Quantifying the intensity of high-frequency mechanical impact treatment

High-frequency mechanical impact (HFMI) is a user-friendly and efficient mechanical post-weld treatment method, and the achieved fatigue life improvement is statistically proved and is attributed to HFMI-induced compressive residual stresses amongst other effects. Several studies have shown in the past that the process parameters (treatment time and working speed) have an influence on the stress state introduced by the HFMI treatment. Thus far, however, only device-specific quantitative recommendation for the HFMI treatment exists based on the instructions of each HFMI device manufacturer. It is not clear if a certain treatment time for a given intensity leads to optimum results regarding the enhanced fatigue life and the treatment parameters of the several HFMI devices cannot be directly compared with each other. For these reasons, a universal and simple definition of the HFMI treatment’s intensity based on the kinetic energy of the HFMI pin was used to quantitatively correlate the HFMI-induced compressive residual stress with the HFMI-process parameters for two different HFMI devices: pneumatical impact treatment (PIT) and high-frequency impact treatment (HiFIT). To this purpose, data from former studies of HFMI-treated base material and welded specimens are revaluated. It is shown, that the compressive residual stresses show only slight changes after reaching a threshold value of the applied kinetic energy ( 50 to 100 J/mm). This energy-based approach for the quantification of the treatment intensity was also used for a case study with PIT- and HiFIT-treated transverse stiffeners with different treatment intensities (2 J/mm and 7 J/mm). A high influence of the treatment intensity on the residual stress state was determined.