Experimental investigation on the fatigue strength assessment of welded joints made of S1100 ultra-high-strength steel in as-welded and post-weld treated condition

In this paper, a fatigue test program including 24 tested specimens made of S1100 steel was carried out to investigate the fatigue performance of the ultra-high-strength steel (UHSS) welded joints in the as-welded (AW) and post-weld treated (PWT) condition. The fatigue test series consisted of butt joints, load-carrying cruciform joints, and non-load-carrying T-joints. Both high frequency mechanical impact (HFMI) treatment and tungsten inert gas (TIG) dressing were considered as PWT methods in the non-load carrying joints. Fatigue strength was assessed using the conventional stress-based methods, i.e. nominal stress, structural hot spot (HS) stress and effective notch stress (ENS) approaches, and using a novel multi-parametric fatigue design approach, named as the 4R method. The 4R method considers a mean stress level of cyclic loading, including material ultimate strength (R_m ), weld toe quality in terms of toe radius (r_true ), residual stress (σ_res ), and applied stress ratio (R) in the elastic-plastic analysis at the notch root. The results showed a good correspondence between the experimental results and computed fatigue lives when using the 4R method. The 4R method with the local mean stress correction adopts a single S-N curve for all welded specimens regardless of joint or load configuration, resulting in a high accuracy of fatigue strength assessment in these tests. The conventional methods showed conservative results in R = 0.1 loading, and unconservative results in R = 0.5 loading, respectively.