Influence of effective stress ratio on the fatigue strength of welded and HFMI-treated high-strength steel joints

Abstract This paper contributes to quantify the influence of the local mean stress condition and the local weld geometry on the high-cycle fatigue strength of welded and high frequency mechanical impact (HFMI) treated S960 high-strength steel joints. A calculation procedure is presented, which bases on the effective stress ratio at the weld toe considering the local residual stress condition as well as the specimen clamping-induced and load-dependent notch mean stress state to assess a final influence fatigue factor. Fatigue tests incorporating butt joint, T-joint, and longitudinal stiffener specimens present the effect of different welding parameters and the HFMI technique as post-treatment method. The results show that firstly, by an application of optimized welding parameters, such as filler material and shielding gas, an increase in fatigue strength of 7% for the butt joint and of 30% for the T-joint specimens is experimentally observed. Secondly, it is shown that all investigated specimen types reveal a significant increase in fatigue strength due to the HFMI-treatment up to an enhancement of factor 2.46 in case of the longitudinal stiffener specimens. A study on the effect of stress-relief annealing as post-weld heat treatment (PWHT) process highlights a decrease of about 25% in the high-cycle fatigue strength in case of HFMI-treated T-joint specimens. Applying the introduced calculation method, an effective stress ratio of Reff = 0.05 for the butt joint and T-joint specimens, and Reff = −0.81 for the longitudinal stiffener specimens in the as-welded condition is calculated. Comparing the computed fatigue factors with the statistically evaluated nominal fatigue strengths it is observed that the effect by the effective stress ratio is dominant, but not fully covers the fatigue enhancement. Furthermore, it reveals that the influence by the effective stress ratio may be related to the fundamental change in the local residual stress state. Especially the HFMI-treatment leads to a reduction of the local residual stress condition up to a difference of 650 MPa for the S960 high-strength steel, which significantly contributes to this influence.