Publications

This work offers a detailed examination of the phase field approach for modeling brittle fracture, emphasizing its theoretical foundations, mathematical descriptions, and computational strategies. Central to our discussion is an in-depth analysis of strain energy decomposition methods integral to phase field models. We introduce an innovative technique using a cleavage plane based degradation that has shown promising results under various loading scenarios. We meticulously evaluate each method’s inherent limitations and challenges to highlight their respective advantages and drawbacks across different loading scenarios. This review aims not only to catalog existing knowledge but also to pave the way for future research directions in the application of phase field approach to fracture analysis.

A method of generating a machining program with a programming system comprising a programming module and a man-machine communication interface having a display surface and selection means. The method comprises : - displaying on the display surface an image illustrating a representation of a real part to be machined and a virtual model corresponding to the real part to be machined;- selecting in the image, using the selection means, at least one given virtual zone of the predetermined virtual model corresponding to a given real zone of the part to be machined and a desired machining intensity parameter for the selected virtual zone; then- generating the machining program comprising a machining intensity characteristic to be applied as a function of the selected parameter.

Procédé de génération d'un programme d'usinage avec un système de programmation comprenant un module de programmation et une interface de communication homme-machine comportant une surface d'affichage et des moyens de sélection. Le procédé comprend : - l'affichage sur la surface d'affichage d'une image illustrant une représentation d'une pièce réelle à usiner et d'un modèle virtuel correspondant à la pièce réelle à usiner ; - la sélection dans l'image, à l'aide des moyens de sélection, d'au moins une zone virtuelle donnée du modèle virtuel prédéterminé correspondant à une zone réelle donnée de la pièce à usiner et d'un paramètre d'intensité d'usinage souhaité pour la zone virtuelle sélectionnée ; puis - la génération du programme d'usinage comprenant une caractéristique d'intensité d'usinage à appliquer en fonction du paramètre sélectionné.

Dissipative components of tool-workpiece interaction are of major importance in cutting-related vibrations. At the macroscopic vibration scale, such dissipation is usually accounted for by additional generalized damping forces in the equation of motion of the system’s elastodynamics. A finer consideration at cutting edge scale would bring up a line-distributed force mostly of ploughing nature. These two scales are usually linked by analytical integration, involving simplifying kinematical assumptions. In the present work a comparative investigation is proposed, for a machining operation, considering both representations in a detailed time domain modeling framework. Tool’s cutting edges are represented in a discretized manner, i.e. split into numerous elementary cutters allowing for detailed tool-workpiece interaction force distribution. The matter removal process is modeled via dexel-based surface discretization coupled with finite element-based modal shapes, enabling a consistent machined surface generation representation. Finally, the equations of motion are formulated for modal degrees of freedom and solved by a time marching algorithm. Based on these analyses, the limitations of resulting process damping force terms representations are considered regarding vibrations and interaction force magnitudes.

The austenitic stainless steels are characterised by remarkable mechanical properties, combining high ductility and high strength. Their plastic deformation involves a wide variety of strain induced deformation mechanisms, strongly related to the alloy stacking fault energy. With increasing SFE, martensitic transformation sequences (gamma to alpha', gamma to epsilon to alpha'), deformation twinning, glide of dissociated or perfect dislocations can occur. A quantitative modelling of such a deformation behaviour, with real predictive capabilities, has been developed recently. It is formulated in terms of finite strains and takes the various inelastic strains encountered in the material into account. In particular, two inelastic deformations are considered, either epsilon martensite transformation strain or twinning strain, depending on the testing temperature, and the alpha' transformation strain. Thermomechanical couplings are realised between these two inelastic deformation modes. The model which uses a self-consistent method for the scale transition, allows us to calculate the global behaviour of the polycrystal. In this contribution, this new model is applied to foresee the behaviour of a 304 stainless steel, tensile tested in a temperature range of -60 degrees Celcius to room temperature. Besides, large experimental investigations were performed on various 304 specimens. In particular, X-ray diffraction was used to quantify the volume fraction of the alpha' and epsilon martensite and to determine the texture evolution of the parent and the product phases. The local microtextures were analysed with the EBSD technique in a SEM FEG. The predictions obtained with the micromechanical model are compared to the different experimental results, notably the mechanical behaviour and associated deformation mechanisms, the transformation kinetic as well as the texture evolution.

Geometric errors in amachine tool structure are mainly responsible for the volumetric error in theworkspace. They occur at the attachment of each link between axis joints, but also along each axis in their joint frame. Reducing the impact of these errors is a key factor in guaranteeing the functional requirements of high value-added parts. Unlike mechanical correction, software compensation strategies are often chosen for their ease of implementation and versatile nature. In this study, a correction method by modifying the position measurement in real time is introduced and compared to compensation tables. The reaction response of the numerical controller (NC) to the modification of its position feedback is studied, and a 5-axis machining experiment to validate the proposed solution is performed. The principle of the experiment is to impose a virtual volumetric error in the workspace by modifying a machining program, then to test separately the ability of compensation tables and the proposed method to correct the chosen virtual geometric errors. The aim is to obtain a corrected workpiece similar to the one machined with a nominal program. In this way, it is not necessary to identify the geometric errors of the machine’s structure to test the performance of software compensationmethods. The machined workpieces feature geometries that are easy to control, but the tool paths generated to produce them were complex enough to challenge the compensation methods. The ability of the proposed solution to correct the virtual volumetric error introduced by a modified machining program is evaluated at 98%. Indeed, roundness measurements show that over 99% of the added error has been corrected, with residuals lower than 5 μm. Furthermore, the joint trajectories monitored during machining are studied through a contouring error estimation. Nominal and compensated trajectories are 98% similar with the proposed solution, compared with 35% for compensation tables.

The presence of heterogeneities and significant property mismatches in soft composites lead to complex be­ haviors that are challenging to model with conventional analytical or numerical homogenization techniques. The present work introduces a micromechanics-informed deep learning framework to characterize microscopic dis­placements and stress fields in soft composites with periodic microstructures undergoing finite deformation. The main obstacle we address is the construction of specific loss functions incorporating intricate knowledge of finite strain homogenization theory, which is valid for arbitrary macroscopic deformation gradients. Notably, a multi-network model is utilized to describe the discontinuities in material properties and solution fields within the composites. These neural networks communicate with each other through interface traction and displacement continuity conditions within the loss function. In addition, to exactly impose the periodicity boundary in hex­agonal and square unit cells, the neural network architectures are modified by incorporating a number of trainable harmonic functions. A significant advantage of the current framework is that it allows for a straight­ forward solution of the governing partial differential equations expressed in terms of the first Piola-Kirchhoff stresses, eliminating the need for iterative formulations of the residual vector and tangent matrix required by classical numerical methods. We extensively assess the effectiveness of the proposed approach upon extensive comparison with isogeometric analysis to determine the displacement and Cauchy stress fields in square and hexagonal arrays of fibers/porosities, demonstrating neural networks as a powerful alternative to the conven­tional numerical approaches in finite deformation analysis of microstructural materials.

Bien qu’ils présentent des fortes capacités en matière de prédiction des effets de taille, les modèles de plasticité à gradient développés dans la littérature manquent encore de maturité pour être appliqués dans le monde industriel. Il a été démontré que ces modèles peuvent conduire à des phénomènes méconnus sous certains chargements complexes, comme l’apparition de ‘gaps’ élastiques qui caractérisent la classe de modèles à gradient la plus utilisée dans la littérature. Ce phénomène, qui consiste en un retardement de l'écoulement plastique suite à un changement infinitésimal dans les conditions aux limites, n’a pas encore été observé dans la réalité et sa nature physique ne cesse de susciter la controverse [1,2]. Dans le but d’élucider le mystère des ‘gaps’ élastiques, ce travail propose d’appliquer la dynamique des dislocations discrètes en 3D (DDD-3D) pour faire une étude approfondie des effets de tailles dans les monocristaux sous chargements proportionnels et non-proportionnels complexes. Les résultats obtenus ont permis de reproduire des effets intéressants prouvés expérimentalement, comme l’effet « Hall-Petch », l’écrouissage non-linéaire de type III d’Asaro [3] ou encore la plasticité réversible [4]. Toutefois, aucun signe de ‘gaps’ élastiques n’a été obtenu, même sous des chargements non-proportionnels générant de tels ‘gaps’ en appliquant des modèles de plasticité à gradient. Ceci constitue une première preuve que ces ‘gaps’ peuvent ne pas être physiques.

In this paper, the performance of the solid-shell finite element SHB8PS is assessed in the context of sheet metal forming simulation using anisotropic elastic-plastic behavior models. This finite element technology has been implemented into the commercial implicit finite element code Abaqus/Standard via the UEL subroutine. It consists of an eight-node three-dimensional hexahedron with reduced integration, provided with an arbitrary number of integration points along the thickness direction. The use of an in-plane reduced integration scheme prevents some locking phenomena, resulting in a computationally efficient formulation when compared to conventional 3D solid elements. Another interesting feature lies in the possibility of increasing the number of through-thickness integration points within a single element layer, which enables an accurate description of various phenomena in sheet forming simulations. A general elastic-plastic model has been adopted in the constitutive modeling for sheet forming applications with plastic anisotropy. As an illustrative example, the performance of the element is shown in the earing prediction of a cylindrical cup drawing process.

This contribution concerns the implementation in the FE code Abaqus/Explicit of anisotropic elastoplastic behaviour laws for sheet metal forming. Several hardening laws are considered: isotropic hardening (Swift, Voce), kinematic hardening (Armstrong-Frederick), combinations of both as well as the microstructural model of Teodosiu-Hu. The initial anisotropy is described by the Hill'48 yield function. A specific, explicit time integration scheme is developed for use with shell elements. The validation of the implementation and the comparison between the different hardening laws is made using rheological tests. Several different monotonic tests are used: tensile test, shear test and plane strain test, together with two-path tests: tensile+shear and shear-to-shear (Bauschinger). A cross-shape cup drawing test is used as an application for a deeper comparison of the real benefit of the different hardening laws.