Fatigue lifetime assessment of as-welded and HFMI treated lean duplex steel butt joints subjected to constant and variable amplitude axial loading according to the peak stress method

High-Frequency Mechanical Impact (HFMI) is widely adopted as a post-weld treatment to increase fatigue strength, as it produces a beneficial compressive residual stress state at the weld toe, promotes surface work hardening, and smooths the local geometry by enlarging the weld toe radius. The Peak Stress Method (PSM) proved reliable for fatigue assessment of as-welded steel joints subjected to constant amplitude (CA) as well as variable amplitude (VA) uniaxial or multiaxial loading conditions. However, for HFMI treated joints, the recent PSM applications have addressed only CA uniaxial loading with positive load ratios. In this study, the transferability of the existing PSM framework is assessed for HFMI treated joints subjected to uniaxial loadings including both negative load ratios and VA cases. To this aim, new experimental fatigue results were generated on full-penetration butt joints made of lean duplex steel, tested both in the as-welded condition and after HFMI treatment. Fatigue tests were carried out under uniaxial CA loadings at nominal load ratios of R = 0.1 and R = −1 and VA loadings at R = −1. The combined impact of HFMI treatment, load ratio, and residual stresses on fatigue response were investigated. The experimental results were subsequently analysed by the PSM approach and assessed against the relevant fatigue design curves. The results are consistent with the curves, indicating that PSM provides an effective basis for fatigue life prediction of HFMI-treated duplex steel joints under both CA and VA loading conditions.