THE EFFECT OF UNDERCUT AND RESIDUAL-STRESSES ON FATIGUE BEHAVIOR OF MISALIGNED BUTT JOINTS

A numerical model is developed to predict the combined effect of a wel d toe undercut, residual stresses and misalignment on the fatigue stre ngth and fatigue life of butt welded joints subjected to pulsating ten sile loading. Linear Elastic Fracture Mechanics (LEFM), Finite Element Analysis (FEA) and superposition approaches have been used for the mo delling. It has been found that the effect of undercut is the most sig nificant geometrical effect affecting the fatigue behaviour of butt we lded joints and is governed by the effect of other weld geometry param eters. The reduction of fatigue life and fatigue strength, in comparis on 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 Bush-ground welded plate. It is also found that a presence of misalignment of 5% of plate thickn ess and undercut of 2% of plate thickness are fairly representative fo r 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 p erfect stress-relieved conditions of aligned and misaligned joints, re spectively, once high tensile residual stresses of yield stress magnit ude are present. As a result, the slope of S-N is no longer constant a nd the effect of misalignment is more significant than that of undercu t. However, when the magnitude of tensile residual stress becomes smal ler or it changes into compressive stresses the effect of undercut wil l play a dominant role in comparison with that of misalignment. The ef fect of misalignment may be beneficial for high cycle fatigue life of the order of greater than 2 x 10(5) cycles, especially when compressiv e residual stresses are introduced into the weld toe surface by using surface treatment (i.e. shot peening, hammering). In this case, the im provement of fatigue strength of misaligned joints and undercut-free j oints can be 50% more effective than that of aligned undercut joints. Copyright (C) 1996 Elsevier Science Ltd.