International audienceDuring a manufacturing process, the ultrasonic shot peening (USP) technique can be used as the final surface treatment. The aim of this operation is to introduce surface compressive residual stresses in order to prevent crack propagation advancement. Although the numerical simulation method is able to predict the level of residual stresses in a peened part, the 3D modelling of the real USP process, in which many successive and shifted impacts take place, is very delicate to perform and costly in terms of computing time and memory space required. In this paper, a two step method based at first on the calculation of the averaged plastic strain tensor in a half-space by using a semi-analytical method and in a second time on the transfer of this plastic strain field to a finite element model is proposed in order to simulate the effects of the USP process in thin structures. The accuracy and advantages of the semi-analytical method are validated by a benchmark with several finite element codes. Experiments, similar to the Almen test, are performed on thin plates of Inconel 600. Numerical results in terms of distortions and residual stresses are compared with the experimental data

is an open access repository that collects the work of Arts et Métiers ParisTech researchers and makes it freely available over the web where possible. a b s t r a c tA new semi-phenomenological model is developed based on a mean-field description of the TRIP behavior for the simulation of multiaxial loads. This model intends to reduce the number of internal variables of crystalline models that cannot be used for the moment in metal forming simulations. Starting from local and crystallographic approaches, the mean-field approach is obtained at the phase level with the concept of Mean Instantaneous Transformation Strain (MITS) accompanying martensitic transformation. Within the framework of the thermodynamics of irreversible processes, driving forces, martensitic volume fraction evolution and an expression of the TRIP effect are determined for this new model. A classical self-consistent scheme is used to model the behavior of multiphased TRIP steels. The model is tested for cooling at constant loads and for multiaxial loadings at constant temperatures. The predictions reproduce the increase in ductility, the dynamic softening effect and the multiaxial behavior of a multiphased TRIP steel.

is an open access repository that collects the work of Arts et Métiers ParisTech researchers and makes it freely available over the web where possible. AbstractPhase transformations during nitriding of a ternary carbon iron-based alloy Fe-3%Cr-0.345%C were studied, aiming for a better understanding of residual stresses generation and evolution. The relationship between the precipitation of Cr 7 C 3 carbides and CrN nitrides, the induced volume change and the mechanical properties were investigated at three distinct depths of the diffusion zone. The relaxation of residual stresses arose through phase transformations according to the diffusion of nitrogen but also of carbon.

This paper investigates the relevance of the representation of polycrystalline aggregates using Radical Voronoï (RV) tessellation, computed from Random Close Packs (RCP) of spheres with radius distribution following a lognormal distribution. A continuous relationship between the distribution of sphere radii with that of RV cell volumes is proposed. The stereology problem (deriving the 3D grain size distributions from 2D sections) is also investigated: two statistical methods are proposed, giving analytical continuous relationships between the apparent grain size distribution and the sphere radius distribution. In order to assess the proposed methods, a 3D aggregate has been generated based on a EBSD map of a real polycrystalline microstructure.

The estimation of the grain size in granular materials is usually performed by 2Dobservations. Unfolding the grain size distribution from apparent 2D sizes is commonly referred as the corpuscle problem. For spherical particles, the distribution of the apparent size can be related to that of the actual size thanks to the Wicksell’s equation. The Saltikov method, which is based on Wicksell’s equation, is the most widely used method for resolving corpuscle problems. This method is recursive and works on the finite histogram of the grain size. In this paper, we propose an algorithm based on a minimizing procedure to numerically solve the Wicksell’s equation, assuming a parametric model for the distribution (e.g. lognormal distribution). This algorithm is applied on real material and the results are compared to those found using Saltikov or Saltikov-basedstereology techniques. A criterion is proposed for choosing the number of bins in the Saltikov method. The accuracy of the proposed algorithm, depending on the sample size, is studied.

International audienceManufacturing industries perform mechanical surface treatments like shot peening at the end of the manufacturing chain to protect important working parts. This treatment modifies the near surface of the treated part with the introduction of compressive residual stresses due to the repeated impacts of the shot. Then, the treated part exhibits, not only a longer life, but also a better fretting behavior, an improved resistance to corrosion… The objective of the present paper is first to study the relation between the process parameters and the material state (residual stress and plastic variables…) for a complex geometry. Next, a numerical tool is proposed, able to predict this material state in a time frame that is consistent with industrial constraints. The originality of the proposed approach thus consists in the chaining of the different steps. The first step is to choose the process parameters for the shot peening process considering conventional or ultrasonic shot peening and model the shot dynamics for a complex geometry. Once the impact velocity field is known, the objective is to compute the local incompatible plastic deformation field due to the repeated impacts using analytical methods. Then, a finite element model is used to compute the residual and deformation fields in the considered mechanical part. The complete method has been performed on the model of a gear, a mechanical part that is most often shot peened and exhibits a complex geometry

During conventional shot peening on metastable austenitic steels, martensitic transformation occurs in addition to plastic straining. In this work, the impact of a single spherical steel shot on a AISI 301LN steel was studied. The volume fraction of martensite, residual stresses in both phases were determined in the vicinity of the dent as a function of the shot velocity and diameter. An elasto-plastic two phase model that includes martensitic phase transformation was adapted to model mechanical and microstructural fields and implemented in Abaqus Explicit for the 2D simulation of a single shot impact. It was found, for instance, that the martensitic transformation takes place only under the dent and that martensite is in tension at the surface while austenite is in compression. Simulation results of stress levels showed a good agreement with experimental stresses determined by X-ray diffraction.

Background The slitting method is a widely used destructive technique for the determination of residual stresses. Because of the rich data content of the full-field methods, optical techniques such as digital image correlation (DIC) are replacing strain gages for surface measurements. Objective The objective of the current paper is to overcome the difficulties that arise in using the DIC technique combined with the slitting method. The present noise, low signal-to-noise ratio, and systematic errors are the main impediments to the use of DIC in the slitting method. Methods An approach based on the eigenstrain concept was exploited to ascertain the optimum region of interest (ROI) for the analysis. After that, a robust procedure was implemented to utilize the DIC method while excluding the rigid body motion and rotation artifacts from the obtained displacements. Results Different slitting steps may cause dissimilar rigid body motions and rotations of the specimen. The proposed method was able to eliminate all of these different shears and stretches in the images simultaneously. The slitting experiment was conducted on a symmetric cross-ply composite specimen, and the slit progressed down to half the thickness. Although some rigid body motions were large, the method managed to exclude all of them for eight slitting steps. Conclusion A comparison made between the results of the current method and those of the strain gage technique shows that they are in acceptable agreement with each other, and this full-field method can be extended to smaller scales or other destructive techniques.