Enhancing Fatigue Strength by Ultrasonic Impact Treatment

Enhancement in fatigue performance of welded joints by Ultrasonic Impact Treatment (UIT) was evaluated in large-scale specimens having a nominal yield strength of 345 to 690 MPa. Eighteen rolled-beam specimens having welded details at cover plates and transverse stiffeners and eight built-up specimens having only transverse stiffener details were fatigue tested after treating them by UIT. A partial factorial experiment was conducted at various levels of stress range between 52 and 201 MPa, and at various levels of minimum stress, resulting in stress ratios not exceeding 0.6. Test results indicated that UIT enhanced the fatigue performance of all treated details by improving the weld toe profile, changing microstructure, and introducing beneficial compressive residual stresses at the treated region. The treatment effectively elevated the fatigue limit without changing the slope of the S-N curve in the finite life region. Fatigue design guidelines for the AASHTO Category E ́ and Category C ́ details were proposed and fatigue strength of a treated detail for infinite life was estimated by a simplified stress-life approach using Finite Element Analysis.