Relaxation behavior of compressive residual stress induced by UIT under cyclic loading

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Abstract
Ultrasonic Impact treatment (UIT) is an effective method of inducing compressive residual stress at weld toes in order to improve the fatigue strength of welded joints. In this study, compressive residual stresses introduced by UIT and its relaxation behavior under cyclic loading were investigated for different steel grades. The compressive residual stress in UIT grooves showed the maximum value at the bottom, which corresponded to 50 to 60% of the yield strength of base metal. The compressive residual stress was relieved by cyclic loading and it was observed that approximately 90% of residual stress relaxation occurred within 10 cycles. The amount of residual stress relaxation due to tensile cyclic loading was dominated by ratio of the maximum stress to the static strength of the base material. Therefore, it is considered that the combined use of high-strength steel and UIT is effective for improving fatigue properties from the viewpoints of the introduction of higher compressive residual stress and the suppression of compressive residual stress relaxation under cyclic loading. Compressive cyclic loading showed a tendency to relax more than tension cyclic loading. It was considered that compressive stress exceeding the assumed value was applied due to slight bending deformation of the specimen.

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