A comparison has been made between the fatigue lives of welded specimens a) in the as welded condition b) after heat treatment, and c) after post-weld vibration. By comparison with the aswelded specimens, the fatigue lives of the thermally relieved specimens were found to decrease by 43%, while the vibration treated specimens showed an increase of between 17% and 30%. While these findings are interesting in that they offer a way of extending the fatigue lives of welded joints, they confirm the view that the mechanism of residual stress relieving in the vibratory stress relief (VSR) method, and its relationship with fatigue life is poorly understood.Tel: 0 I4 I 577 6297, Fax: 0 I 4 I 577 628 I

Previous studies suggested that the state of residual stress in a weld may be modified by vibration during and after the welding process. In this study, the effect of rigid body motion vibration on welding residual stresses was investigated. The specimens were welded while they were being vibrated in a rigid body motion mode. The specimens were vibrated using two different frequencies (50 Hz and 500 Hz). At the lower frequency vibration (50 Hz), small changes in the residual stresses were found, with no particular trend. At the high frequency vibration, no significant reductions in the residual stresses were observed in the longitudinal stresses or in the transverse direction. Some initial results are presented also regarding flexural vibration effects.

In this study the effect of torsional vibration on residual stresses was investigated. Three types of shaft specimen were processed, namely (a) a homogeneous shaft, (b) a shaft welded on a circumferential line and (c) a spot-welded shaft. The first two types of shaft showed some redistribution in the residual stresses under applied torsional loads. On the spot-welded shafts the residual stresses were found to decrease significantly at a very low level of vibration induced stress.

Previous studies have suggested that welding residual stresses can be reduced by the application of controlled vibration. In the present study welded specimens were processed after being cooled to room temperature, with varying amplitude of applied stress and time of vibration. An increase in the applied stress led to a signi®cant decrease in the residual stresses. The effect of time of vibration was found to be very small for a lower range of applied stresses (v230 MPa); an increase in the time of vibration had no in¯uence on residual stresses. At higher applied stresses (w230 MPa), the residual stresses were found to redistribute with increasing time of vibration. It is shown that the energy concept of the vibratory stress relief mechanism was not validated.MST/4693

Previous studies have suggested that weld microstructure may be modi ed by the presence of static stresses. In this investigation, vibratory stress was applied to mild steel specimens while they were being welded to observe its effect on the residual stress, microstructure and hardness of the material. Residual stresses were found to decrease in response to vibration whether it was applied during welding or after welding. It was found that the applied stress in uenced the grain growth process in the weld. As a result of the treatment the hardness of the material was found to be increased by 25 per cent.