Cracking Resistance in a 4.5-Inch (114-mm) Plate of 7075-T7351 Aluminum Alloy

摘要

After an anodizing treatment, peculiarities were observed in the appearance of structural components for the Space Shuttle, thought to be indicative of flaws in the Al-7075-T7351 material. Consequently, work was undertaken to explore structural integrity aspects of this material as well as the nature of any material defects. The material contains a number of macro- and microstructural inhomogeneities which could be classed as defects or potential crack-like defects. viz. stringers of brittle constituent particles, shrinkage cavities, and in one instance, a long line of sponge-like porosity. Nonetheless, uniaxial strength level was found to exceed design requirements. Similarly, results for cracking resistance- in terms of fracture toughness, fatigue crack growth rates (DA/DN) and rotating-beam fatigue life (S/N), appear to meet or exceed handbook levels. However, the very largest defects are so infrequently observed that their presence in the crack-tip process zone of an individual fracture mechanics specimen is something less than likely. The unusual macroappearance of anodized components may be attributable to entrapment of anodizing solution by microshrinkage cavities and/or microcracks along constituent particle stringer interfaces, with subsequent seepage and staining adjacent to these and/or other flaws.