Fatigue of 7075-T651 aluminum alloy

Extensive fatigue experiments were conducted using 7075-T651 aluminum alloy under uniaxial, torsion, and axial-torsion loading. Detailed fatigue results were reported. Different mean stresses were applied in the experiments and the mean stress was found to have a significant influence on the fatigue strength of the material. A tensile mean stress decreased the fatigue strength dramatically. Fatigue damage was found to occur under compression–compression loading. In addition, axial-torsion experiments using tubular specimens were conducted under different loading paths to study the multiaxial fatigue behavior. Fatigue cracking behavior was found to be dependent on the loading path as well as the loading magnitude. When the loading magnitude was high, the material displayed shear cracking. When the loading stress was below a certain level, the material exhibited tensile cracking. For most loading cases under investigation, the material displayed a mixed cracking behavior. A kink was found in the shear strain versus fatigue life curve from the pure torsion experiments, and it was associated with a distinctive transition of cracking behavior. The Smith–Watson–Topper (SWT) parameter with a critical plane interpretation was found to correlate well with most of the experiments conducted in terms of fatigue lives. However, the SWT parameter cannot deal with the uniaxial fatigue conditions where the maximum stress is low or negative. More importantly, the model fails to correctly predict the cracking behavior observed experimentally on the material. A critical plane criterion based on a combination of the normal and shear components of the stresses and strains on material planes was found to better correlate the fatigue experiments in terms of both fatigue life and cracking behavior. The characteristics of the multiaxial fatigue criterion were discussed based on the experimental observations on 7075-T651 aluminum alloy.