Stress components and local effects in the fatigue strength assessment of fillet weld joints made of ultra-high-strength steels

Abstract

Ultra-high-strength steels (UHSSs) enable a significant reduction in material usage compared to mild steels and, thus, provide energy-efficient solutions for structural applications. In welded steel components, an increase in material strength does not necessarily result in enhanced fatigue strength, unless post-weld treatments (PWTs) are introduced. Consequently, the fatigue strength of welded UHSS components is amongst the most important design criteria, and fatigue assessments should be conducted using appropriate and accurate approaches.

This dissertation’s objective is to evaluate the effect of stress components and local behavior on the fatigue strength assessments of fillet-welded UHSS joints. In this thesis, the effects of the cyclic membrane and bending loads on the fatigue strength of loadcarrying (LC) and non-load carrying (NLC) fillet weld joints are investigated considering both weld root and weld toe failures. Furthermore, the local effects on the fatigue performance are addressed. The local effects are evaluated at a structural level, i.e. the effect of geometrical symmetry and asymmetry on the fatigue performance of fillet weld joints. In addition, the effect of stress components on notch stress is evaluated. Notch stress analysis applies a generic fatigue strength assessment model – namely the 4R method – to evaluate the combined effects of notch stress, residual stress, material strength and applied stress ratio on the fatigue strength of welded UHSS components. The thesis applies both experimental and numerical methods to examine the fatigue performance of fillet-welded LC and NLC joints. Experiments are conducted for joints made of S960MC and S1100QL UHSS grades, and numerical studies carried out for investigating structural stresses, notch stresses, and crack propagation of these joints.

The results demonstrate the importance of stress components and structural symmetry when assessing the fatigue strength of fillet-welded UHSS joints. Furthermore, PWT and its effect on the fatigue strength of UHSS components requires particular attention to notch geometry in association with the residual stresses and applied stress ratio in the fatigue strength assessments – for which the 4R method provides an efficient means of assessing the influencing factors.