Fatigue behavior of load-carrying cruciform fillet weld joints under variable amplitude load

The objective of the current work is to study the mean stress effect of variable amplitude (VA) loading on the fatigue strength of load-carrying fillet-welded joints (LCFWJs). Experimental fatigue tests were carried out on LCFWJs made of S700 high-strength steel, considering constant amplitude (CA) and VA loads. In the case of VA loads, a Gaussian load spectrum shape with both constant minimum and maximum stress levels was considered to study varying mean stress conditions. Numerical analyses were conducted to determine the effective stresses at the weld root. The fatigue behavior of LCFWJs was evaluated using the 4R method, which takes into account the local stress ratio (Rlocal, i), material ultimate strength (Rm), residual stress (σres), and weld toe radius (r). Lower fatigue strength was found for the VA tests with the constant maximum stress tests compared to the CA tests when using damage parameter of D = 1.00, while the VA tests with the constant minimum stress tests showed higher fatigue performance. Nonetheless, employing damage parameters of D = 0.2 and D = 0.5 for variable amplitude loads with and without fluctuating mean stress conditions, respectively, provided conservative assessments compared to the design standards. By employing the 4R method, the precision of the assessments could be significantly improved, and the CA and VA fatigue test results fit into a single scatter band with scatter index of Tσ = 1.06 with damage sum of D = 1.00.