Fatigue assessment of as-welded and HFMI-treated high-strength steel joints under variable amplitude loading using local approaches

Among various post-weld treatment techniques, the high-frequency mechanical impact (HFMI) treatment is a robust technique to introduce an enhancement in the fatigue strength of welded high-strength steel joints. In engineering applications, however, structures usually undergo fluctuating cyclic load conditions resulting in variable amplitude load (VAL) conditions. The objective of this work is to validate the use of local notch stress approaches to assess the fatigue strength of as-welded (AW) and HFMI-treated joints made of high-strength and ultra-high-strength steels under VAL conditions. The 4R method is applied as an effective stress-based concept considering mean stress correction via four parameters, i.e., material strength R_m, applied stress ratio R, residual stresses σ_res, and weld toe radius r_true. To validate the 4R method, the study extracts and re-analyzes fatigue test data of published literature on CAL and VAL studies. Finite element analyses are carried out to determine effective notch stresses (ENS) for the evaluation of the data using the ENS concept and 4R method. The results of the re-evaluation conducted using the ENS method showed that the existing design curves provide conservative assessments. However, applying equivalent stress concept for VAL data, the experimental data showed damage parameters of D > 1.0 for joints in the AW conditions with respect to the CAL data. The fatigue data was analyzed with the 4R Master Curve, which obtained conservative assessments for both joints in the AW and HFMI-treated conditions, as well as CAL and VAL data and, thus applicability of the 4R method for the fatigue assessments of welded joints was thus further validated by the experimental data.