Assessment of Weld Fatigue by Effective Notch Stress Approach

Abstract

Applicability of different approaches, i.e. nominal stress-, structural hot spot- and effective notch stress approach, for fatigue assessment of welded structures has been discussed. The effective notch stress approach was validated for a cruciform fillet welded and fully penetration welded joint, according to DNV guideline [1]. The same fillet welded joint was further investigated for varying weld sizes. The results show that with changing weld size, there are considerable differences between calculated stresses, and specially calculated fatigue lives, when comparing with nominal stress approach. Further investigations revealed that there is a nonlinear relation between effective notch stress and weld sizes, while comparison of the notch stress and nominal stress approaches indicate that a linear relation is to be expected. Based on the established methodology for the cruciform joint, another joint, i.e. a knee plate located in a horizontal brace of a drilling vessel, was assessed for weld root fatigue. Based on the proposed equations and the linear relation found between the nominal- and the notch stress S-N curves, a weld size providing a longer fatigue life at the weld root, rather than that at the toe, was proposed.