Failure analysis of aluminum structural members on the space shuttle Columbia

Various aluminum parts, both castings and rolled sheet, were found among the Columbia debris. At first glance, these parts seemed to have failed in a brittlelike manner, contrary to normal aluminum fracture behavior. Light optical microscopy on the etched aluminum samples examined in this study revealed second-phase particles along the grain boundaries. It was also observed that at the elevated temperatures encountered during re-entry, some of the particles melted and wetted the grain boundaries. Large voids could be found between the grains near the crack tips. Energy-dispersive spectroscopy showed particles that were rich in iron. During re-entry of the shuttle, elevated temperatures caused melting of the second-phase particles, allowing them to wet the grain boundaries, a process called liquation. Cracks initiated and propagated through the grain boundaries, thus resulting in intergranular rupture that had the macroscopic characteristics of brittle failure of the aluminum.