Microstructure and Strain-Based Fatigue Life Approach for High-Performance Welds

Abstract

Microstructure and pre-existing surface flaws in smooth notch geometries significantly affect the fatigue life of welded joints. Traditionally, a welded joint is assumed to incorporate crack-like defects and the crack propagation dominates the total fatigue life. For a smooth weld notch geometry, the macro crack initiation period becomes more significant, and this difference cannot be modelled with the existing stress or fracture mechanics ‑based approaches. In this paper, a microstructure and strain ‑based fatigue life approach is presented. In the approach, the fatigue damage process is modelled as a repeated crack initiation process within a material volume related to the microstructure. The novelty of the developed approach is that the size of the damage zone is defined from the grain size statistics without using fracture mechanics. The approach is able to consider the changes in the stress gradient, stress triaxiality and plasticity during the fatigue crack initiation and growth. The developed approach has been validated with experiments on submerged-arc and laser-hybrid welded joints. The predicted fatigue life, crack growth path and rate showed good agreement with the experiments. For a welded joint with smooth and favourable notch shape, the short crack growth, i.e. macro crack initiation period is dominant and it has a significant influence on the fatigue life.