Removing Residual Stress and Increasing Fatigue Life by Ultrasonic Method

In this paper, a new method is introduced for evaluating effects of residual stress on fatigue life. The ability of ultrasonic method using longitudinal wave with critical angle of refraction or LCR wave in measuring and removing residual stresses due to welding was used. Two plates of alloy 2024-T351 were welded to each other. To measure their residual stress field acoustoelastic property was used and the changes in the speed of ultrasonic propagation of elastic waves when passing through the residual stress fields was investigated. In order to exert the effects of residual stress on fatigue life, the relations between the coefficients of effective stress intensity (SIF) and Fatigue Crack Propagation (FCP) rate in a state that the parts were welded together with residual stress under cyclic loading were obtained. Finally, ultrasonic waves with a certain frequency were used to remove the residual stresses. Also, the relationships between stress intensity factor and fatigue crack propagation rate were modified to predict fatigue life after removal of residual stresses. This method resulted in a 31% increase in fatigue life. The main reason for the increase in life was the plastic area created by the ultrasound wave. Therefore, it can be said that introduced method are suitable for using to remove residual stress due to welding.