Finite life fatigue strength
The region of the S-N curve where a component can survive only a limited number of cycles before failure. It describes strength under stresses higher than the endurance limit.
Every load cycle deforms the material at stress peaks. At notches, pores, or slag inclusions, stresses concentrate, and after many cycles tiny cracks form there. They grow a bit with each load cycle until the remaining cross-section becomes too weak and the component suddenly breaks. We call that fatigue, and the finite life fatigue strength describes exactly this crack growth region. When welding, you always have notches: the transitions between weld and base material, the root, and undercuts act like crack starters. Add the coarse-grained or martensitic weld structure and the residual stresses from welding, which act as additional mean stress. That's why a welded joint under alternating load withstands significantly less than the base material. In the workshop, that means: grind weld transitions flat, work out the root, and apply a back weld. Grind away undercuts. Shot peening or hammering the weld transitions introduces compressive residual stresses and helps enormously. For thick plates, preheat, maintain interpass temperature, and for parts under fatigue loading, stress-relieve anneal. And for dynamically loaded structures, you must limit permissible stresses according to the codes.