A FATIGUE ASSESSMENT METHOD BASED ON WELD STRESSES

In this paper, a method for extending the applicability of structural stress method for fatigue assessment of welded structures is discussed. The structural stress method for plate structures, as currently presented in commonly used design guidance documents, cannot account the effect of weld size and load carrying fillet welds are assessed using a different S-N curve as compared to non-load carrying fillet welds. The current proposal is to linearize the local stress distribution through the plate thickness in the plane of the weld toe and, thus, partially capture the local effect. A bilinear stress distribution is derived from the actual non-linear stress distribution based on equilibrium. A simple procedure is presented to determine the bilinear curve from the nominal weld stress. This is a great advantage in finite element analysis when only nominal base plate stress and nominal weld stresses need to be determined. The same weld stresses can be used also in the analysis of the root cracks. The proposed method was scaled to correspond to the traditional structural stress method using detailed linear elastic fracture mechanics simulations. The method is here applied only to fully load-carrying welds but can also be used for partial load-carrying welds. A symmetric splice plate having a fully loaded fillet weld is presented as an example case. The influence of base plate thickness is also studied.