In order to investigate the effect of post weld treatment on the fatigue life of welded details of ultra high strength steels, experimental investigations are necessary. Within a test series on butt welded specimens, the influence of the high frequency hammer peening method HiFIT on fatigue behaviour is investigated. Results of fatigue tests under pure bending show, that the fatigue strength in the low cycle fatigue range could be increased considerably by post weld treatment. The fatigue life of HiFIT-treated specimens could be extended by a factor of 5 compared to as welded ones. To analyse the influence of the type of loading, fatigue tests under axial tension are carried out and compared to those subjected to pure bending.
Welded steel structures can be significantly improved with respect to fatigue behavior and durability. This is important for applications in offshore and crane constructions. Using post weld treatment methods such as High Frequency Impact Treatment (HiFIT), the fatigue resistance and the resulting fatigue life of welded structures can be considerably increased. The functional principle of this method is based on the transfer of mechanical impulses to the material surface using a post weld treatment device. Effects of the treatment are a strain hardening of the surface layer as well as the introduction of residual compressive stresses (Ummenhofer et al. 2005). Within the scope of the research project REFRESH (Ummenhofer et al. 2009), a significant improvement of fatigue resistance of simple joints has been shown. Using high frequency hammer peening methods, the fatigue strength at 2 million load cycles could be increased by 80% to 100%. Furthermore, a higher efficiency of the method was observed with increasing yield strength of the base material. Up to now, the steel grades S355J2 and S690QL have been mainly investigated.
