Issue 65

M. Zhelnin et alii, Frattura ed Integrità Strutturale, 65 (2023) 100-111; DOI: 10.3221/IGF-ESIS.65.08

According to the results of the fatigue test, the LSP pattern № 1 doesn’t have a significant influence on the fatigue life of the treated specimens. The average number of cycles before fracture of the base material and specimens treated by LSP pattern № 1 is 130·10 3 cycles and 142·10 3 cycles, respectively. At the same time, the LSP pattern № 2 induces a significant grow in the fatigue life (more than 10 6 cycles). Two specimens were broken near the grip (i.e. not in the treated semi circular notched area). Another two specimens indicated by arrow in Fig.4 weren’t broken at all. This experimental observation is probably related to the most optimal configuration of residual stress field generated by LSP in the stress concentrator region. In this case, the applied stress level, at which the fatigue crack grows in the base material, is not enough for a fatigue crack initiation in specimens after LSP by the pattern № 2. To confirm this assumption, it is necessary to analyze the residual stress distribution in the stress concentrator region. Experimental measurements of residual stress distribution in this region are difficult. Therefore, to analyze the distribution of residual stresses in the specimen after the application of the LSP pattern № 2 and explain the fatigue life improvement after LSP a numerical simulation has been carried out. s it was shown above, LSP pattern № 1 didn’t improve the fatigue life of the notched specimens. Similar results were obtained by other researchers [10, 13]. They obtained no improvement in the fatigue life of samples with stress concentrators. These results were related to the deteriorating effect of tensile residual stresses which occur in the specimen after the treatment. It was observed [10, 13] that tensile residual stresses produced in the mid-section of the specimen with stress concentrator during LSP of its front and rear sides contribute to negative effect on its fatigue performance. The LSP scheme № 2 is different and laser shots are applied directly at the concentrator. This peening pattern leads to a significant improvement in the fatigue performance of specimens. To analyze distribution of residual stresses in the sample after application of this pattern and explain these results the numerical simulation has been carried out. The numerical simulation was conducted using a finite-element model of a multi-shot LSP process which was developed in [14]. Finite-element model In this work, only a brief description of the material model and numerical simulation process is given. More detailed information can be found in [14]. Similar to [15-19], LSP was considered as a purely mechanical process. Therefore, the formation of high-pressure plasma and surface ablation were neglected during the simulation. The effect of the laser pulse was taken into account by imposing mechanical pressure on the treated surface. Numerical simulation was performed in finite-element software Abaqus. Fig. 5 shows the geometry of the computational domain. The geometry corresponds to the tested samples. Boundary conditions were assigned according to the experimental procedure. The half-circled zone near both ends of the gage at the front side (shaded areas at Fig. 5(a)) as well as half of the opposite side of the sample (without stress concentrator) (shaded area at Fig. 5(b)) were fixed. These conditions correspond to the gripping of the specimen by robotic arm in the LSP process. The specimen was discretized by 8-node linear brick elements with reduced integration (C3D8R). A more refined mesh was applied at the peened area. Convergence study had shown that optimum size of the sample in this region was equal to 0.1 mm. In other regions mesh size was increased with the maximum element size which didn’t exceed 2.5 mm. The peening strategy was the same as in the LSP pattern № 2. Only the bore of the notch was subjected to LSP. The peening zone was a semicircular domain with chords at a distance of 4 mm from the external stress concentrator edges. Each laser shot was modeled by the two-step approach. The first step was dynamic which simulated elasto-plastic stress wave propagation induced by pulse loading. At this step, an explicit time integration scheme was applied. Laser pulse was imposed as a pressure boundary condition acting at the square region with an edge size of 1 mm of the stress concentrator surface. Following [16,20,21], it was assumed that pressure function for square spots is spatially homogeneous. A simple triangular approximation of dependence of the pressure P on time t was used as it has been proved to be valid for LSP simulation [15,22,23]: A N UMERICAL SIMULATION

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