TWI Industrial Member Report Summary 62/1978
By T R Gurney and G O Johnston
The Report gives the results of further finite element analyses of transverse non-load-carrying fillet welded joints for a range of weld leg lengths varying from 0.1 to 0. 8 times plate thickness (=2B). The value of Mk (the factor by which the stress concentration at the weld toe increases K) decreases rapidly with increasing crack depth in the initial stages of growth and becomes equal to 1.0 at approximately a = 0.7B.
Using the calculated values of Mk a fracture mechanics analysis has been made of the effect of continuous and semi-elliptical toe cracks on fatigue strength. These show that the initial defect size necessary to cause through thickness cracks in a given life tends to decrease rapidly with increasing plate thickness over the range of plate thicknesses which are of most interest to structural engineers. This implies that fatigue design rules, which are inevitably based largely on data obtained with relatively thin (typically 12.7mm thick) specimens, may not be strictly applicable to joints in thicker members. Qualitatively it is easy to see that this 'size effect' is inevitable, since the rate of crack propagation is a function of crack size, while the stress distribution is a function of the ratio between crack size and plate thickness.