Validity of Crystal Plasticity Models Near Grain Boundaries: Contribution of Elastic Strain Measurements at Micron Scale

Synchrotron Laue microdiffraction and digital image correlation measurements were coupled to track the elastic strain field (or stress field) and the total strain field near a general grain boundary in a bent bicrystal. A 316L stainless steel bicrystal was deformed in situ into the elasto-plastic regime using a four-point bending setup. The test was then simulated using finite elements with a crystal plasticity model comprising internal variables (dislocation densities on discrete slip systems). The predictions of the model are compared with both the total strain field and the elastic strain field obtained experimentally. While activated slip systems and total strains are reasonably well predicted, elastic strains appear overestimated next to the grain boundary. This suggests that conventional crystal plasticity models need improvement to correctly model stresses at grain boundaries

Numerical Modeling of Damage in Nanostructured Materials Obtained by Severe Plastic Deformation

Les matériaux nanostructurés obtenus notamment par déformation plastique sévère (SPD), également appelés matériaux à grains ultrafins, constituent une classe émergente de matériaux avancés qui offrent de nouvelles possibilités en termes de propriétés fonctionnelles et structurelles en combinant résistance et ductilité élevées. La combinaison d'une taille de grain ultrafine et de densités de dislocation élevées permet d'améliorer paradoxalement à la fois la résistance et la ductilité des métaux, contrairement aux méthodes de formage conventionnelles telles que le laminage ou l'étirage. Les matériaux obtenus par un processus de déformation plastique sévère semblent donc très intéressants pour les applications structurelles avancées.La simulation du processus SPD est assez difficile car il implique une déformation plastique excessive et une non-linéarité due aux conditions de contact. De nombreuses études ont été réalisées sur la modélisation des processus SPD. Cependant, un aspect important de ces processus, l'endommagement, a toujours été négligé. De nombreux modèles d'endommagement physiques et phénoménologiques ont été développés mais aucun n'a été implémenté dans un cas sévère tel que les processus SPD.Dans cette thèse, nous essayons d'implémenter des modèles microstructuraux récents basés sur l'évolution de la densité de dislocation dans les processus SPD et en implémentant des lois d'évolution de l'endommagement pendant la simulation de ces processus, un cadre de calcul sera développé afin de prédire l'évolution de la microstructure et de l'endommagement pendant le SPD. Cela permet d'améliorer la compréhension du compromis résistance-ductilité dans le SPD et d'optimiser les conditions de traitement afin de minimiser les dommages et d'améliorer les propriétés du matériau traité.Nanostructured materials obtained notably through severe plastic deformation (SPD), alternatively called bulk ultrafine grained materials (UFG), are an emerging class of advanced materials that bring new possibilities in terms of functional and structural properties by combining high strength and ductility. The combination of ultrafine grain size and high dislocation densities permits to improve paradoxically both the strength and the ductility in metals, in contrast with conventional forming methods such as rolling or drawing. Materials obtained by intense plastic deformation processing thus appear very attractive for advanced structural applications.Simulation of SPD process is quite challenging as it involves excessive plastic deformation and nonlinearity due to contact conditions. Many studies have been done on modelling the intensive plastic deformation during SPD processes. However, an important aspect of these processes, namely, damage has always been neglected. Many physical and phenomenological damage models have been developed but none have been implemented in a severe case such of SPD processes.In this thesis, we try to implement recent microstructural models based on dislocation density evolution in SPD processes and by implementing damage evolution laws during the simulation of these processes a computational framework will be developed in order to predict the evolution of microstructure and damage during SPD. This permits to improve the understanding of strength-ductility trade-off in SPD and optimize the processing conditions in order to minimize the damage and enhance the properties of the processed material

Modélisation numérique de l’endommagement dans les matériaux nanostructurés obtenus par déformation plastique sévère

Nanostructured materials obtained notably through severe plastic deformation (SPD), alternatively called bulk ultrafine grained materials (UFG), are an emerging class of advanced materials that bring new possibilities in terms of functional and structural properties by combining high strength and ductility. The combination of ultrafine grain size and high dislocation densities permits to improve paradoxically both the strength and the ductility in metals, in contrast with conventional forming methods such as rolling or drawing. Materials obtained by intense plastic deformation processing thus appear very attractive for advanced structural applications.Simulation of SPD process is quite challenging as it involves excessive plastic deformation and nonlinearity due to contact conditions. Many studies have been done on modelling the intensive plastic deformation during SPD processes. However, an important aspect of these processes, namely, damage has always been neglected. Many physical and phenomenological damage models have been developed but none have been implemented in a severe case such of SPD processes.In this thesis, we try to implement recent microstructural models based on dislocation density evolution in SPD processes and by implementing damage evolution laws during the simulation of these processes a computational framework will be developed in order to predict the evolution of microstructure and damage during SPD. This permits to improve the understanding of strength-ductility trade-off in SPD and optimize the processing conditions in order to minimize the damage and enhance the properties of the processed material.Les matériaux nanostructurés obtenus notamment par déformation plastique sévère (SPD), également appelés matériaux à grains ultrafins, constituent une classe émergente de matériaux avancés qui offrent de nouvelles possibilités en termes de propriétés fonctionnelles et structurelles en combinant résistance et ductilité élevées. La combinaison d'une taille de grain ultrafine et de densités de dislocation élevées permet d'améliorer paradoxalement à la fois la résistance et la ductilité des métaux, contrairement aux méthodes de formage conventionnelles telles que le laminage ou l'étirage. Les matériaux obtenus par un processus de déformation plastique sévère semblent donc très intéressants pour les applications structurelles avancées.La simulation du processus SPD est assez difficile car il implique une déformation plastique excessive et une non-linéarité due aux conditions de contact. De nombreuses études ont été réalisées sur la modélisation des processus SPD. Cependant, un aspect important de ces processus, l'endommagement, a toujours été négligé. De nombreux modèles d'endommagement physiques et phénoménologiques ont été développés mais aucun n'a été implémenté dans un cas sévère tel que les processus SPD.Dans cette thèse, nous essayons d'implémenter des modèles microstructuraux récents basés sur l'évolution de la densité de dislocation dans les processus SPD et en implémentant des lois d'évolution de l'endommagement pendant la simulation de ces processus, un cadre de calcul sera développé afin de prédire l'évolution de la microstructure et de l'endommagement pendant le SPD. Cela permet d'améliorer la compréhension du compromis résistance-ductilité dans le SPD et d'optimiser les conditions de traitement afin de minimiser les dommages et d'améliorer les propriétés du matériau traité

Modélisation numérique de l’endommagement dans les matériaux nanostructurés obtenus par déformation plastique sévère

Nanostructured materials obtained notably through severe plastic deformation (SPD), alternatively called bulk ultrafine grained materials (UFG), are an emerging class of advanced materials that bring new possibilities in terms of functional and structural properties by combining high strength and ductility. The combination of ultrafine grain size and high dislocation densities permits to improve paradoxically both the strength and the ductility in metals, in contrast with conventional forming methods such as rolling or drawing. Materials obtained by intense plastic deformation processing thus appear very attractive for advanced structural applications.Simulation of SPD process is quite challenging as it involves excessive plastic deformation and nonlinearity due to contact conditions. Many studies have been done on modelling the intensive plastic deformation during SPD processes. However, an important aspect of these processes, namely, damage has always been neglected. Many physical and phenomenological damage models have been developed but none have been implemented in a severe case such of SPD processes.In this thesis, we try to implement recent microstructural models based on dislocation density evolution in SPD processes and by implementing damage evolution laws during the simulation of these processes a computational framework will be developed in order to predict the evolution of microstructure and damage during SPD. This permits to improve the understanding of strength-ductility trade-off in SPD and optimize the processing conditions in order to minimize the damage and enhance the properties of the processed material.Les matériaux nanostructurés obtenus notamment par déformation plastique sévère (SPD), également appelés matériaux à grains ultrafins, constituent une classe émergente de matériaux avancés qui offrent de nouvelles possibilités en termes de propriétés fonctionnelles et structurelles en combinant résistance et ductilité élevées. La combinaison d'une taille de grain ultrafine et de densités de dislocation élevées permet d'améliorer paradoxalement à la fois la résistance et la ductilité des métaux, contrairement aux méthodes de formage conventionnelles telles que le laminage ou l'étirage. Les matériaux obtenus par un processus de déformation plastique sévère semblent donc très intéressants pour les applications structurelles avancées.La simulation du processus SPD est assez difficile car il implique une déformation plastique excessive et une non-linéarité due aux conditions de contact. De nombreuses études ont été réalisées sur la modélisation des processus SPD. Cependant, un aspect important de ces processus, l'endommagement, a toujours été négligé. De nombreux modèles d'endommagement physiques et phénoménologiques ont été développés mais aucun n'a été implémenté dans un cas sévère tel que les processus SPD.Dans cette thèse, nous essayons d'implémenter des modèles microstructuraux récents basés sur l'évolution de la densité de dislocation dans les processus SPD et en implémentant des lois d'évolution de l'endommagement pendant la simulation de ces processus, un cadre de calcul sera développé afin de prédire l'évolution de la microstructure et de l'endommagement pendant le SPD. Cela permet d'améliorer la compréhension du compromis résistance-ductilité dans le SPD et d'optimiser les conditions de traitement afin de minimiser les dommages et d'améliorer les propriétés du matériau traité

On the strength-ductility modifications in pure copper after severe plastic deformation

The aim of this work is to investigate experimentally and numerically the modifications of both strength and ductility after processing by severe plastic deformation, using several passes of repetitive corrugation and straightening on sheets made of pure copper. Experimental stress–strain curves are determined before and after processing in order to study the influence of the process on the mechanical properties. The modeling of the mechanisms responsible of the mechanical properties evolution is done through the development of an extended Gurson model including a dislocation-based modeling of hardening. The model developed is implemented into a finite element code and applied to the numerical prediction of the repetitive corrugation and straightening followed by a tensile test. The modifications of strength and ductility predicted numerically are qualitatively in good agreement with the experimental observations

Reconstruction of heterogeneous surface residual-stresses in metallic materials from X-ray diffraction measurements

The aim of this paper is to provide spatially resolved distributions of residual stresses. X-ray diffraction measurements provide an intrinsic average of the residual stress due to the diffracted volume analyzed during the measurement. When the irradiated area is higher than the characteristic length of stress gradients, strong averaging effects are observed. A spatial deconvolution technique is developed to reconstruct the local residual stress field, based on the inversion of a linear system constructed from the average datasets. The method is first applied to the reconstruction of residual stresses in two reference cases inducing heterogeneous plastic strains (laser shot peening and repetitive corrugation and straightening processing), in which the average datasets are constructed from the local stress profiles determined numerically by the finite element method. In both processes, a very good agreement is observed between the reference stress profiles and the reconstructed ones. Finally, the method is applied to experimental X-ray diffraction measurements on a specimen processed by repetitive corrugation and straightening in similar conditions than the numerical simulations. A strong averaging effect is observed on the collected data and a good agreement is observed between the local stress profile reconstructed from the experimental measurements and that predicted numerically

Analysis of shear ductile damage in forming processes using a micromechanical model with void shape effects

The aim of this work is to investigate and predict ductile failure in forming processes. Experimental results of deep drawing and corrugation processing on aluminum alloys suggest that in some cases failure can be due to shear-dominated loadings. In order to simulate numerically failure during forming, we use the micromechanical Madou–Leblond model, which permits to account for void shape effects that are important under shear loadings. In the case of deep drawing, the model is able to reproduce failure either due to bottom or shear cracks, depending on the processing conditions. In the case of corrugation processing, the model reproduces accurately the occurrence of failure as well as the crack shape. Comparisons with the GTN model show the importance of void shape effects upon failure

Characterization and modeling of the damage mechanisms in ductile steel metal-matrix composites: Application to virtual forming

The aim of this work is to investigate the damage mechanisms and stiffness loss in Fe-TiB metal-matrix composites during plastic deformation. First, experimental results of interrupted tensile tests are performed to quantify the evolution of damage, using SEM observations, as well as the decrease of Young’s modulus as a function of the tensile strain. The experimental results are then used to calibrate a two-step homogenization model for metal-matrix composites in which the nucleation and growth of voids modify incrementally the overall elastic properties. The model is finally applied to the numerical prediction of stiffness loss in a problem of metal forming based on Nakazima tests. Overall, the stiffness loss predicted before the onset of coalescence is moderate and its distribution is homogeneous, emphasizing that Fe-TiB metal-matrix composites could be used in applications requiring metal forming

Validity of Crystal Plasticity Models Near Grain Boundaries: Contribution of Elastic Strain Measurements at Micron Scale

International audienceSynchrotron Laue microdiffraction and digital image correlation measurements were coupled to track the elastic strain field (or stress field) and the total strain field near a general grain boundary in a bent bicrystal. A 316L stainless steel bicrystal was deformed in situ into the elasto-plastic regime using a four-point bending setup. The test was then simulated using finite elements with a crystal plasticity model comprising internal variables (dislocation densities on discrete slip systems). The predictions of the model are compared with both the total strain field and the elastic strain field obtained experimentally. While activated slip systems and total strains are reasonably well predicted, elastic strains appear overestimated next to the grain boundary. This suggests that conventional crystal plasticity models need improvement to correctly model stresses at grain boundaries