Paul E. Magnusen

Analysis and prediction of microstructural effects on long-term fatigue performance of an aluminum aerospace alloy

Abstract A program of experimental and analytical tasks has been conducted to define the linkage(s) between microstructural characteristics and fatigue performance in an aluminum alloy typically used for airframe structural applications. The first goal was to develop data for quantitatively linking measurable characteristics of material microstructure with long-term fatigue performance. The second goal was to develop models to predict fatigue performance based on the microstructural characteristics. The work focused on several process variants of aluminum alloy 7050-T7451 plate. This material was chosen because of its widespread use for flight-critical airframe structural components, and the particular characteristics associated with the manufacturing, service and maintenance of thick section components. Within the framework of this objective, life-limiting microstructural features have been identified and ranked by severity, and models to quantitatively describe the evolution and growth of macrostructural cracks from those features have been developed. The modeling framework has been applied to predict the cyclic lifetime of the 7050 alloy process variants based on the populations of life-limiting microstructural features. In addition, the models have been used to show how changes in the material characteristics may affect the fatigue performance. This includes predictions of the effect of changing the life-limiting microfeature size and shape distributions, and the effect of changing material strength properties. The use of this modeling approach to probabilistically describe the implications of changes in the microstructure has been demonstrated, thereby allowing the effects of material pedigree to be predictively linked with the structural integrity of end components. The modeling framework has potential applications in airframe design support processes, and as a tool for use in material and product form selection processes.

Development of High Toughness Sheet and Extruded Products for Airplane Fuselage Structures

High specific ultimate strength and high plane stress fracture toughness are primary requirements of aircraft fuselage skins. The performance of alloys/products used in high performance fuselage applications is first reviewed. The specific fracture toughness for products such as 2017-T3, 2024-T3, 2524-T3 and 6013-T6, is discussed as a function of their composition and microstructure. Then the performance of modern Al-Li alloys/products such as 2199 and 2060 sheet and 2099 and 2055 extrusions is examined. It is concluded that the performance of Li containing alloys/products offer significant improvements over non-Li containing conventional fuselage products because of the optimization of strengthening precipitates and grain microstructures. The role of chemical composition on resulting microstructures is discussed.

The influence of material quality on airframe structural durability

Previous work on Al-Zn-Cu-Mg alloy 7050-T7451 has shown that improved processing to reduce the size of microporosity results in longer fatigue lifetimes. To differentiate this improvement, Alcoa has implemented smooth specimen fatigue testing on a lot release basis to warranty 7050-T7451 thick (5–6 in., 127–153 mm) plate initial fatigue quality. The present study employs a probabilistic fracture mechanics analysis to demonstrate that the material quality improvement verified in the smooth specimen fatigue tests translates to durability performance in actual structures. Microflaw size distributions measured from failed smooth coupon tests serve as the starting point for probabilistic crack growth analysis. Using crack exceedance probability as a basis for durability performance comparison, a hypothetical calculation shows that a fighter aircraft wing fabricated from improved quality metal outperforms the same wing fabricated from unimproved metal.