Fatigue strength assessment of ultra-high-strength steel fillet weld joints using 4R method

Abstract

A fatigue analysis on experimentally tested transverse fillet-welded non-load-carrying T- and X-joints made of S960 and S1100 ultra-high-strength steel was carried out in the present study. The test data consisted of welded joints in the as-welded, high frequency mechanical impact-treated, and tungsten inert gas-dressed conditions, that were fatigue tested using uniaxial constant amplitude loading with an applied stress ratio of R = 0.1–0.5. The weld geometry and residual stress measurements were carried out, and for each joint, the fatigue strength was assessed using a multiparametric notch stress approach, entitled the 4R method, which incorporates the consideration of four parameters, i.e. notch stress range Δσk(r), applied stress ratio R, material ultimate strength Rm, and residual stress σres in the fatigue assessment. In the 4R method, the local cyclic elastic-plastic behavior at the notch root is obtained, and the Smith-Watson-Topper parameter is applied to conduct a mean stress correction to commeasure all results into a single S-N curve. The results showed that the applied stress ratio had a distinct influence on the fatigue strength capacity for both joints in the as-welded and post-weld treated conditions when using the conventional stress-based approaches, i.e. nominal stress, structural hot-spot stress and effective notch stress concepts. Nevertheless, the 4R method resulted in a good agreement between the experimental test results and the fatigue strength assessments, regardless of the load and joint conditions.