Estimating the fatigue strength of HFMI-treated weldments by means of numerical analyses

A thorough methodology for the estimation of fatigue strength of components post-weld treated with the HFMI (High Frequency Mechanical Impact) method is presented. Previously experimentally validated models, for the simulation of the welding process and HFMI treatment are enhanced and used to estimate the HFMI groove geometry and the residual stress state at the weld toe. A reference S-N curve is translated for predicting the fatigue strength of the HFMI-treated weld toes, as it was suggested earlier by the 4R fatigue estimation method, which provided sufficient estimates of the fatigue strength using analytical estimation and engineering assumptions for defining the material state at the weld toe. However, this translation of the reference S-N curve is carried out in the present study based on the findings of the elaborate numerical estimations, dropping in this way the need for assumptions and enabling a spatial estimation of the location of fatigue crack initiation, improving the overall accuracy. The results seem to validate both the findings of the 4R method as well as the FAT classes proposed by the IIW recommendations for the HFMI-treated weld toes made of S355. The methodology for the weld and HFMI simulations is outlined as well in the present study, so that a clear synopsis of the estimation path is provided to the reader.