Applications of the 4R method for the fatigue assessment of welded joints considering different steels, joint types, post-weld treatments, and load conditions

Abstract

The use of high-strength steels (HSS) and ultra-high-strength steels (UHSS) is a way to reduce the mass of a structure while increasing mechanical performance. However, a high strength-to-weight ratio does not guarantee better fatigue strength. In welded structures, fatigue is typically the most significant failure mechanism and can cause considerable damage. HSS and UHSS materials enable higher stress levels in static loading, which also increases the fatigue loading level. Therefore, welded structures made of HSS and UHSS materials require high-quality welds to achieve better fatigue resistance. However, better fatigue strength cannot be fully numerically proven by using conventional fatigue assessment methods. For this reason, a multi-parametric fatigue analysis method, called the 4R method, has been developed at LUT University. The 4R method can consider the effect of the material’s ultimate strength, joint geometry, applied stress ratio, and residual stresses. The 4R method enables consideration of fatigue effective parameters, thus improving the accuracy of fatigue assessments, particularly in high-quality weldments.

The objective of this thesis is to extend the 4R method to a generic fatigue analysis method that can be applied to different joint types, loadings, conditions, and materials. The thesis focuses on studying HSS and UHSS structural steels, including duplex and super-duplex stainless steels, in as-welded and post-weld-treatment conditions.

The thesis applies both experimental and numerical methods to investigate the applicability of the 4R method. The fatigue test results were analysed using the 4R method and compared with the results obtained using conventional methods. The results showed that the scatter of the fatigue test results analysed with the 4R method was significantly lower than that obtained by the other methods used. According to the study, the 4R method is an accurate and comprehensive method of approaching fatigue design. The research showed that the 4R method is suitable for all the studied joint types and materials. For welded joints in different post-treatment conditions, the 4R method can be applied according to certain parameter recommendations.