The effect of undercut and residual stresses on fatigue behaviour of misaligned butt joints

Abstract

A numerical model is developed to predict the combined effect of a weld toe undercut, residual stresses and misalignment on the fatigue strength and fatigue life of butt welded joints subjected to pulsating tensile loading. Linear Elastic Fracture Mechanics (LEFM), Finite Element Analysis (FEA) and superposition approaches have been used for the modelling. It has been found that the effect of undercut is the most significant geometrical effect affecting the fatigue behaviour of butt welded joints and is governed by the effect of other weld geometry parameters. The reduction of fatigue life and fatigue strength, in comparison to flush-ground welded plate, caused by an introduction of weld toe undercut is twice that due to welded joint without undercut. It means that by elimination of undercut, fatigue strength can be improved up to 50% of the level that can be obtained by flush-ground welded plate. It is also found that a presence of misalignment of 5% of plate thickness and undercut of 2% of plate thickness are fairly representative for the lower boundary of S- N curves of butt joints in a real situation. Fatigue life can be decreased by up to 10 and 100 times compared to perfect stress-relieved conditions of aligned and misaligned joints, respectively, once high tensile residual stresses of yield stress magnitude are present. As a result, the slope of S- N is no longer constant and the effect of misalignment is more significant than that of undercut. However, when the magnitude of tensile residual stress becomes smaller or it changes into compressive stresses the effect of undercut will play a dominant role in comparison with that of misalignment. The effect of misalignment may be beneficial for high cycle fatigue life of the order of greater than 2 × 10 5 cycles, especially when compressive residual stresses are introduced into the weld toe surface by using surface treatment (i.e. shot peening, hammering). In this case, the improvement of fatigue strength of misaligned joints and undercut-free joints can be 50% more effective than that of aligned undercut joints.