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The existing methods of assessing the fatigue life of welded joints fail to consider local strain ranges and mean stress at the weld toe. The present work proposes a novel approach to assessing the fatigue life of welded joints by conducting measurements with digital image correlation (DIC) and X-ray diffraction (XRD) in combination. Local strain ranges at the weld toe of gusset welded joints were measured by DIC. Hammer peening was conducted on the welded joints to introduce different initial stresses. The influence of mean stress was investigated by considering initial residual stress measured by XRD and a perfect plastic material model. The fatigue experiment was carried out on specimens with and without hammer peening. The results showed that hammer peening could offset adverse welding deformation effectively, and introduce significant residual compressive stress. The fatigue failure life increased by more than 15 times due to hammer peening. The fatigue initiation life assessed by the proposed method was close to that based on nominal stress, indicating that the proposed method is reliable for predicting the fatigue initiation life of welded joints.

The effect of hammer peening on the fatigue resistance of high strength welded steel S690 has been investigated. Cyclic four points bending tests have been performed on butt weld samples, manually and automatically hammered. Microstructural and mechanical modifications induced by both methods and their subsequent influences on fatigue life have been studied using several characterization techniques. A comparison of residual stress measurements using X-ray diffraction shows also differences between methods. On the other hand, hardness profile of the strain hardened region is very similar for both types of samples. The ultra-fine grain microstructure of the heat-affected zone (HAZ) induced by the hammer peening process has been studied by Field Emission Scanning Electron Microscopy and Electron Back Scattered Technique (EBSD). In fatigue as expected, hammer peening increases the fatigue strength of weld structures in good agreement with the latest proposal of the International Institute of Welding (IIW). Compressive residual stresses play a key role. Microstructure does not seem to affect fatigue strength in spite of nanostructured layer. On the contrary to what is usually admitted, the local geometry of the weld toe is deteriorated because the process induces crack-like folds. In this paper, some recommendations are proposed for the hammer peening process in terms of sweeping strategy.

The effect of hammer peening on the fatigue resistance of high strength welded joints in high strength structural steel S690 has been investigated. Cyclic four points bending tests have been performed on butt weld samples, manually and automatically hammered. Microstructural and mechanical modifications induced by both methods and their subsequent influences in fatigue life have been studied using several characterization techniques. It was found that the roughness of manually hammered samples is different than that of automatically hammered samples for which a lack of correct treatment in some regions of the weld cordon has been observed. A comparison of residual stress measurements using X-ray diffraction shows also differences between both methods. On the other hand, hardness profile of the strain hardened region is very similar for both types of samples. The ultra-fine grain microstructure of the heat-affected zone (HAZ) induced by the hammer peening process has been studied by Field Emission Scanning Electron Microscopy and Electron Back Scattered Technique (EBSD). The strain-hardened depth determined by Kernel Average Misorientation (KAM), microhardness and residual stress are compared.

This paper presents the results of the development process for a Finite Element Analysis of the Hammer Peening Fatigue Life Improvement Technique. The Fatigue Life of welded structures is still in need for improvement. The sheer number of Fatigue Live Improvement Techniques parameters leads to the need of simulating and predicting their results. For this study, two different materials were used, an Austenitic Stainless Steel and a Duplex Stainless Steel. Non-load carrying cruciform weld joints were produced and fatigue tested, with and without the Hammer Peening treatment. Finally a FEA code ( ABAQUS® ) was used to simulate the Hammer Peening technique. A fully dynamic model was used, combined with the Chaboche Kinematic-hardening material model and different Hammering parameter experimentally determined. Alongside the residual stresses introduced by the Hammer Peening Technique, the predicted Fatigue Life using the FEA model were compared with the experimental results, showing a very good agreement between them. Also the effect of several parameters, like the hammering impact load, the hammer positioning or the number of hammering passages, were analysed as a way to validate the FEA model. The most important result was of course the Fatigue Strength Gain factor, for the Hammer Peening Technique, that in both cases was found to be superior to 1.3.