Multiaxial variable amplitude fatigue life analysis including notch effects

This paper evaluates the methodology for, and effectiveness of, common equivalent stress- and strain-based fatigue life analysis approaches when applied to variable amplitude multiaxial service loading conditions. Experimental results for both un-notched and notched specimens of 2024-T3 aluminum alloy, tested under axial, torsion, and combined axial–torsion loadings, are compared to predictions based on von Mises equivalent stress and strain. Mean stresses were considered using Smith–Watson–Topper, modified Goodman, and modified Morrow models. Variable amplitude life predictions are compared to constant amplitude predictions to highlight similarities and differences in life prediction trends. Overall, mixed results were obtained. Both constant and variable amplitude fatigue lives are predicted well for notched specimens, where a local uniaxial stress state always exists for the geometry considered. However, for un-notched specimens, where multiaxial loading effects come into play, no approach considered is able to predict more than 60% of fatigue data within a factor of ±3 of experimental life. A consistent trend of non-conservative life predictions for variable amplitude loading conditions is observed. The importance of considering stress gradient effects in notched specimen life prediction is also demonstrated. An extensive discussion on areas where improvements can be made with respect to fatigue damage quantification completes the paper