Fatigue of aluminum alloy 7075-T651

Fatigue of 7075-T651 aluminum alloy was systematically investigated. 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 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 parameter 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.;Crack propagation experiments were conducted on both standard and nonstandard compact specimens. The crack growth behavior of 7075-T651 aluminum alloy was experimentally studied in ambient air. The effects of the stress ratio ( R), overloading, underloading, and high-low sequence loading on fatigue crack growth rate were investigated. Significant R-ratio effect was identified. At the same R-ratio, the influence of specimen geometry on the relationship between crack growth rate and stress intensity factor range was insignificant. A single overload retarded the crack growth rate significantly. A slight acceleration of crack growth rate was identified after a single underload. The crack growth rate resumed after the crack propagated out of the influencing plastic zone created by the overload or underload. A parameter combining the stress intensity factor range and the maximum stress intensity factor can correlate the crack growth at different stress ratios well when the R-ratio ranged from -2 to 0.75. The parameter multiplied by a correction factor can be used to predict the crack growth with the influence of the R-ratio, overloading, underloading, and high-low sequence loading. Wheeler's model cannot describe the variation of fatigue crack growth with the crack length being in the overload influencing zone. A modified Wheeler's model based on the evolution of the remaining affected plastic zone was found to predict well the influence of the overload and sequence loading on the crack growth.