Modeling mean stress relaxation in variable amplitude loading for 7075-T6511 and 7249-T76511 high strength aluminum alloys

Mean stresses in fatigue life calculations for critical structural components are a source of great uncertainty in any engineering design. Correction of life predictions in strain or stress based approaches to fatigue are necessary when dealing with mean stresses; the Goodman, Morrow, Smith–Watson–Topper and Walker models are widely used mean stress models for mean stress correction in life calculation. Aluminum alloys 7075-T6511 and 7249-T76511 are tested in fatigue. Relaxation tests are performed with the intent of characterizing the cyclic relaxation of mean stresses for variable amplitude loading when loading conditions approach the irregularity of service loadings. Several plasticity models are studied to reproduce the observed behavior, and simulations are compared to identify the best candidate for implementation in a life prediction software. An incremental plasticity model introduced by Jiang and Sehitoglu in 1996 is fitted to test data for both alloys to obtain all the model parameters. Constant and variable amplitude loading simulations are run to compare the results with experimental data. A multisurface plasticity model developed by Wetzel in 1971 is also implemented to simulate material behavior. Then modifications are suggested on the model formulation so that the mean stress relaxation response is closer to the experimental data. Life calculations for the variable amplitude loading patterns are performed without considering relaxation effects, and with the use of the plasticity models described, to compare the advantages of introducing transient effects in the analysis.