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Dascalu C.,CNRS Study of Microstructures, Mechanics and Material Sciences lab
Mechanics of Materials | Year: 2017

The contribution presents results on the modeling of fracture instabilities with a dynamic two-scale damage approach. The evolution law for damage is deduced by homogenization from small-scale descriptions of mode I dynamic propagation of micro-cracks. The model is sensitive with respect to the size of the microstructure through a length parameter present in the damage law. We study the macroscopic failure evolution in brittle solids and the development of associated instabilities like micro-branching. The results of numerical simulations for a reduced-size specimen are compared with experimental results for tensile fracture of PMMA and it is shown that the damage model is able to reproduce the observed micro-branching instabilities and the corresponding crack speed evolutions. © 2017 Elsevier Ltd.

Gu C.F.,Monash University | Toth L.S.,CNRS Study of Microstructures, Mechanics and Material Sciences lab
Acta Materialia | Year: 2011

Numerical simulations of texture development in polycrystalline oxygen-free high conductivity (OFHC) copper have been studied using the Taylor, viscoplastic self-consistent (VPSC) and a recent Taylor type polycrystal grain refinement (GR) model for the forward and reverse strain conditions. The predictions were compared with each other and with experimental results. OFHC copper was deformed by equal channel angular pressing (ECAP) for up to two passes in Route C. In this Route the deformation mode is shear and is reversed every second pass, however, shear texture is still observed experimentally. For this reason this case is suitable to test the predictive capacity of polycrystal models. The simulation results demonstrate that the magnitude of the shear strain increment plays an important role in predicting texture evolution during strain reversal. If the strain increment is large the three models predict a strain reversal shear texture in acceptable agreement with the experimental results. However, when the strain increments are sufficiently small (as they should be to obtain high precision), both the Taylor and VPSC models return the texture to its initial state; only the GR model can produce a strain reversal texture in accord with the experimental results. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Forquin P.,CNRS Study of Microstructures, Mechanics and Material Sciences lab
European Physical Journal: Special Topics | Year: 2012

In the present work, an experimental method named "the rocking spalling test" is proposed to investigate the crack-propagation velocity in concrete and rock-like materials under dynamic tensile loading. This method is based on the use of double-notched specimens loaded in spalling tests. A compressive pulse is transmitted to a rectangular specimen by means of a Hopkinson bar. It is reflected as a tensile wave on the opposite free surface of the sample. A large notch provides a rocking effect of the rear part of the specimen whereas a short notch is used to trigger a single unstable crack. This experimental configuration has been optimized through a series of numerical simulations. Finally, a series of tests have been conducted on dry and wet concrete specimens. Crack gauges and ultra-high speed camera coupled to Digital Image Correlation have been used to characterize the crack speed in dry and wet concrete samples. © 2012 EDP Sciences and Springer.

Mareau C.,Arts et Metiers ParisTech | Berbenni S.,CNRS Study of Microstructures, Mechanics and Material Sciences lab
International Journal of Plasticity | Year: 2015

The modeling of heterogeneous materials with an elasto-viscoplastic behavior is generally complex because of the differential nature of the local constitutive law. Indeed, the resolution of the heterogeneous problem involves space-time couplings which are generally difficult to estimate. In the present paper, a new homogenization model based on an affine linearization of the viscoplastic flow rule is proposed. First, the heterogeneous problem is written in the form of an integral equation. The purely thermoelastic and purely viscoplastic heterogeneous problems are solved independently using the self-consistent approximation. Using translated field techniques, the solutions of the above problems are combined to obtain the final self-consistent formulation. Then, some applications concerning twophase fiber-reinforced composites and polycrystalline materials are presented. When compared to the reference solutions obtained from a FFT spectral method, a good description of the overall response of heterogeneous materials is obtained with the proposed model even when the viscoplastic flow rule is highly non-linear. Thanks to this approach, which is entirely formulated in the real-time space, the present model can be used for studying the response of heterogeneous materials submitted to complex thermo-mechanical loading paths with a good numerical efficiency. © 2014 Elsevier Ltd. All rights reserved.

Charpentier I.,CNRS Study of Microstructures, Mechanics and Material Sciences lab
Optimization Methods and Software | Year: 2012

Modelling often involves nonlinear parametric problems and bifurcation analysis. This interdisciplinary paper reviews higher-order numerical methods for the solution of nonlinear problems, and proposes a synthesis of two different conceptual frameworks, namely automatic differentiation and the asymptotic numerical method. Various mechanical problems and references illustrate the presentation. © 2012 Taylor & Francis.

Sutter G.,CNRS Study of Microstructures, Mechanics and Material Sciences lab | List G.,CNRS Study of Microstructures, Mechanics and Material Sciences lab
International Journal of Machine Tools and Manufacture | Year: 2013

The chip formation for a Ti-6Al-4V alloy was studied at high cutting speeds combined with large uncut chip thicknesses (0.1-0.25 mm). Orthogonal cutting tests were conducted by using uncoated carbide tools on a specific ballistic set-up with cutting speeds from 300 m/min to 4400 m/min (5-75 m/s). A hypothesis on the mechanism of chip generation is proposed for this speed range validated by high-speed imaging system enabled direct observation of cutting process. A transition, from serrated more or less regular with localized shearing and possible presence of cracking, to discontinuous at very high speed is observed. The inclination of the segment Φseg is shown as resulting from the primary shear angle Φ that can be modified by compression between the tool and the uncut part. A maximum value of 60° for Φseg is reached with increasing speed after which it decreases to 45° at very high speed. The cutting speed appears as the most important factor when compared with the uncut chip thickness, in determining the formation of chips by affecting the frequency of segmentation, the shear angles and the crack length. The significant reduction of cutting forces occurring with increases in cutting speed was firstly explained by the conflicting work hardening-thermal softening processes and then depended on whether the deformation phase of the chip segment is occurred. © 2012 Elsevier Ltd.

Erzar B.,CNRS Study of Microstructures, Mechanics and Material Sciences lab | Forquin P.,CNRS Study of Microstructures, Mechanics and Material Sciences lab
Mechanics of Materials | Year: 2011

Due to their large aggregates size and their heterogeneous microstructure, concretes are difficult materials to test at high strain-rates. Direct tensile tests, spalling tests and edge-on impact experiments have been especially developed and performed on a standard concrete (max grain size of 8 mm). The influence of free water on the high strain rate behaviour has been carefully evaluated. Numerical simulations of dynamic testing have been also performed using a mesoscopic approach in which the matrix and the aggregates are differentiated. Numerical and analytical homogenization methods have been employed to define a model-concrete which fits experimental data of simple and œdometric compression tests. Then, the numerical simulations with several random distributions of aggregates were conducted to validate the processing methods applied to the experimental data of the dynamic tests. Moreover an anisotropic damage model coupled to the mesoscopic approach has been used to simulate the dynamic behaviour of concrete under impact. It allows predicting the increase of strength and cracking density with strain-rate and the free water influence on the dynamic behaviour of concrete. © 2011 Elsevier Ltd. All rights reserved.

Beausir B.,CNRS Study of Microstructures, Mechanics and Material Sciences lab | Fressengeas C.,CNRS Study of Microstructures, Mechanics and Material Sciences lab
International Journal of Solids and Structures | Year: 2013

The aim of the paper is to show experimental evidence of the rotational defects referred to as disclinations in polycrystalline aggregates. Using orientation maps obtained from electron backscattered diffraction or transmission electron microscopy, a method for the recovery of components of the disclination density tensor is presented and applied to various polycrystalline materials. Mapping the disclination densities reveals their extensive presence at intra-granular low-angle boundaries, low and high-angle grain boundaries and triple junctions, irrespective of the material symmetry and grain size. A significant level of rotational incompatibility, with dipolar distribution of the disclinations, is detected in all cases investigated. Since high-angle rotational incompatibility cannot be accounted for consistently by dislocation-based models, the present results support considering disclinations in addition to dislocations in the interpretation of grain boundaries and triple junctions. © 2012 Elsevier Ltd. All rights reserved.

Richeton T.,CNRS Study of Microstructures, Mechanics and Material Sciences lab | Berbenni S.,CNRS Study of Microstructures, Mechanics and Material Sciences lab
European Journal of Mechanics, A/Solids | Year: 2013

In the present paper, the static Field Dislocation Mechanics (FDM) theory is first recalled and next formulated for heterogeneous elasticity and plasticity with respect to a reference homogeneous medium. The issue of an infinite bicrystal with planar boundary undergoing piecewise uniform plastic distortions and elastic properties is then considered. The static FDM theory is accordingly used to derive explicit closed-form solutions of stress and lattice misorientation fields in the general context of heterogeneous anisotropic elasticity. These expressions are validated by Crystal Plasticity Finite Element simulations performed on a bicrystal with periodic boundary conditions. As a result of analytical expressions, it is possible to quantify the contribution of the different incompatibility sources, namely the contribution arising from elastic incompatibilities alone, that due to plastic incompatibilities alone and the one due to couplings between elastic and plastic incompatibilities. Besides, residual stresses and lattice misorientations are accordingly calculated in elastic/plastic bicrystals for different orientations of the elastic crystal. Results are compared with the corresponding homogeneous isotropic elasticity approximation. It is shown that elastic-plastic coupling incompatibilities may have consequent effects on residual stresses and lattice misorientations depending on the degree of elastic anisotropy of the material and the relative orientation of both crystals. As a conclusion, the consideration of coupling incompatibilities in multiscale plasticity models when predicting internal stresses and texture developments is discussed. © 2012 Elsevier Masson SAS. All rights reserved.

Taupin V.,CNRS Study of Microstructures, Mechanics and Material Sciences lab | Capolungo L.,Georgia Institute of Technology | Fressengeas C.,CNRS Study of Microstructures, Mechanics and Material Sciences lab
International Journal of Plasticity | Year: 2014

The shear-coupled boundary migration of 〈0 0 1〉 symmetric tilt boundaries is investigated within the framework of an elasto-plastic theory of disclination and dislocation fields. The tilt boundaries are built from periodic partial wedge-disclination dipole arrays, on the basis of their atomistic topography. Non-locality of the elastic response of the adjacent crystals stems from the defected structure of their boundary. Upon applying a shear strain to the bicrystal, couple stresses are generated, which set the disclination dipole array into motion normal to the boundary. In the process, edge dislocation densities with partial Burgers vector lying along the boundary are nucleated, whose glide parallel to the boundary and annihilation produces plastic shear. The misorientation dependence of the shear coupling factor predicted by the model is in full agreement with data from atomistic simulations and experiments. It is found to depend on the polarity and the magnitude of the wedge disclination dipoles composing the grain boundary. © 2013 Elsevier Ltd. All rights reserved.

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