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Rousselier G.,MINES ParisTech Center of materials | Luo M.,Massachusetts Institute of Technology
International Journal of Plasticity | Year: 2014

This paper deals with the modeling of ductile fracture in the whole range of stress triaxiality. At high stress triaxiality, a classical void damage based model is used. At low stress triaxiality, the Mohr-Coulomb model at the slip system scale combines the resolved normal and shear stresses for each slip plane and direction. These two models are fully coupled in the framework of classical polycrystalline plasticity. A Reduced Texture Methodology (RTM) is used to provide the computational efficiency needed for numerical applications. The RTM approach involves a significant reduction of the number of representative crystallographic orientations. The model is applied to a 6260 thin-walled aluminum extrusion. With RTM, a special hybrid experimental-numerical procedure is used to identify all plasticity parameters (including texture) from mechanical experiments. The fracture parameters are calibrated with fracture experiments on a flat notched tensile specimen and the so-called butterfly shear specimen. Fractographic examinations show a combination of dimples and large smooth areas in the notched specimens (mixed fracture) and flat smooth areas only in the shear specimens. It highlights the need of combined fracture models. With the embedded new fracture model, finite element analyses of the notched specimen can model the through-the-thickness slant fracture propagating from the center towards the edges. Because of the very large strains in the shear specimen tests/analyses, small edge cracks first appear in the tensile areas before main shear cracks initiate and propagate along the width of the specimen. The experimental and numerical results are in good agreement with regard to fracture strains and locations, macroscopic and microscopic features. © 2014 Elsevier B.V.

Luo M.,Massachusetts Institute of Technology | Rousselier G.,MINES ParisTech Center of materials
International Journal of Plasticity | Year: 2014

This paper works on the macroscopic modeling of the anisotropic plasticity of a 6260-T6 thin-walled aluminum extrusion with a focus on the large strain multi-axial deformation with Strength-Differential Effect (SDE). Based on the framework of the self-consistent polycrystalline plasticity, the recently developed Reduced Texture Methodology (RTM) (Rousselier et al.; 2012) is employed to provide the computational efficiency needed for industrial applications while keeping the physically-based nature of the plasticity model. In particular, the new model features a novel hardening law at slip-system level to better capture large strain behaviors, as well as a generic method designed to describe the stress/strain history effect. All model parameters (including texture) are identified from mechanical experiments using a special optimization procedure. An extensive experimental program covering more than 30 distinct multi-axial stress states with both proportional and non-proportional loadings is used to calibrate and validate the present model. Both full- and reduced-thickness specimens are tested to capture the through-thickness heterogeneity of texture and grain size. It is shown that the present model predicts well the stress-strain responses in most of the multi-axial loading conditions which have been tested. Moreover, the model is able to capture various interesting behaviors of the present material during plastic deformation, including anisotropy, through-thickness heterogeneity, SDE of tension/compression or shear, and cross-hardening during non-proportional loadings. Furthermore, successful simulation of two structural level tests including a circular punch indentation and a three-point bending shows the applicability and potential of the new model in industrial practices. © 2013 Elsevier Ltd. All rights reserved.

Pineau A.,MINES ParisTech Center of materials
Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences | Year: 2015

The size and the character (low and large angle, special boundaries, tilt and twist boundaries, twins) of the grain boundaries (GBs) in polycrystalline materials influence their strength and their fracture toughness. Recent studies devoted to nanocrystalline (NC) materials have shown a deviation from the Hall-Petch law. Special GBs formed by Ó3 twins in face-centred cubic metals are also known to have a strong effect on the mechanical behaviour of these metals, in particular their work-hardening rate. Grain orientation influences also crack path, the fracture toughness of body-centred cubic (BCC) metals and the fatigue crack growth rate of microstructurally short cracks. This paper deals both with slip transfer at GBs and with the interactions between propagating cracks with GBs. In the analysis of slip transfer, the emphasis is placed on twin boundaries (TBs) for which the dislocation reactions during slip transfer are analysed theoretically, experimentally and using the results of atomic molecular simulations published in the literature. It is shown that in a number of situations this transfer leads to a normal motion of the TB owing to the displacement of partial dislocations along the TB. This motion can generate a de-twinning effect observed in particular in NC metals. Crack propagation across GBs is also considered. It is shown that cleavage crack path behaviour in BCC metals is largely dependent on the twist component of the GBs. A mechanism for the propagation of these twisted cracks involving a segmentation of the crack front and the existence of intergranular parts is discussed and verified for a pressure vessel steel. A similar segmentation seems to occur for short fatigue cracks although, quite surprisingly, this crossing mechanism for fatigue cracks does not seem to have been examined in very much detail in the literature. Metallurgical methods used to improve the strength of the materials, via grain boundaries, are briefly discussed. © 2015 The Author(s) Published by the Royal Society. All rights reserved.

Coupez T.,MINES ParisTech Center of materials | Hachem E.,MINES ParisTech Center of materials
Computer Methods in Applied Mechanics and Engineering | Year: 2013

The objective of this paper is to show that anisotropic meshes with highly stretched elements can be used to compute high Reynolds number flows. In particular, it will be shown that boundary layers, flow detachments and all vortices are well captured automatically by the mesh. We present an anisotropic meshing based on a posteriori estimation for the incompressible Navier Stokes equations. The proposed a posteriori estimate is based on the length distribution tensor approach and the associated edge based error analysis. The Finite Element flow solver is based on a Variational MultiScale (VMS) method, which consists in here of decomposing both the velocity and the pressure fields into coarse/resolved and fine/unresolved scales. This choice of decomposition is shown to be efficient for simulating flows at high Reynolds number. The stabilization parameters are determined taking into account the anisotropy of the mesh using a directional element diameter. The adaptation algorithm is applied to high Reynolds number flows inside the 2D and 3D lid-driven cavities and compared to accurate reference solutions. © 2013 Elsevier B.V.

Taleb L.,CNRS Material Physics Group | Cailletaud G.,MINES ParisTech Center of materials
International Journal of Plasticity | Year: 2010

In this paper we consider the elastoplastic behavior of the 304L stainless steel under cyclic loading at room temperature. After the experimental investigations presented in Taleb and Hauet (2009), the present work deals with modeling in the light of the new observations. An improved version of the multimechanism model is proposed in which the isotropic variable is revisited in order to take into account the non-proportional effect of the loading as well as the strain memory phenomenon. A particular attention has been paid to the identification process in order to capture the main important phenomena: relative parts of isotropic and kinematic hardening, time dependent effects, non-proportionality effect, strain amplitude dependence. Only strain controlled tests have been used for the identification process. The capabilities of the model with "only" 17 parameters are evaluated considering a number of proportional and non-proportional stress and strain controlled tests. © 2009 Elsevier Ltd. All rights reserved.

Morgeneyer T.F.,MINES ParisTech Center of materials | Besson J.,MINES ParisTech Center of materials
Scripta Materialia | Year: 2011

Tomography examination of arrested cracks at crack initiation, flat to slant transition and slant propagation indicates a clear change in micromechanisms from high-stress triaxiality void growth to shear-dominated coalescence in the slant regime. A Gurson-type model is used and a shear void nucleation term based on the Lode parameter for strain rate is suggested for simulation of a fully meshed sample, which achieves flat to slant crack transition at loads close to experimental results for aluminium. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Taleb L.,CNRS Material Physics Group | Cailletaud G.,MINES ParisTech Center of materials
International Journal of Plasticity | Year: 2011

The main objective of the work is to investigate the importance of time dependent phenomena in the cyclic behavior of the 304L SS at room temperature. The contribution of creep in the cyclic evolution of the inelastic strain observed under stress control is particularly studied through various tests using proportional and non proportional loading paths. The test results demonstrate that the behavior is strongly rate dependent. Moreover, most of the cyclic accumulation of the inelastic strain is due to creep which implies that ratcheting is very small for the different conditions considered. The study clearly shows that a correct description of the behavior of the 304L SS at room temperature cannot be obtained without time-dependent constitutive equations. The tests performed have been simulated with the multimechanism constitutive equations after the identification of its 17 parameters. The quality of the simulations is in good agreement with the experimental results especially for the non proportional loading paths. © 2011 Elsevier Ltd. All rights reserved.

Ryckelynck D.,MINES ParisTech Center of materials | Missoum Benziane D.,MINES ParisTech Center of materials
Computer Methods in Applied Mechanics and Engineering | Year: 2010

This paper concerns the adaptation of reduced-order models during simulations of series of elastoviscoplastic problems. In continuation with previous works, this paper aimed at extending the A Priori Hyper-Reduction method (APHR method) for nonlinear thermal problems to nonlinear mechanical problems involving internal variables. This method is an a priori approach because full incremental responses of detailed models are not forecasted in order to build reduced-order models. The recent extension of the Hyper-Reduction method to reduction of mechanical models involving internal variables makes possible the reduction of degrees of freedom and the reduction of integration points. A multi-level formulation is introduced to focus on the capability of the method to perform efficient parallel computations to adapt reduced-order models. © 2009 Elsevier B.V.

Liu M.,MINES ParisTech Center of materials
International Journal of Solids and Structures | Year: 2014

Normal contact deformation of an asperity and a rigid flat is studied within an axisymmetric finite element model. The asperity features a sinusoidal profile and is elastic-plastic with linear strain hardening. Influences of geometrical (asperity height and width) and loading (the maximum interference) parameters on frictionless contact responses are explored for both loading and unloading. Dimensionless expressions for contact size and pressures covering a large range of interference and asperity ratio values are obtained in power-law forms. Results show the mean contact pressure after fully-plastic contact reaches a plateau only for small asperity ratios, while it continues increasing for large asperity ratios. The residual depth is found to be associated with plastically dissipated energy. © 2014 Elsevier Ltd. All rights reserved.

Cao T.-S.,MINES ParisTech Center of materials
International Journal of Solids and Structures | Year: 2014

Damage to fracture transition has become a popular topic in the ductile fracture scientific community. Indeed, the transition from a damage continuous approach to a discontinuous fracture is not straightforward both from mechanical and numerical points of view. In the present study, a new improved Lode dependent phenomenological coupled damage model is used to investigate the ductile fracture in different mechanical tests. The remeshing and elements erosion techniques are employed to propagate the ductile cracks in 3D models using Forge® finite element code. This code is based on a mixed velocity-pressure formulation using the MINI element P1+/P1. In addition, the plasticity behavior is modeled by a Lode-dependent plasticity criterion. Applications to different mechanical tests at different loading configurations, using identified damage model parameters, show good agreement in terms of fracture prediction between experimental and numerical results. © 2014 Elsevier Ltd. All rights reserved.

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