Influence of cyclic loading on the relaxation behavior of compressive residual stress induced by UIT

Abstract

Fatigue strength of welded joints is improved by inducing compressive residual stress at weld toes. The ultrasonic impact treatment (UIT) is regarded as an effective method for inducing compressive residual stress at weld toes. However, excessive loads that cause plastic deformation at weld toes relieve the induced compressive residual stress. In addition, stress fluctuation affects the relaxation of residual stress even it is smaller than the yield strength. The relaxation of compressive residual stress decreases the fatigue strength improvement. Therefore, in order to evaluate the fatigue strength of welded joints enhanced by UIT, it is necessary to quantify the relaxation of local residual stresses at weld toes under cyclic loading. In this study, we measured a residual stress at the UIT groove using the X-ray diffraction method with a small-diameter collimator and an irradiation angle that was corrected based on the shape effect of the UIT groove, and we investigated its relaxation behavior under cyclic loading. In addition, we estimated the fatigue test results of the out-of-plane gusset joints improved by UIT using the modified Goodman diagram considering the relaxation of residual stress.