Proposal for a more accurate physically based S–N curve for welded steel joints

The present article proposes a more accurate S–N curve in the high cycle fatigue regime for fillet welded joints in steel subjected to constant amplitude loading. The S–N curves are constructed based on a physical model of the fatigue damage evolution. It is a two phase model where the crack initiation is treated by a local weld notch approach. The subsequent growth is based on the concept of the stress intensity factor at the crack front by applying the Paris law. According to the proposed model, the time to crack initiation becomes the dominant part of the fatigue life at low stress range levels. The resulting S–N curves are non-linear for a log–log scale and they do not predict any fatigue limit. The curves change slope gradually and the stress range continue to decrease with increasing number of cycles. The curves fit collected life data far better than the conventional bi-linear curves found in rules and regulations. The curves predict considerably longer fatigue lives at low stresses than the conventional curves do, despite the non-existence of a fatigue limit.