Role of Microstructure on the Fatigue Durability of Aluminum Aircraft Alloys

Abstract : Although the role of microstructure on the initiation and growth of fatigue cracks, and thus the fatigue performance of a structural metal, has long been recognized, quantitative models of the linkage(s) between microstructural characteristics and structural fatigue performance have not been widely available. The absence of such models, coupled with the compositional basis of material specifications, has limited the ability to exploit improvements in material performance which have been generated by microstructural consequences of manufacturing process improvements. The potential performance benefits of quality-improved materials have thus remained inaccessible to airframers, who lack the means of specifying such materials or characterizing their merits without committing massive testing resources. Opportunities for reducing airframe weight and/or cost have thus been lost. A program of experimental and analytical tasks has been conducted to develop a path for defining the linkage(s) between microstructural characteristics and fatigue performance in an aluminum alloy typically used for airframe structural applications. Within the framework of this overall objective, life-limiting microstructural features have been classified and ranked by severity, and models to quantitatively describe the evolution and growth of macrostructural cracks from those features have been developed. The use of these models to probabilistically describe the structural implications of different levels of microstructural quality has been demonstrated, thereby allowing the efforts of material pedigree to the predictively linked with the structural integrity of end components. The focus of this work was on several process variants of aluminum alloy 7050-T7451 thick plate.