Influence of the target material constitutive model on the numerical simulation of a shot peening process
Introduction
Shot peening (SP) is one of the most widely used mechanical surface treatments generally applied to improve the fatigue behaviour of metallic components. During this process, a cold plastic deformation in the surface of the work piece is generated by peening the surface with small spherical shots impacting at high speed. The large localized plastic deformation on the surface of the material produces a field of compressive residual stresses [1], [2], responsible for the improvement in fatigue behaviour. The factors involved in shot peening process are so numerous and the extent of their individual influence may become so complex that it sometimes becomes economically unfeasible to conduct analysis via experimentation alone.
This is the reason why numerical modelling of the process using the finite element method is an increasingly common option. The complexity of the numerical model is clear, as shot peening is a dynamic problem in which multiple impacts are involved. Accordingly, a great computational cost is required. Nonetheless, one of the most important aspects to perform a reliable simulation of the shot peening process is the constitutive model used to describe the target material behaviour. On the one hand, the material is subjected to the action of shots that impact the surface at high speeds, so it is obvious that the constitutive material model used in simulations should take into account the effect of the strain rate. These types of models are described by isotropic hardening laws. However, isotropic models are only valid if the load process is monotonic, or in case of cyclic loads when Bauschinger effects are not present [3]. As the shot peening process involves the action of repeated loads, the cyclic response of the material must be assessed. If the material does not exhibit the Bauschinger effect, the aforementioned isotropic model may be correct; otherwise, a kinematic hardening model should be used [4].
Even though the importance of using an adequate model to describe the mechanical behaviour of the material to be treated has been highlighted, scientists working on the modelling of shot peening process have yet to address the problem in all its magnitude. Most studies use isotropic hardening models, which do not take into account the cyclic behaviour of the target material [5], [6], [7], [8], [9], [10], [11], [12], [13], while others use only kinematic models, which do not take into account strain rate effects [3], [14], [15], [16], [17].
The aim of the present study is to analyse the importance of the constitutive material model used to describe the mechanical behaviour of the target material on the results obtained from the numerical modelling of the shot peening process. This will involve a comprehensive experimental characterization of a duplex stainless steel, as well as the proposal of different constitutive models based on the type of action assessed: monotonic dynamic load or cyclic loading. Once incorporated in finite element analyses, the goodness of each constitutive model will be analysed by comparing the numerical predictions obtained for residual stresses and surface roughness with experimental results.
Section snippets
Material and experimental characterization
The material used in this study was a duplex (AISI 2205 grade) stainless steel. This material was supplied as hot rolled bars, with a nominal diameter of 16 mm. The chemical composition of this steel is shown in Table 1. Metallographic analyses performed on longitudinal and transversal sections of the bars show a duplex α/γ microstructure (50%/50%).
Static tension tests were carried out according to ASTM E8M [16] in order to obtain the monotonic properties of the material (Young's Modulus, E;
Experimental characterization
Duplex stainless steel discs (16 mm diameter and 10 mm thickness) extracted from the bars were subjected to a shot peening process using a GUYSON Euroblast 4PF direct pressure pneumatic machine and S-230 steel shots with a nominal diameter of0.6 mm. The air pressure, shot flow and distance between the nozzle and specimen were chosen to achieve a13A Almen Intensity SAE J442 [30] and SAE J443 [31].
In order to obtain the other test parameters needed to define the numerical model, accurate
Axisymmetric model: prediction of shot velocity
In the case of the axisymmetric model, only the effect of a single impact will be assessed. In this case, the desired result is the size of dimple generated after impact, instead of the residual stress field induced below the impacted surface. Thus, if the former stress profile is represented as in Fig. 6, a tensile stress zone can be observed. This is in good agreement with the results of other researchers [44], [45], [46], but it is not representative of the final state of the material after
Conclusions
In this paper, an AISI 2205 duplex stainless steel was described by means of a novel non-linear kinematic isotropic model which also takes into account the effect of the strain rate through its isotropic component.
By means of a 3D numerical simulation, it has been proven that the proposed model can reproduce the behaviour of the material subjected to an actual shot peening treatment, providing a very accurate prediction of both the residual stress field and the surface roughness generated by
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