Investigation of broom-straw fracture behavior of aluminum alloy 2024 debris recovered from Space Shuttle Columbia

Abstract

Materials investigations of Space Shuttle Columbia debris hold great relevance for understanding materials response to atmospheric re-entry. Sections of the Xo 582 ring-frame bulkhead, recovered from the Columbia accident, exhibited highly localized thermal degradation and material loss. Unique delamination features were observed on fracture surfaces of the bulkhead, as well as on other aluminum debris. The unique nature of the fracture surfaces, termed broom-straw, is not a well described phenomenon, and takes central focus of this research. This paper presents a forensic materials characterization of the bulkhead that substantiates and expands upon the previous notions that broom-straw fracture is the result of grain boundary liquation. Observed features were consistent with material exposure to elevated temperatures and subsequent material property degradation, followed by mechanical overload. This research attempts to advance the understanding of broom-straw fracture and determine the influence of microstructure and thermo-mechanical processing on the broom-straw fracture mechanism.