According to the IIW recommendation, the fatigue strength of welded steel joints is defined as independent of the base material in case of the as-welded condition. However, post-treatment techniques can improve the fatigue performance of welded structures, especially for increased base material strengths. Therefore, this paper investigates the effect of TIG dressing, as common post-weld treatment method, on the fatigue strength of high-strength steel S700 cruciform joints. The statistically evaluated fatigue test results reveal a significant increase of the nominal fatigue strength from FAT 90 for the as-welded up to 182 MPa for the TIG-dressed state. The experiments are further compared to recommended and suggested design curves applying both nominal as well as local stress approaches. Focusing on the TIG-dressed state, the suggested increase in nominal stresses is well validated leading to a conservative assessment. In addition, the proposed slope in the finite life region with a value of m₁ = 4 shows a sound fit to the statistically evaluated value of m₁ = 4.7 for the test results. The local fatigue strength estimation is performed based on a recent proposal using the theory of critical distances. Therefore, linear elastic numerical analysis of the investigated specimens is performed. Again, the resulting S-N curves agree well to the experiments validating the proposed local design approach.

The focus of this work is to investigate the fatigue strength of a cruciform joint one-sided through-welded by a HV seam with a backing layer made of a 10-mm-thick S355 mild steel. The aim is firstly to analyse the effect of the deformation state incorporating angular distortion and axial misalignment on the fatigue strength of the tested specimens and secondly, the analysis of the benefit by grinding as post-treatment method on the resulting fatigue performance. The deformation impact is numerically investigated utilizing geometry measurements of the specimens as well as strain gauge measurements for validation. Thereby, the near notch stress under clamping condition without any load and under an additional static nominal load is analysed. Based on these investigations, the applied stress range for each tested specimen is converted to a “deformation-free” condition by means of the IIW-recommended k_m factor. The statistical analyses of the data points with and without this modification reveal a significant impact by the deformation state on both the evaluated fatigue strength and the scatter band. Moreover, this affects the evaluated benefit by grinding as post-treatment, whereas an increase of more than 40% in the fatigue strength at 2 million cycles is observed using the modified data points.

This study deals with the validation of the fatigue design values given in the IIW Recommendations for the HFMI Treatment for HFMI-treated steel joints under bending loading. In total, ten data sets involving T-joint specimens under bending loading with varying specimen geometries and base material yield strengths are investigated. The load stress ratio was R = 0.1 in all test series. The corresponding FAT-classes are defined on the basis of the IIW Recommendations for the HFMI Treatment in dependence of the structural detail and the yield strength of the base material. Furthermore, the thickness as well as certain thinness effect is covered by an IIW-recommended factor f(t) in one case leading to a design curve HFMI-IIW with f(t). In another case, a factor k _tb based on the British Standard BS 7608 is additionally considered, which combines the thickness as well as bending effect. The corresponding design curve is denoted as HFMI-IIW with k _tb . A comparison of the fatigue test data points and the related statistically evaluated S/N-curve for each data set with the two approaches reveals that the design curve HFMI-IIW with f(t) leads to a conservative assessment for all data sets involved in this study. Also, a certain thinness effects is well covered and still a proper fatigue design should be ensured. Focusing on the design curve HFMI-IIW with k _tb , which additionally covers the bending effect, a conservative assessment is observed for almost all data sets. However, it is concluded that further test data especially for reduced plate thicknesses should be assessed to provide additional comparison results and ensure a conservative applicability for HFMI-treated steel joints under bending loading in any case.