Numerical and Theoretical Study of Shot Peening and Stress Peen Forming Process

RESUME Le grenaillage de precontrainte est un procede d’ecrouissage largement utilise dans les industries aerospatiale et automobile pour ameliorer la resistance a la fatigue de composants metalliques et ainsi prolonger leur duree de vie utile. La performance et la repetabilite de cette amelioration dependent largement des parametres de grenaillage, tels que l’intensite Almen, le taux de recouvrement, la duree du traitement de grenaillage, etc. La methode d’analyse par elements finis a ete adoptee pour simuler le procede de grenaillage en raison du developpement de plus en plus pousse des methodes numeriques et de l’augmentation des ressources et capacites informatiques disponibles. La plupart des modeles par elements finis du procede de grenaillage, tels que les modeles 2D ou les modeles a impact unique, ne peuvent representer adequatement ce procede qui implique un grand nombre d’impacts de billes distribuees de facon aleatoire. Par consequent, un des objectifs principaux de cette etude a ete de developper un modele 3D capable de simuler ce procede aleatoire, et d’utiliser ce nouveau modele afin d’etudier de facon quantitative les relations entre les parametres de grenaillage et les resultats de ce traitement. La mise en forme par grenaillage avec application d’un pre-chargement externe est une utilisation particuliere du grenaillage de precontrainte lors de la fabrication de composantes a paroi mince. Ce procede est largement utilise dans l’industrie aeronautique afin de produire des composantes minces avec des formes geometriques complexes, telles que des revetements de voilure. La plupart des etudes publiees portant sur ce procede consistent en des etudes experimentales d’essais et erreurs. Par consequent, un autre objectif de cette etude a ete de developper un modele numerique afin de simuler ce procede, et d’investiguer quantitativement les relations entre les conditions de pre-chargement (moment de flexion ou force en tension appliquee) et la forme resultante (fleche maximale resultante ou rayon de courbure). Un nouveau modele 3D par elements finis capable de simuler les impacts de billes distribues de facon aleatoire a ete developpe afin de simuler de facon dynamique le grenaillage de precontrainte. En utilisant ce modele aleatoire, des relations quantitatives ont ete etablies entre l’intensite du jet de billes, le taux de recouvrement, la rugosite induite et le nombre d’impacts, pour une composante en aluminium 2024. Les resultats de simulation montrent que ce nouveau modele par elements finis aide a bien comprendre le procede et permet de mieux predire les----------ABSTRACT Shot peening is a cold working process widely used to improve the fatigue life of metallic components in the aerospace and automobile industries. The performance and repeatability of this improvement depend greatly on the shot peening parameters, such as Almen intensity, surface coverage, peening time, etc. Finite Element Analysis (FEA) has been adopted to simulate the shot peening process based on the development of computer resources and numerical methods. Most of the existing Finite Element (FE) models, however, such as 2D models and one shot impact models, cannot describe numerous randomly distributed shot flow. Therefore, one goal of this study was to establish a 3D shot peening model that can simulate a random shot peening process and to study the quantitative relationship between peening parameters and peening results. Stress peen forming is a special application of shot peening that deals with thin target components. Stress peen forming has been widely used in the aeronautics industry to produce thin components with complex shapes, such as wing skins. Most studies of the stress peen forming process have been based on trial and error experiments. Therefore, another objective of this study was to develop a numerical model in order to simulate this process, and to investigate the quantitative relationship between the prebending conditions (prebending moment or prestress) and the forming results (resulting arc height or radius of curvature). A novel 3D FE model that describes randomly distributed shots was developed in order to simulate the dynamic shot peening process. Using this 3D random FE model, a quantitative relationship was established between peening intensity, surface coverage and roughness, with respect to the number of shots, based on the same target component, aluminum 2024. The simulated results show that the novel FE model can help us to understand and predict shot peening results better than the existing conventional FE models. An experimental study of shot peening and stress peen forming was carried out in order to validate the novel FE model. In shot peening experiments, the quantitative relationships between saturation, surface coverage and roughness, with respect to peening time, were established for aluminum 2024 test strips. A pre-stressing device was designed to apply prebending moments on the strip in order to perform stress peen forming. The principal conclusion was that with increasing prebending moment, the deformed arc height following the prebending direction increases.