Probabilistic fatigue-life assessment model for laser-welded Ti-6Al-4V butt joints in the high-cycle fatigue regime

The present paper focuses on the effect of inherent welding-induced defects on the high-cycle fatigue behaviour of laser-welded Ti-6Al-4V butt joints. The transition of the crack origin from the surface to the subsurface occurs upon removal of the surface stress concentrators. Under these circumstances, fatigue cracks nucleate at subsurface porosity and show a typical fish-eye pattern of fracture surface. A fatigue-life assessment model has been developed for internally flawed materials based on a fracture-mechanics approach, which takes effects of short-cracks into account. A novel approach for the simplified construction of the cyclic resistance curve of internal cracks is proposed herein. Using statistical methods, the experimentally determined porosity distribution has been incorporated into the model to predict the fatigue scatter range. The presented methodology can potentially be used to achieve a required reliability of the welded joints with respect to fatigue as a design criterion.