Fatigue Algorithm for Modeling Many Weld Toe Cracks Propagating at Welded Joints

Abstract

Fatigue crack growth at welded joints often propagates from as many as tens to hundreds of small weld toe cracks along the weld toe line in offshore welded structures. This paper will present a fatigue algorithm for modeling many small weld toe cracks propagating from a welded joint. Cracks usually initiate at the weld toe region of the structures and propagate as surface cracks at the stress concentration regions of the weld-toe line. The presence of such weld defects or crack-like flaws can have a severe detrimental effect on their fatigue life and fracture resistance. Currently, there is a lack of studies that considers the effects of multiple cracks and their distribution density in welded joints. This work focuses on the fatigue analysis and modeling of multiple weld toe cracks, specifically in T-butt joints. Fatigue crack growth prediction is usually determined by the stress intensity factor range and crack propagation rate through Paris law. To predict the Stress Intensity Factor (SIF) of a weld toe crack, the magnification (Mk) factor was used. The Mk factor is influenced by the size of the welded attachment, as well as the size and depth of the weld toe crack. Simplified solutions for practical prediction of Mk factors were determined from 3D extended finite element method (XFEM) by modelling a semi-elliptical weld toe crack in a T-butt weld for cracks of different dimensions. The accuracy of the Mk factor solutions was verified by comparison to HSE fatigue data on 16 mm thick tubular joints. The Mk factor solutions were used to predict the growth of fatigue cracks using a model based on Paris Law and SIF solutions by Newman and Raju with plastic zone size corrections. Fatigue life was predicted for plates with and without attachments. It could be seen that the predicted life of a weld toe crack was severely reduced with the addition of a welded attachment. The model was extended to the multiple surface cracks commonly observed at the weld toe, where each crack is treated as independent, following established code procedures. The multiple cracks will coalesce as they propagate, until a single dominant crack emerges and fracture occurs. In this paper, the relationship between the fatigue life and the number and density distribution of the initial cracks was investigated. Fatigue life was predicted for plates with attachments with 1, 2, 10 and 100 cracks initially. The results show that as the number of cracks increases, the predicted fatigue life decreases.