Showing papers by "Marc Bernacki published in 2016"

Advancing layer algorithm of dense ellipse packing for generating statistically equivalent polygonal structures

Abstract: A new constructive algorithm, called Advancing layer algorithm, for the generation of dense ellipse packing is proposed. Compared to existing algorithms for filling a 2D domain by elliptical particles, the method allows to respect the imposed size, shape and spatial orientation distributions (i.e. the inertia tensor) and achieve high packing densities. In particular case of disk packing, the comparison with Optimized Dropping and Rolling method shows that the computational cost of the proposed methodology is lower for moderate polydispersities of particle size while achieving higher packing densities and more homogeneous placing of particles in the domain. Thanks to an approximation of each ellipse by a set of circles, polygonal structures are constructed on the base of obtained ellipse packing by Laguerre–Voronoi Tessellation method in good agreement with desired characteristics of cells (polygons).

Improvement of 3D mean field models for capillarity-driven grain growth based on full field simulations

Abstract: In the present study, mean field models of grain growth (Hillert and Burke–Turnbull models) are compared with 3D full field simulations considering an isotropic grain boundary energy and mobility and under the absence of second-phase particles. The present 3D full field simulations are based on a level set description of the grain interfaces within a finite element framework. The digital initial microstructures are generated using a coupled “Voronoi–Laguerre/dense sphere packing” algorithm. Based on full field simulation results, new formulations of Burke–Turnbull and Hillert models are proposed. In contrast with classical formulations, the new ones account for the possible heterogeneity of the initial grain size distribution.

Full field modeling in a level set framework of Zener pinning phenomenon - discussion of classical limit mean grain size equation

Abstract: Pinning of grain boundaries by second phase particles is widely used to control the grain size during forming process of superalloys

Full Field Modeling of the Zener Pinning Phenomenon in a Level Set Framework - Discussion of Classical Limiting Mean Grain Size Equation

Abstract: The dragging effect exerted by second phase particles on grain growth in two-dimensional systems is investigated Full field simulations were performed to highlight the influence of the size and the surface fraction of the precipitates on the limiting mean grain size A modified version of the 2D classical Zener equation is finally proposed based on these numerical experiments It is shown that the proposed model is in good agreement with other works from the literature

Permeability Computation on a Representative Volume Element (RVE) of Unidirectional Disordered Fiber Arrays

Abstract: An efficient method to compute the permeability of disordered fibrous arrays is proposed. A stabilized mixed finite element method is used with an immersed domain approach to represent the porous material at its microscopic scale. Therefore, the Stokes equations are solved in the whole domain (including solid part) using a penalization method. The accuracy is controlled by refining the mesh around the fluid-solid interface defined by a level-set function. Using homogenization techniques, the permeability of an RVE is obtained. Furthermore, a new method to generate disordered fibers in function of the porosity, Φ, and other microstructural parameters is proposed and a study of the effect of inter-fiber spacing on K, the permeability tensor, is performed. This task was achieved using parallel computation and over 460 simulations were carried out in two-dimensional RVEs consisting of over 555 fibers

A new algorithm for dense ellipse packing and polygonal structures generation in context of FEM or DEM

Abstract: A new constructive ellipse packing algorithm is presented. It allows to respect the imposed area, shape and spatial orientation distribution (i.e. the inertia tensor) and achieve high packing densities. The packing density decreases with increasing particles aspect ratio what is in agreement with results reported in the literature. The generated packings with complex imposed area, shape and spatial orientation distributions with densities in the range of 0.74 and 0.8 are presented. The efficiency of the algorithm is demonstrated by comparison with the Optimized Dropping and Rolling method for disk packing. Moreover, the proposed packing strategy enables to generate very easily non-equiaxed polygonal structures by using Laguerre-Voronoi tessellation of the generated disk-based ellipse packing.

Full field modeling of dynamic recrystallization in a global level set framework, application to 304L stainless steel

Abstract: A new full field numerical approach for the simulation of dynamic and post-dynamic recrystallization will be detailed. A level Set framework is employed to link a crystal plasticity finite element method with the modeling of recrystallization. Plasticity is calculated through the activation of slip systems and provides predictions for both SSDs and GNDs densities. These predictions control the activation and kinetics of recrystallization. All the developments are applied on 304L stainless steel.

Etablissement et évolution des interfaces lors du soudage diffusion -- Modélisation en champ complet du mécanisme de croissance de grains

Abstract: La genese des microstructures des interfaces soudees diffusees implique plusieurs mecanismes qui ont lieu simultanement et dont le developpement au cours du cycle de soudage depend des parametres appliques et des proprietes des surfaces initiales. L'elimination des pores est obtenue par la plastification immediate des asperites sous l'effet de la pression, leur fluage et finalement la diffusion. Certains alliages sont tres sensibles a l'oxygene residuel et forment facilement des oxydes ou des carbures en surface, au chauffage. L'evolution globale du volume du materiau, en particulier le grossissement de grain, affecte aussi l'interface mais celle-ci ne peut disparaitre si elle presente des defauts susceptibles d'epingler les joints de grains.

Understanding and modeling of void closure mechanisms in hot metal forming processes: a multiscale approach

Abstract: The presence of voids after casting processes of large metal workpieces requires the use of adapted hot metal forming processes to deliver sound products. Yet, there is at present a lack of knowledge regarding void closure mechanisms and there is no reliable model that can accurately predict void closure. A new model accounting for both stress triaxiality ratio and Lode angle is proposed. Based on an advanced multiscale approach, this model also accounts for voids shape and orientation with respect to loading direction.

Towards a mesh independent fracture modeling method using cohesive elements

Abstract: Prediction and understanding of material failure is of major importance fordifferent industrial and natural processes. Linear Elastic Fracture Mechanics(LEFM) is a very useful tool when dealing with brittle and quasi-brittle materials. Indeed, LEFM approach enables to identify a characteristic length where the fracture process will take place. It is well known that the softening experienced by materials once the failure process has started leads to an ill-posed problem if no regularization technique is used. Many different techniques have been developed in order to overcome this issue. It is possible to distinguish three main categories, namely, gradientenhanced models, non-local models and cohesive zone models (CZM). Among these numerical techniques, CZM represents a very interesting alternative regarding the modeling of fracture processes since it allows to control in a very accurate way the fracture energy while keeping a rather low computational cost. They also allow to represent sharp cracks (in contrast to non local models), but their numerical implementation can be complex, especially within a parallel computing framework. In order to avoid the change on the macroscopic response of the material (in particular while the fracture has not yet started) dynamic insertion of cohesive elements is often used. Although it is possible to obtain a macroscopic behavior that is in agreement with experiments, the main drawback of CZM is related to the fact that obtained fracture patterns are mesh dependent since the cohesive elements are present at the facets between two bulk elements. We will address this issue by using advanced remeshing techniques. Within the context of fracture modeling at microscopic scale, we have developed a numerical approach that allows to perform constrained remeshing steps during the resolution of the mechanical problem. These constrained remeshing steps consist in automatically generating a conforming mesh that suits the matrix and the inclusions of the material. This constrained remeshing tool has been enhanced in order to allow the dynamic insertion of cohesive elements (also known as extrinsic cohesive elements). We will present the numerical approach and its validation by looking into the convergence of the fracture pattern for different prescribed crack paths. Finally we will compare the results obtained for different crack propagation criteria for 2D problems in terms of fracture pattern and energy dissipation.

Ductile fracture – Influence of heterogeneous microstructure on nucleation, growth and coalescence mechanisms

Abstract: Investigations at the microstructure scale are essential if one wants to predict accurately ductile fracture of metallic materials under complex multiaxial and non-proportional loadings. An advanced finite element approach based on a level set formalism and body-fitted immersed meshing capabilities enable the modeling of nucleation, growth and coalescence mechanisms for complex 3D microstructures and under large plastic strain. Applications on real microstructures (coming from in-situ laminography) and exact boundary conditions (thanks to Digital Volume Correlation) are studied and discussed for different loading conditions.

Full field modeling of recrystallization and grain growth thanks to a level set approach: towards modeling by industry

Abstract: Metal forming modeling can be predictive only if the strain rate, strain and temperature dependency of the flow behaviour are correctly described. The mechanical properties and behaviour of metallic materials mainly depends on the content and structure of dislocation network, this points out the need to incorporate microstructure concepts into the numerical models. The goal is to correctly describe the main physical mechanisms occurring in metals during thermomechanical processes i.e. work-hardening, recovery, grain boundary migration, nucleation and grain growth related to dynamic, static or metadynamic recrystallization. Macroscopic and homogenized models are widely used in the industry, mainly due to their low computational cost. If this mean field framework is quite convenient, it can be synonymous, for a given material, with a large amount of experiments with advanced laboratory devices. Moreover, the homogenization of the microstructure does not permit to capture some very local phenomena.

An extended mean field approach for modelling realistic grain size distribution evolutions during Discontinuous Dynamic RX and Post-Dynamic RX

Abstract: Mean-field models for Discontinuous DRX (DDRX) have demonstrated for years their ability to reproduce stress and grain size evolutions during hot deformation. However most of them fail to predict grain size distributions, which is necessary to couple DDRX models with Post-Dynamic RX models. Such failure is a consequence of mean field assumptions that prescribe grains to follow the same path in the diameter dislocation-density space. In this paper, an alternative model based on a stochastic approach which permits to deal with the main advantages of mean field models without this limitation is proposed. Results of this new model are successfully compared with experimental data of DDRX and PDRX obtained on a home-made AISI304 deformed up to large strains by hot torsion tests within the range 950 °C – 1150 °C. A comparison with a level-set based full field method of PDRX run on a simplified case is also discussed.

Numerical modeling of failure mechanisms in complex heterogeneous microstructures

Abstract: This work focuses on the development of an efficient finite element approach for the modeling of failure mechanisms of complex 3D microstructures under large plastic strain and multi-axial loading conditions. Heterogeneous microstructures are represented by a matrix containing particles and voids. The first step consists in the definition and meshing of real microstructures based on X-ray tomography or laminography pictures. This step is facilitated by the use of level-set functions to represent interfaces between each phase and local mesh refinement to capture them accurately [1]. Recent improvements of this methodology enable to explicitly mesh interfaces and constrain remeshing operations in order to address volume conservation issues. Indeed, topological mesh adaptation is applied automatically during mesh motion to reach large plastic strain while maintaining an appropriate discretization and preventing element flipping [2]. Then, failure is modeled for the void nucleation and coalescence stages. Stress-based criteria are used for particles failure and particles/matrix debonding whereas coalescence can be activated either through a local damage parameter or a minimum distance between neighboring voids. The finite element framework described above enables the dynamic insertion of cracks during the computation and their remeshing throughout void growth. This technique also allows accounting for complex topological events such as void coalescence. This new method is applied to study the influence of particles orientation and of loading path on micro mechanical ductile failure mechanisms. These micromechanical failure mechanisms require material’s threshold values that need to be identified. The identification stage is based on the following methodology: - A representative volume element (RVE) corresponding to the real microstructure of the studied material is meshed; - An in-situ tensile test is carried out with X-Ray laminography observations at multiple steps; - Digital Volume Correlation (DVC) is applied to these 3D images in order to extract the exact boundary conditions that should be applied to the initial RVE [3]; - A finite element modeling of the RVE with these exact boundary conditions is done and comparisons with experimental pictures are used for the identification of failure criteria parameters. Applications using nodular cast iron samples are presented for the validation of the overall strategy. [1] E. Roux, M. Shakoor, M. Bernacki, and P.-O. Bouchard, A new finite element approach for modelling ductile damage void nucleation and growth – analysis of loading path effect on damage mechanisms, Modelling and Simulation in Materials Science and Engineering , 22, 1-23, 2014. [2] M. Shakoor, P.-O. Bouchard and M. Bernacki, An adaptive level-set method with enhanced volume conservation for simulations in multiphase domains, Computer Methods in Applied Mechanics and Engineering , Submitted, 2015. [3] A. Buljac, T. Taillandier-Thomas, T.F. Morgeneyer, L. Helfen, S. Roux, F. Hild, Slant strained band development during flat to slant crack transition in AA 2198 T8 sheet: in situ 3D measurements, International Journal of Fracture, Article In Press, 2015

Introduction to the level-set full field modeling of laths spheroidization phenomenon in α/β titanium alloys

Abstract: Fragmentation of α lamellae and subsequent spheroidization of α laths in α/β titanium alloys occurring during and after deformation are well known phenomena. We will illustrate the development of a new finite element methodology to model them. This new methodology is based on a level set framework to model the deformation and the ad hoc simultaneous and/or subsequent interfaces kinetics. We will focus, at yet, on the modeling of the surface diffusion at the α/β phase interfaces and the motion by mean curvature at the α/α grain interfaces.