An Investigation into Fatigue Crack Growth through a Weld Toe Residual Stress Field

Abstract

Welds are a common method of joining metallic components in buildings, bridges and offshore structures, but they are susceptible to fatigue crack initiation and slow but accelerating growth arising due to fluctuating service loads, eventually resulting in fracture unless detected. Recently developed and unique Hellier-Brennan-Carr (HBC) parametric equations give the surface stress concentration factor (SCF) and stress distribution through the main plate thickness (i.e. the potential Mode I crack plane) at a T-butt weld toe as a function of a wide variety of weld geometries, for tension (membrane) loading. An existing Brennan-Dover-Karé-Hellier (BDKH) parametric equation is available for T-butt weld toe stress intensity factor (SIF) geometric Y-factor at the deepest point of a semi-elliptical surface crack, also subject to tension (membrane) loading. Three fatigue crack propagation programs have been written in FORTRAN. FATIGUE1 utilises the BDKH tension equation and the Paris Law to predict the (Stage 2) fatigue propagation life of a T-butt weld containing an initial semi-elliptical surface flaw. FATIGUE2 utilises the BDKH tension equation in conjunction with the Forman Equation to predict the semi-elliptical flaw fatigue life in the presence of a mean applied stress (i.e. with fixed stress ratio R > 0). FATIGUE3 utilises both the BDKH tension equation and the HBC tension equation in conjunction with the Forman Equation to predict the semi-elliptical flaw fatigue life in the presence of a known residual stress distribution from the literature (for 350 grade structural steel). The maximum and minimum stress values are calculated at each incremental crack length, from the applied stress at the crack tip given by the HBC tension equation added to the discretised value of residual stress at the same point. Thus the R-ratio is recalculated at the crack tip for every crack increment. Positive R-ratios increase the fatigue crack propagation rate; negative values result in some crack closure, reducing the propagation rate