Enhanced strain-based fatigue methodology for high strength aluminum alloys

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Virginia Tech


The design of any mechanical components requires an understanding of the general statical, dynamical and environmental conditions where the components will be operating to give a satisfactory results in terms of performance and endurance. The premature failure of any components is undesirable and potentially catastrophic, therefore predictions on performances and endurances of components to proceed with repair or substitution is vital to the stability of the structure where the component is inserted. The capability of a component of withstanding fatigue loading conditions during service is called fatigue life and the designed predictions can be conservative or non conservative.

Improvements to a strain based approach to fatigue were obtained in this study, studying the effects of mean stresses on fatigue life and investigating cyclic mean stress relaxation of two aluminum alloys, 7075-T6511 and 7249-T76511, used in structural aircraft applications. The two aluminum alloys were tested and their fatigue behavior characterized. The project, entirely funded by NAVAIR, Naval Air Systems Command, and jointly coordinated with TDA, Technical Data Analysis Inc., was aimed to obtain fatigue data for both aluminum alloys, with particular interest in 7249 alloy because of its enhanced corrosion resistance, and to give guidelines for improving the performances of FAMS, Fatigue Analysis of Metallic Structures, a life prediction software from the point of view of both mean stress effects and mean stress relaxation.

The sensitivity of engineering materials to mean stresses is of high relevance in a strain based fatigue approach. The performance of the most common models used to calculate mean stress correction factors was studied for the two aluminum alloys 7075 and 7249 to give guidelines in the use of those for life predictions. Not only mean stresses have a high influence on fatigue life, but they are also subjected to transient cyclic behaviors. The following study considered both an empirical approach and a plasticity theory approach to simulate and include these transient effects in life calculations. Results will give valid directions to a successful modification of FAMS like any other life calculation software to include in the picture transient phenomena.



variable amplitude loading, relaxation, mean stress, strain-based approach, Fatigue, Aluminum alloys