Abstract
The induction of near-surface compressive residual stress is an important factor for fatigue life improvement of HFMI-treated welded joints. However, the relaxation of these beneficial residual stresses under single overload peaks under variable amplitude and service loads may significantly reduce fatigue life improvement. For this reason, several recommendations exist to limit the maximum applied load stress for this kind of post-treated welded joints. In this work, the effect of single tension and compression overloads on the relaxation behavior of HFMI-induced residual stresses was studied experimentally by means of X-ray and neutron diffraction techniques complemented by numerical simulation at transverse stiffeners made of mild S355J2 steel and high strength S960QL steel. Loads were applied close to the real yield strength of the base material. Significantly different relaxation behavior was observed for S355J2 and S960QL steel. Furthermore, high compression loads lead to full residual stress relaxation at the weld toe of S960QL and moderate relaxation for S355J2. High tension loads lead only to slight relaxation.
Highlights
High-frequency mechanical impact (HFMI) is a user-friendly and effective post-weld surface treatment method for improving the fatigue strength of welded steel joints and components
Further investigations under variable amplitude (VA) loading that is more related to industrial applications revealed that the fatigue life improvement might decrease significantly for both mild and high strength steel [2, 3]
The relaxation behavior of HFMI-induced residual stresses was studied by means of X-ray and neutron diffraction techniques at transverse stiffeners made of mild S355J2 steel and high strength S960QL steel
Summary
High-frequency mechanical impact (HFMI) is a user-friendly and effective post-weld surface treatment method for improving the fatigue strength of welded steel joints and components. The benefit of the HFMI treatment is related to compressive residual stresses They can relax under high mean stresses and single stress peaks if the sum of residual stress and load stress exceeds the local material yield strength [4]. For this reason, a reduction in the number of FAT classes with respect to the stress ration of R > 0.15 up to R = 0.52 was proposed by Marquis et al [5] and transferred to the IIW Recommendation [1]. The IIW Recommendation for post weld treatment methods [6] limits the allowable stress for hammer and needle peened joints to 0.8fy, where fy is the base material nominal yield strength.
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