Numerical implementation for fatigue assessment of butt joint improved by high frequency mechanical impact treatment: A structural hot spot stress approach

• High-speed camera was used to determine the initial velocity of the HFMI process.
• The effective SCF depends on material strength and external applied stress.
• The slope m of S-N curve for HFMI-treated joint is related to material strength.
• The corresponding characteristic fatigue strength has been proposed.

In the study, three-dimensional finite element modelling of high frequency mechanical impact (HFMI) treatment is presented for butt joints of four strength grade materials (Q235B, AISI 1006, 45 Steel and AISI 2205). The fatigue assessment of butt joints by HFMI is implemented by structural hot spot stress approaches, i.e., linear surface extrapolation (LSE) and through thickness at the weld toe (TTWT). The effective structural hot spot stress concentration factors for HFMI-treated joints are determined, and the dependence of HFMI improvement on material strength and external stress is well captured by TTWT method. Following structural hot spot stress S-N curves for as-welded joints recommended by IIW, the structural hot spot stress S-N curves for HFMI-treated joints are suggested if the slope m of S-N curves for HFMI-treated joints is designated to vary from 5 to 10, inversely proportional to material strength. At final, the characteristic fatigue strength (FAT) has been determined and verified with the available experimental data.