to Atmospheric Re-entry Exposure for Sub-structural Ti-6Al-4V, Inconel 718, A-286 and Al 2024 Components Recovered from the Space Shuttle Columbia Accident
Investigations of Space Shuttle Columbia debris hold great relevance for understanding material response to re-entry environment in the case of thermal protection system (TPS) compromise. The Xo 582 ring-frame bulkhead and attached payload bay door (PLBD) rollers exhibited highly localized thermal degradation and material loss. This research presents examination of these components that provided a unique opportunity to examine multiple alloy systems located in close proximity of one another. These artifacts were of great interest due to their possible involvement in the thermal event responsible for the char layer deposition discovered on overhead windows. Located adjacent to the PLBD roller and bulkhead, post-accident investigation of the char-layer deposition on overhead windows revealed indications of unexpected material response. Inconsistent with the relative melting temperatures of titanium and aluminum, it was observed that titanium melted and deposited onto the windows before aluminum. The unusual arrangement of the char layer was hypothesized to have occurred due to localized elevated heating and deposition of titanium due to shock-shock interaction or titanium combustion. The latter hypothesis takes central focus of the PLBD roller investigation, as the ignition and combustion response of titanium in a re-entry environment was previously investigated by arc-jet testing of Ti-6Al-4V plates in simulated low-pressure, high-enthalpy conditions. Researchers observed and classified titanium reactivity, identifying discriminatory behavior among the reacting specimens. Analysis of the PLBD roller revealed features suggestive of titanium combustion and possessed microstructural similarities to the arc-jet specimens that reacted more aggressively to the simulated environment. Observation of the ring-frame bulkhead, and other aluminum debris recovered from the Columbia accident, revealed unique delamination features on fracture surfaces. The unique nature of the fracture surface, termed broom-straw fracture, is not broadly described in literature. Examination of the bulkhead substantiated and expanded upon the previous notions that broom-straw fracture is the result of grain boundary liquation and the influence of microstructure and thermo-mechanical processing was postulated. The findings included in this dissertation, along with other post-accident analyses, further demonstrate the critical need for a comprehensive characterization of behavioral properties for reactive aerospace materials in the high-temperature, low-O2, low-pressure re-entry environment.
Ochoa, Ngozi Chinoyerem Ubani, "Investigation of Material Response to Atmospheric Re-entry Exposure for Sub-structural Ti-6Al-4V, Inconel 718, A-286 and Al 2024 Components Recovered from the Space Shuttle Columbia Accident" (2018). ETD Collection for University of Texas, El Paso. AAI10815938.