High Frequency Mechanical Impact treatment (HFMI) is a newer version of hammer and needle peening, where higher frequency impacts are employed, resulting in smaller spacing between the indentations. The main mechanisms favoring fatigue life improvement in welded components are: the tensile residual stresses that usually exist at the weld toe due to welding are substituted by compressive residual stresses in the order of magnitude of the material yield strength, the transition from base to weld material is smoothened and the strain hardening that results from cold working, improve the fatigue resistance of the material at the weld toe. The method has gained popularity over conventional hammer peening, due to the greater treatment depths achieved, is user friendly and produces a uniform treated region with good repeatability. The beneficial effects of this PWT method on the fatigue life extension have been extensively reported in the literature. The fatigue life of the specimen treated by HFMI was reported to be lengthen between two and ten times depending on the weld configuration and the base material. 

Current study aims to investigate the influence of HFMI on the residual stresses field of a GMAW butt-weld joint of the HSLA steel S700MC 10mm thick. For the numerical simulation of the HFMI-treatment, a suitable material hardening model was employed and stress relaxation during fatigue loading was taken into account. PIN was simulated as a discrete body in order to save computational time. Displacement-controlled simulations were carried out to estimate the local residual stress evolution considering the effect of combined isotropic-kinematic hardening law.

Typical results are depicted in Fig.1 and allow the correlation between experimental settings of the HFMI tool and the quantification of the residual stresses reduction in the weld area. Evaluation points out that HFMI can be used as a mitigation tool as it modifies residual stress fields radically and can play a vital role on the fatigue life extension of the weld studied.