Material spatial randomness: From statistical to representative volume element (original) (raw)
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Europhysics Letters (EPL), 2007
Spatial heterogeneity in the elastic properties of soft random solids is investigated via a twopronged approach. First, a nonlocal phenomenological model for the elastic free energy is examined. This features a quenched random kernel, which induces randomness in the residual stress and Lamé coefficients. Second, a semi-microscopic model network is explored using replica statistical mechanics. The Goldstone fluctuations of the semi-microscopic model are shown to reproduce the phenomenological model, and via this correspondence the statistical properties of the residual stress and Lamé coefficients are inferred. Correlations involving the residual stress are found to be long-ranged and governed by a universal parameter that also gives the mean shear modulus.
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The computationally random homogenization analysis of a two-phase heterogeneous materials is addressed in the context of linear elasticity where the randomness of constituents' moduli and microstructural morphology together with the correlation among random moduli are fully considered, and random effective quantities such as effective elastic tensor and effective stress as well as effective strain energy together with their numerical characteristics are then sought for different boundary conditions. Based on the finite element method and Monte-carlo method, the RVE with randomly distributing particles determined by a numerical convergence scheme is firstly generated and meshed, and two types of boundary conditions controlled by average strain are then applied to the RVE where the uncertainty existing in the microstructure is accounted for simultaneously. The numerical characteristics of random effective quantities such as coefficients of variation and correlation coefficients are then evaluated, and impacts of different factors on random effective quantities are finally investigated and revealed as well.
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International Journal of Solids and Structures, 1998
One of the main challenges in solid mechanics lies in the passage from a heterogeneous microstructure to an approximating continuum model. In many cases (e.g. stochastic finite elements, statistical fracture mechanics), the interest lies in resolution of stress and other dependent fields over scales not infinitely larger than the typical microscale. This may be accomplished with the help of a meso-scale window which becomes the classical representative volume element (RVE) in the infinite limit. It turns out that the material properties at such a mesoscale cannot be uniquely approximated by a random field of stiffness/compliance with locally isotropic realizations, but rather two random continuum fields with locally anisotropic realizations, corresponding, respectively, to essential and natural boundary conditions on the meso-scale, need to be introduced to bound the material response from above and from below. We study the first-and second-order characteristics of these two meso-scale random fields for anti-plane elastic response of random matrix-inclusion composites over a wide range of contrasts and aspect ratios. Special attention is given to the convergence of effective responses obtained from the essential and natural boundary conditions, which sheds light on the minimum size of an RVE. Additionally, the spatial correlation structure of the crack density tensor with the meso-scale moduli is studied.
Scale effects in plasticity of random media: status and challenges
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When the separation of scales in random media does not hold, the representative volume element (RVE) of classical continuum mechanics does not exist in the conventional sense, and various new approaches are needed. This subject is discussed here in the context of plasticity of random, microheterogeneous media. The first principal topic considered is that of hierarchies of mesoscale bounds, set up over a statistical volume element (SVE), for elasticplastic-hardening microstructures; these bounds, with growing mesoscale, tend to converge to RVE responses. Following a formulation of the said hierarchies from variational principles and their illustration on two specific examples of power-law hardening materials, we turn to rigid-perfectly-plastic materials. The latter are illustrated by simulations in the setting of a planar random chessboard. The second principal topic is the analysis of spatially non-uniform response patterns of randomly heterogeneous plastic materials. We focus here on the geodesic properties of shear-band patterns, and then on the correlation of strain fields to the underlying microstructures. In the case of perfectly-plastic materials, shear-bands become slip-lines, but their spatial disorder is still present, and is described in ensemble sense by wedges of randomly scattered characteristics.
Scale-dependent homogenization of random composites as micropolar continua
European Journal of Mechanics - A/Solids, 2015
A multitude of composite materials ranging from polycrystals to rocks, concrete, and masonry overwhelmingly display random morphologies. While it is known that a Cosserat (micropolar) medium model of such materials is superior to a Cauchy model, the size of the Representative Volume Element (RVE) of the effective homogeneous Cosserat continuum has so far been unknown. Moreover, the determination of RVE properties has always been based on the periodic cell concept. This study presents a homogenization procedure for disordered Cosserat-type materials without assuming any spatial periodicity of the microstructures. The setting is one of linear elasticity of statistically homogeneous and ergodic two-phase (matrix-inclusion) random microstructures. The homogenization is carried out according to a generalized Hill-Mandel type condition applied on mesoscales, accounting for non-symmetric strain and stress as well as couple-stress and curvature tensors. In the setting of a two-dimensional elastic medium made of a base matrix and a random distribution of disk-shaped inclusions of given density, using Dirichlet-type and Neumann-type loadings, two hierarchies of scale-dependent bounds on classical and micropolar elastic moduli are obtained. The characteristic length scales of approximating micropolar continua are then determined. Two material cases of inclusions, either stiffer or softer than the matrix, are studied and it is found that, independent of the contrast in moduli, the RVE size for the bending micropolar moduli is smaller than that obtained for the classical moduli. The results point to the need of accounting for: spatial randomness of the medium, the presence of inclusions intersecting the edges of test windows, and the importance of additional degrees of freedom of the Cosserat continuum.
Micromechanically based stochastic finite elements: length scales and anisotropy
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The present stochastic finite element (SFE) study amplifies a recently developed micromechanically based approach in which two estimates (upper and lower) of the finite element stiffness matrix and of the global response need first to be calculated. These two estimates correspond, respectively, to the principles of stationary potential and complementary energy on which the SFE is based. Both estimates of the stiffness matrix are anisotropic and tend to converge towards one another only in the infinite scale limit; this points to the fact that an approximating meso-scale continuum random field is neither unique nor isotropic. The SFE methodology based on this approach is implemented in a Monte Carlo sense for a conductivity (equivalently, out-of-plane elasticity) problem of a matrix-inclusion composite under mixed boundary conditions. Two versions are developed: in one an exact calculation of all the elements' stiffness matrices from the microstructure over the entire finite element mesh is carried out, while in the second one a second-order statistical characterization of the mesoscale continuum random field is used to generate these matrices.
Mesoscale bounds in viscoelasticity of random composites
Under consideration is the problem of size and response of the representative volume element (RVE) of spatially random linear viscoelastic materials. The model microstructure adopted here is the random checkerboard with one phase elastic and another viscoelastic, perfectly bonded everywhere. The method relies on the hierarchies of mesoscale bounds of relaxation moduli and creep compliances obtained via solutions of two stochastic initial boundary value problems, respectively, under uniform kinematic and uniform stress boundary conditions. In general, the microscale viscoelasticity introduces larger discrepancy in the hierarchy of mesoscale bounds compared to elasticity, and this discrepancy grows as the time increases.