Fatigue Master Curve Approach for Arc-Welded Aluminium Joints: Mean Stress Effects

Mean stress affects the crack propagation and fatigue performance of arc-welded joints. However, it is a tough phenomenon because of a complex combination of properties in the alternating material zones: weld material, Heat Affected Zone (HAZ) material and base material. First, modeling steps from weld notch stress distributions to weld stress intensity factors, to a non-similitude two-stage crack propagation model, to a fatigue master curve formulation are summarized. Focusing on base and HAZ material, Walker’s mean stress model is adopted as a result of a concise review and superior results shown in literature. However, its model coefficient γ is determined using a rational approach rather than curve fitting and a micro- and macro-crack propagation effect is distinguished. Subsequently, for base material, the crack propagation model is modified to incorporate loading induced mean stress effects. Validation using experimental crack propagation data shows promising results. In the HAZ, except loading induced mean stress, the welding process induced residual stress acts as high-tensile mean stress as well. The latter dominates the former in the micro-crack propagation region. Fatigue performance improvement, e.g. a result of Ultrasonic Impact Treatment (UIT), that reduce the high-tensile mean stress is included correcting the loading induced macro-crack propagation mean stress parameter. Finally, the fatigue master curve formulation is modified accordingly and mean stress effects in the HAZ are satisfactorily validated using weld toe failure fatigue test data, including some UIT results.