K.L. Yuan et alii, Frattura ed Integrità Strutturale, 34 (2015) 476-486; DOI: 10.3221/IGF-ESIS.34.53 476 Focussed on Crack Paths Modelling of ultrasonic impact treatment (UIT) of welded joints and its effect on fatigue strength K.L. Yuan, Y.Sumi Yokohama National University, Japan A BSTRACT . Ultrasonic impact treatment (UIT) is a remarkable post-weld technique applying mechanical impacts in combination with ultrasound into the welded joints. In the present work, a 3D simulation method including welding simulation, numerical modelling of UIT-process and an evaluation of fatigue crack growth has been developed. In the FE model, the actual treatment conditions and local mechanical characteristics due to acoustic softening are set as input parameters. The plastic deformation and compressive stress layer are found to be more pronounced when acoustic softening takes place. The predicted internal residual stress distributions of welded joint before and after UIT are compared with experimental results, showing a fairly good agreement with each other. Finally, simulated results of fatigue crack growth in various residual stress fields are well compared with test results, so that the proposed model may provide an effective tool to simulate UIT-process in engineering structures. K EYWORDS . UIT; Residual stress; FEM; Surface crack growth. I NTRODUCTION t is well known that the fatigue strength of steel plates increases in proportion to its static strength. On the other hand, the fatigue strength of the welded joints is little influenced by the static strength level because of the tensile welding residual stress and high stress concentration near the weld toe. Ultrasonic impact treatment (UIT) is a new method for increasing fatigue strength by improving the weld toe geometry, removing defects and introducing beneficial compressive residual stress [1-3]. Recently, new IIW guidance on fatigue strength improvement using high frequency mechanical impact (HFMI) methods including UIT has been prepared by Marquis et al. [4-5]. In parallel to the development of IIW guideline, ship classification societies have also accepted or investigated the UIT as post-weld treatments in ship and offshore structures [6-7]. In order to better understand the effects of UIT on fatigue performance of welded joints, its mechanism has been experimentally investigated in several studies. Cheng et al. [2] measured the residual stress induced by UIT using both X- ray and neutron diffraction (ND) techniques for the first time. The compressive stress layer was found not less than 1mm in depth for welded specimen. A German research project [8, 9] has presented the experimental data that the compressive residual stress is generated down to a depth of 1.5 to 2 mm with maximum values at approximately 0.4 to 0.5 mm below the surface. After treatment the weld toe radius is averaged by 1.5 to 2 mm with its groove depth of 0.1 to 0.2 mm. The study at the University of Waterloo, Canada [10, 11] was carried out to investigate the fatigue performance of structural steel welds subjected to UIT at under-, proper and over-treatment levels, respectively. A close relationship was found between the measured groove depth and local residual stress, indicating the groove depth as an important quality control I