The Peak Stress Method applied to fatigue lifetime estimation of welded steel joints under variable amplitude multiaxial local stresses

Fatigue actions of most engineering welded structures involve variable amplitude (VA) loading cycles. The fatigue design of welded joints in such structures is based on the use of a fatigue strength criterion for constant amplitude (CA) loading in conjunction with a cumulative damage rule. In the present work, the Peak Stress Method (PSM), has been reformulated to estimate the lifetime of welded joints subjected to VA loadings. The PSM is an engineering finite element (FE)-oriented technique to rapidly estimate the mode I, II, and III notch stress intensity factors (NSIFs) at the weld toe or at the weld root, by taking advantage of the singular linear elastic, opening, sliding, and tearing peak stresses, respectively. Such stresses are computed at the V-notch tip by means of 2D or 3D FE models discretized using coarse meshes, provided that some conditions of applicability of the method are satisfied. The fatigue strength under CA loading is then evaluated by combining the simplicity and rapidity of the PSM in evaluating the NSIFs with a robust and validated fatigue strength criterion such as the one based on the averaged Strain Energy Density (SED), which can be written as a function of the relevant NSIFs. To preserve the simplicity of the method, its extension to VA loading conditions has been achieved by assuming the Miner's Linear Damage Rule (LDR) as a cumulative damage rule. The proposed method has been validated against new experimental results generated by testing non-load-carrying (nlc) fillet-welded joints with double inclined attachment and made of structural steel under pure axial loading.