Emmanuelle Rouhaud

3D Finite Element Models of Shot Peening Processes

The principle of shot peening is to hit the surface of a metallic part with spherical shots to introduce a compressive residual stress field by plastification. The compressive stress introduced by shot peening lowers the number of cracks initiation on the materials surface. This process is thus used to increase fatigue life of many mechanical parts. The efficiency of shot peening depends on parameters such as the coverage rate or the size and speed of the shots. The values chosen for these parameters should be accurate to obtain the required fatigue life; investigations on the influence of the shot peening parameters, should be efficient and easy. A few analytical models have been proposed to better evaluate the residual stress field of a shot peened part, but most of the knowledge on shot peening processes today is empirical. In this study we chose to use a finite element model. Indeed, finite element codes are now powerful enough to simulate shot-peening processes extensively and to analyze the influence of the relevant parameters. This paper presents finite element simulations of shot peening processes and the residual stress fields obtained. We validated the model with a confrontation between numerical and experimental results. Two types of materials often used in aircraft and automobile industry were chosen for the study. The mechanical properties of these two materials were experimentally measured. In the same way, the coverage rate, the diameter and the speed of shots used to treat the surface of the materials were evaluated. Selected data were extracted from the literature and experimental measurements carried out in our laboratory. Three-dimensional models reproducing the experimental conditions were then investigated to simulate the shot peening of these materials. Results such as the displacement and the residual stress field are presented and compared to experimental results when possible. The models proposed show how the finite element simulation is able to predict the residual stress field of a shot peened part. The possibility to vary parameters values easily confirms the interest of shot peening numerical simulations.

Modelling of the Ultrasonic Shot Peening Process

The aim of this work is to reach a better understanding of the ultrasonic shot-peening process and, in particular, the evolution of the shot speed distribution. A simple 1D modelling of the interaction between the shots and the sonotrode is carried out. The impact is considered as inelastic with an energy absorption that depends on the speed of the shot. It is found that after about 10 interactions (» 1s) the speed distribution in the chamber follows a Maxwell-Boltzmann distribution, which is the distribution found in a perfect gas at equilibrium. The influence of various process parameters such as the sonotrode amplitude, the vibration frequency on the average speed and on the Almen intensity is studied.

A Kinematic Hardening Finite Elements Model to Evaluate Residual Stresses in Shot-Peened Parts, Local Measurements by X-Ray Diffraction

Experimental analysis can be very costly and time consuming when searching for the optimal process parameters of a new shot-peening configuration (new material, new geometry of the part…). The prediction of compressive residual stresses in shot-peened parts has been an active field of research for the past fifteen years and several finite elements models have been proposed. These models, although they give interesting qualitative results, over-estimate, most of the time, the level of the maximal compressive stresses. A better comprehension of the phenomena and of the influence of the parameters used in the model can only carry a notable improvement to the prediction of the stresses. The fact that the loading path is cyclic and is not radial led us to think that a model including kinematic hardening would be better adapted for the modelling of shot peening. In this article we present the results of a simulation of a double impact for several constitutive laws. We study the effect of the chosen constitutive law on the level of residual stresses and, in particular, we show that kinematic hardening, even identified on the same tensile curve than isotropic hardening, leads to lower stress levels as compared with isotropic hardening. Furthermore, the overall shape of the stress distribution within the depth is significantly different for the two types of hardening behaviour. Further, in order to check the modelisations, local measurements were carried on with X-ray diffraction on a large size impact and correlated with the topography of the impact.

CAD based model of ultrasonic shot peening for complex industrial parts

This paper presents a numerical model developed specifically for ultrasonic shot peening (USP). It allows simulating the shot dynamics (trajectories in the chamber and impacts on the peened sample) in industrial configurations. The model supports complex 3D geometries, rotating parts and employs efficient collision detection algorithms for short computation times. The aim is to improve peening chamber designs and the choice of process parameters. The algorithm and main assumptions are presented. Numerical studies are then conducted to determine the performances of the model, in terms of computation time. Finally, a case study on a spur gear tests the model in an industrial configuration and shows a high correlation between the numerical results and experimental data.

S151 NUMERICAL AND EXPERIMENTAL INVESTIGATION ON SHOT-PEENING INDUCED DEFORMATION: APPLICATION TO SHEET METAL FORMING

The peen forming process is commonly used in the aeronautical industry to form large wing skin panels. This process presents many advantages in terms of cost, production time, and beneficial induced residual stresses. Setting the accurate process parameters to form a given pattern requires however a certain experience and sometimes many trials and errors. The authors propose to model the shot peening induced deformations by elastically equilibrating the real plastic strain gradient present in the whole structure. A complete numerical and experimental protocol is proposed to determine the plastic strain gradient assuming the knowledge of some experimental data. The proposed approach consists in determining the plastic strain field by inverse calculation based on experimentally measured residual stresses fields. A second approach is based on the inverse calculation of shot peening induced plastic strains knowing the global deformation of the treated sheet. A comparison between the proposed approaches applied to partially treated sheets is presented along with experimental data. The numerical results are in good agreement with experimental data and show the strong influence of the peening path on the resulting global deformations of the treated samples.

Study of the effects of grain size on the mechanical properties of nanocrystalline copper using molecular dynamics simulation with initial realistic samples

Abstract Molecular dynamics simulations have been performed to study the mechanical properties of a columnar nanocrystalline copper with a mean grain size between 9.0 and 24 nm. A melting–cooling method has been used to generate the initial samples: this method produces realistic samples that contain defects inside the grains such as dislocations and vacancies. The results of uniaxial tensile tests applied to these samples reveal the presence of a critical mean grain size between 16 and 20 nm, for which there is an inversion of the conventional Hall–Petch relation. The principal mechanisms of deformation present in the samples correspond to a combination of dislocations and grain boundary sliding. In addition, this analysis shows the presence of sliding planes generated by the motion of perfect edge dislocations that are absorbed by grain boundaries. It is the initial defects present inside the grains that lead to this mechanism of deformation. An analysis of the atomic configurations further shows that nucleation and propagation of cracks are localised on the grain boundaries especially on the triple grains junctions.

A new Approach in Residual Stresses Analysis by Speckle Interferometry

The knowledge of residual stresses allows a reliable prediction of structure performances evolution, such as service life [1-3]. In this paper, we develop a new method for residual stresses determination combining Electronic Speckle Pattern Interferometry (ESPI) with the machining of a groove. The internal stress field is perturbed as the depth of the groove is increased incrementally. The structure finds a new equilibrium state generating displacements which are measured using ESPI. This method was tested on an aluminium alloy AU4G plate treated locally by an ultrasonic shot-peening. The investigation of the images obtained with the phase shifting technique and fringe patterns, makes it possible to analyze, simultaneously, the stress profile along two directions: along the depth of the structure, and along the groove direction.

Analysis of the Impact of a Shot at Low Velocity Using the Finite Element Method. Application to the Ultrasonic Shot-Peening Process

Shot-peening is a surface treatment widely used in the industry to improve fatigue life of mechanical components by introducing compressive residual stresses. Ultrasonic shot-peening is a recent development of this process. While the classical shot-peening process uses pneumatic energy to project the shots, ultrasonic peening uses high-power ultrasounds. This energy source allows the use of larger shots projected at lower velocity as compared to classical shot-peening. This work aims at studying the mechanical response (restitution coefficient, residual stress field) of a surface impacted by a shot at low velocity using the finite element method and experimental analysis. This paper presents the simulation of a single elastic steel shot normally impacting an Aluminum alloy plate considered to exhibit a linear-elastic behavior and non-linear isotropic work hardening characteristics. The numerical simulations are carried out for different impact velocities in order to take into account the heterogeneous shot velocity field observed in an ultrasonic shot-peening chamber. We compare the simulated rebound energy and the indentation profiles obtained for different impact velocities to experimental results. The simulated residual stress field topology shows a strong dependence on the shot velocity. While numerical results obtained at high impact energy agree well with literature results, the residual stress distribution simulated for low impact energies shows a tensile layer below the impacted area. The restitution coefficients and the indentation profiles compare well with the experiments.

Modelling of Grain Refinement Induced by SMAT Process, Using a Complete Numerical Chaining Methodology

Surface Mechanical Attrition Treatment (SMAT) is a recent process that enables to nanocrystallise the surface of metallic alloys. It can thus enhance mechanical properties of the treated material by inducing a grain refinement down to the nanometre scale, in the top surface layer. This nanocrystallisation process leads to different effects that were successively studied on several metallic materials. In the present work, investigations are carried out on the modelling of SMAT. A simulation of the shot dynamics is performed using different process parameters, with the aim to obtain the impact velocity field on the treated surface. This field is then used as an input for a finite element model to predict the induced grain refinement. The evolution of the micro and nanostructures are then calculated using a micromechanical approach, which takes into account the dislocations and their interactions. Coupled with a finite element analysis, this approach enables to deduce the influence of the process on the macroscopic material properties, whatever the geometry of the sample.

DFM synthesis approach based on product-process interface modelling. Application to the peen forming process.

Engineering design approach are currently CAD-centred design process. Manufacturing information is selected and assessed very late in the design process and above all as a reactive task instead of being proactive to lead the design choices. DFM approaches are therefore assessment methods that compare several design alternatives and not real design approaches at all. Main added value of this research work concerns the use of a product-process interface model to jointly manage both the product and the manufacturing data in a proactive DFM way. The DFM synthesis approach and the interface model are presented via the description of the DFM software platform.