HFMI: understanding the mechanisms for fatigue life improvement and repair of welded structures

Fatigue resistance of high-frequency mechanical impact (HFMI) treatment is investigated on high-strength welded steel S690. Cyclic four-point bending tests are performed on butt weld samples and HFMI samples with different conditions of treatment with or without stress-relieving heat treatment. Microstructural characterisations with electron backscattering diffraction observations and residual stress analyses are presented. The main objective of this paper is to reveal and identify the parameters (residual stresses, geometry, strain hardening, etc.) which have an effect during the HFMI operation and to understand the phenomena involved during fatigue stressing. The optimisation of the geometry of the fatigue specimen allowed to repetitively obtain failures in the treated areas, thereby showing the relationship between the geometry, the strain hardening, the residual stresses and the fatigue strength. The main conclusions of this paper showed that HFMI treatment improves the fatigue performance of welded joints (in keeping with the International Institute of Welding results). The residual compressive stresses are prominent. The microstructure does not seem to have any effect, despite the nanostructured layer. The geometry is of secondary importance despite a significant change in the local geometry. HFMI treatment is also presented as retrofitting treatment.