Experimental and numerical validation of fatigue crack depth limit for retrofitting using high frequency mechanical impact provided in the updated IIW recommendations

The updated recommendations for High Frequency Mechanical Impact (HFMI) Treatment for Improving the Fatigue Strength of Welded Joints of the International Institute of Welding (IIW) provide fatigue crack depth limit (IIW crack depth limit) for retrofitting using HFMI. The recommended crack depth limit using HFMI of cracked joints has been established based on a limited number of experimental data and weld types, and their applicability requires further validation. This study performed experimental fatigue tests and crack propagation analyses using butt-welded joints made of S355J2 + N steel with the aim of validating the IIW crack depth limit. Four-point bending fatigue tests under a stress ratio of R = 0.1 were conducted on HFMI-retrofitted welds, followed by fracture surface observations. The fatigue tests revealed that fatigue life after HFMI retrofitting exceeded the FAT class specified in the IIW recommendations when the fatigue crack depth was less than 2.0 mm. Three-dimensional finite element simulations of HFMI treatment and subsequent cyclic fatigue bending and axial loadings were carried out to obtain HFMI-induced residual stress distributions for crack propagation analyses based on the weight function approaches. The analyses indicated that fatigue life can be restored to a level comparable to that of newly HFMI-treated welds when the crack depth was less than approximately 2.0 and 1.5 mm under bending and axial loadings. The experimentally and numerically obtained crack depth were consistent with each other. Based on these investigations, the applicability of the IIW crack depth limit was validated for butt-welded joints under R = 0.1.