Effect of preload and stress ratio on fatigue strength of welded joints improved by ultrasonic impact treatment

Ultrasonic impact treatment (UIT) is a post-weld technique to be used for improving the fatigue strength of welded joints. The technique makes use of ultrasonic vibration to impact and plastically deform the weld toe, consequently creating beneficial compressive residual stress near the treated area. Since the compressive residual stress is considered to be a main reason for the improvement of fatigue strength, the improvement effect may be influenced by preload and stress ratio. In the case of ship structures, welded parts may be locally subjected to a stress near the yield strength of the material in heavy weather, and mean stress may change significantly depending on the loading conditions. In the present paper, fatigue tests were conducted using transverse non-load carrying fillet welds to identify the condition that UIT provides substantial benefit for fatigue strength of welded joints. The material used is AH36 shipbuilding high-strength steel. In the investigation of the preload effect, tensile stress of 90 % or compressive stress of 60 % of the yield strength of the base metal is applied prior to fatigue tests, where the stresses are defined as nominal stress. It is found that UIT is more effective than grinding on fatigue strength improvement of welded joints even after application of the preload. The effect of stress ratio was investigated by fatigue tests at stress ratios of 0.1, 0.5, and -1. Though the fatigue strength of the UIT-treated weld joint decreased as the stress ratio increased, the fatigue limits of the UIT-treated weld joint is fairly higher than that of as-welded joints even at the stress ratio of 0.5. Finally, fatigue tests were conducted under random sequence of clustered loading, which may simulate wave-induced load histories. UIT provides substantial benefit for fatigue strength of welded joints under actual service loading in ship structures.