Notch mechanics assessment of fatigue strength reduction in butt welds with defects

The fatigue strength of welded joints is significantly influenced by the presence of geometric imperfections and defects, such as undercuts, pores and lack of fusion. Current technical standards and recommendations typically address these defects through discrete quality classes, which may result in discontinuous strength predictions. The primary objective of this study is to develop a robust yet simplified analytical framework to predict fatigue resistance (FAT class or design strength) as a function of the dimensions and shape of the defect. An extensive parametric investigation covering approximately 6000 defect geometries, analysed using the implicit gradient method, was performed to generate fatigue strength benchmarks. The properties of the stress field at the weld toe and defect tip are addressed within the theoretical framework of sharp open notches and, by adapting the El Haddad–Topper formulation, the proposed model introduces an intrinsic crack depth parameter to bridge the gap between the fatigue limit of the defect-free weld toe and the threshold stress intensity factor range of long cracks. Comparisons with design standards and with numerical data demonstrate that the proposed method provides predictions comparable to the effective fatigue strength by the propagation-based curves derived from Hobbacher’s recommendations for crack-like defects. In addition, strength assessment is extended to rounded defects, and a comprehensive assessment framework is outlined. Furthermore, for practical engineering applications, the proposed approach allows for the derivation of explicit and simple equations, providing a direct tool for fitness-for-service assessments.