Hammer peening is widely adopted for steel structures in order to improve fatigue life of welded joints. The effect of hammer peening, under specific conditions, have been already confirmed in the previous experiments carried out by Morikage (2015). However, two main aspects need further investigations, since, in the current state of art, their contribution to the peening is still unclear. The first one is represented by the presence of a pre-load on the structure (referred as dead load in this paper) whereas the second one is the relaxation of the residual stress field induced by the peening process itself. The purpose of the present study will be the clarification of the interaction among all of this factors by investigating the stress distribution, and its relaxation, by means of FE analyses. The calibration of the material parameters, adopted in the numerical simulation, is done by comparison on the displacement field generated in an actual peening experiment carried out on a steel plate.
Many factors that are known to influence the fatigue life: residual stress, hardening, surface roughness, etc. (Jouno, 1995). Peening is one of the measures widely adopted with the intent to extend the components durability and their service life (Morikage, 2015). The application of this technique generally induces modifications on the geometrical shape, alters the material hardening, and induce a compressive residual stress field as consequence of the impact of the chisel with the surface. A specific form of this technique is the ultrasonic peening (Nose, 2008), easy to control, and well known to perform a uniformly distributed peening effect, however, it will not be treated in the present paper.
Hammer peening can be used directly on structures where the cracks are already opened in order to promote a partial closure and increase the material performances; experiments on this topic were already carried out by Kakiichi (2014). On the other hand, an undesired effect of the application of this process is the generation of a residual stress field inside the material, which can be relaxed through the application of fatigue loading as shown by (Miyashita, 2011). However, nowadays, the effect of the interaction among dead loads, peening and the following application of cyclic loading is still unclear.
Therefore, the present paper aims to investigate the interactions of these factors by means of numerical and experimental evidences. The Subloading Surface theory (Hashiguchi, 1989, 2009) is adopted in the FE analysis to model the elasto-plastic behavior of the material, due to its ability to catch a realistic ratcheting behavior induced by cyclic loads.
