Fatigue life evaluation of root failure in welded joints based on displacement around the weld root

Abstract Unanticipated load transfer in load-carrying cruciform and butt-welded joints with incomplete penetration can initiate fatigue cracks from the weld root. Conventionally, the fatigue life of such joints is assessed using the nominal stress approach. However, this method is often inadequate for geometrically complex joints, particularly when the weld throat nominal stress cannot be clearly defined due to non-uniform stress distribution. Although the effective notch stress (ENS) method provides an alternative, it is computationally demanding and relies on an assumed notch radius which lacks a well-defined physical basis. This study proposes an alternative fatigue evaluation method based on displacement around the weld root, offering a physically meaningful and mesh-insensitive fatigue damage parameter. A linear-elastic finite element model with a 1 mm mesh around the unfused zone was employed to evaluate the displacement at a reference point 1 mm from the weld root. A dataset comprising 398 fatigue test results from specimens with incomplete penetration exhibiting root failure was extracted from the literature to validate the proposed method. A correction factor was introduced to normalize the fatigue damage parameter into a dimensionless form, allowing direct comparisons across specimens of varying sizes and geometries. The relationship between the normalized fatigue damage parameter and fatigue life was presented on logarithmic scales, resulting in the normalized Δd–N curve. This curve not only demonstrates improved predictive accuracy and reliability compared to the conventional S–N curve but also offers a physically interpretable approach applicable to complex welded joint geometries.