Fatigue strength assessment of HFMI-treated steel joints under bending loading

Abstract

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.