Numerical models for the simulation of shot peening induced residual stress fields: from flat to notched targets

Shot peening is a cold-working surface treatment, basically consisting in pelting the surface of the to-be-treated component with a high number of small hard particles blown at relatively high velocity. This causes the plasticization of the surface layer of the substrate, and the generation of a compressive residual stress field beneath the component surface. The surface topology modification can be beneficial for coating adhesion, and the work hardening enhances the fretting resistance of components, but the most commonly appreciated advantage of the process is the increased fatigue resistance in the treated component, due to the compressive residual stress which inhibits the nucleation and propagation of fatigue cracks. In spite of its widespread use, the mechanisms underlying the shot peening process are not completely clear. Many process parameters are involved (material, dimension, velocity of the shots, coverage, substrate mechanical behavior) and their complex mutual interaction affects the success of the process as well as the jeopardizing of any beneficial effect due to the increased surface roughness. Experimental measurements are excessively expensive and time-costly to deal with the wide variability of the process parameters, and their feasibility is not always granted. The effect of shot peening is indeed particularly effective where geometrical details (e.g. notches or grooves) act as stress raisers and where the direct measurement of residual stresses is very difficult. Nonetheless, the knwoledge of the effects of the treatment in this crictical locations would be extremely useful for the quantitative assessment of the effect of shot peening and, ultimately, for the optimization fo the process as well as its complete integration in the design process. The implementation of the finite element method for the simulation of shot peening has been studied since many years. In this thesis the simulation of shot peening is studied, in order to progress towards a simulation approach to be used in the industrial practice. Specifically, the B120 micro shot peening treatment performed with micrometric ceramic beads is studied, which has proven to be very effective of aluminum alloys, such as the aeronautical grade Al7075-T651 alloy considered in this work. The simulation of shot peening on a flat surface is addressed at first. The nominal process parameters are used, to include stochastic variability of the shot dimensions and velocity. A MatLab routine based on the linearization of the impact dent dimension, on the shot dimension and velocity is used to assess the coverage level prior to the simulation and predict the number of shots to full coverage. To best reproduce the hardening phenomena of the substrate material under repeated impacts, the Lemaitre-Chaboche model is tuned on cyclic strain tests. Explicit dynamic finite element simulations are carried out and the statistical nature of the peening treatment is taken into account. The results extracted from the numerical analyses are the final surface roughness and residual stresses, which are compared to the experimentally measured values. A specific novel procedure is devised to account for the effect of surface roughness and radiation penetration in the in-depth residual stress profile. In addition, a static finite element model is devised to assess the concentration effect exerted by the increased surface roughness on an external stress. The simulation of shot peening on an edge is then addressed as a first step towards more complex geometries. Since the true peening conditions are not known in this locations, a synergistic discrete element - finite element method approach is chosen for the correct modelization of the process. A discrete element model of the peening process on a flat surface is used to tune the simulation on the nominal process parameters, i.e. mass flow rate and average shot velocity, and to assess the nozzle translational velocity. Discrete element simulations are used to simulate the process when the nozzle turns around the edge tip. To lower the computing cost, the process is linearized into static-nozzle simulations at different tilting angles. The number of impacting shots and their impact velocity distribution are used to set up the finite element simulations, from which the resulting residual stress field is obtained. In addition to the realistic simulation, two simplified simulation approaches for the practical industrial use are devised. The resulting residual stress fields are compared with the reference residual stress field computed using thermal fields in a finite element simulation, tuned with experimental XRD measurements. The effect of the dimension of the fillet on the edge tip is studied by modifying the finite element model of shot peening on an edge. 3 different fillet radii (up to 40 um) are considered, on the basis of experimental observations. The resulting residual stress field are compared to analyze the effect of the precise geometry of the substrate. Lastly, the simplified simulation approach devised in the case of the edge is used to simulate shot peening on the root of a notch. The resulting residual stress field is again compared to the reconstructed reference one.