Robustness of post-weld treatment HFMI to increase the fatigue strength of welded structures (ROMI)

Purpose and goal

The goal of the feasibility study was to clarify how robust the HFMI (High Frequency Mechanical Impact) processes are today regarding the possibility of improving the fatigue life of weld joints of lower welding quality classes such as C and D. In addition, the effect and robustness of different HFMI parameters on fatigue behavior was studied. The results from this pre-study show good process robustness and that HFMI can be utilized also on lower-class-welds with good results. The goals of the project were achieved.

Expected results and effects

The main result of this pre-study is that HFMI treated weld with weld class D shows fatigue life improvements that fall within the recommendations for fatigue life of HFMI treatment of weld of weld class B. Thus, the HFMI treatment in this pre-study has shown to be robust to deviations in the weld class. The results of this pre-study show a great potential for widening the area of use for the HFMI treatment. If recommendations for using HFMI are revised to include lower welding classes, the number of rejections in welding processes can be significantly reduced.

Planned approach and implementation

Fatigue test specimens with welding quality B and D were manufactured. The specimens with welding quality D were HFMI treated with a total of four different methods to study the effect of variations in HFMI machine, operator and tool radius. Fatigue testing was performed at one load level with six parallel samples per variant and the results were compared to the non-treated references. The welding geometry was measured using laser scanning prior to fatigue testing. These results were used to create 3D models for further FE analyzes of stress concentrations in the welds.