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The hierarchical multi-scale procedure is analysed in this paper. A local multi-scale model has been studied with respect to the macro-level mesh size and meso-level cell size dependency. The material behaviour has been analysed in case of linear-elasticity, hardening and softening. Though the results show no dependency in cases of linear-elasticity and hardening, a strong dependency on both macro-level mesh size and meso-level cell size in case of softening has been found. In order to eliminate both macro-level mesh size and meso-level cell size dependency, a new multi-scale procedure has been proposed. This procedure uniquely links the numerical parameter "macro-level mesh size" with the model parameter "meso-level cell size". The results of this coupled-volume multi-scale model show no dependency on the macro-level mesh size or meso-level cell size.

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a b s t r a c tA new format of anisotropic gradient elasticity is formulated and implemented to simulate stress concentrations in cortical bone. The higher-order effect of the underlying microstructure in cortical bone is accounted for through the introduction of two length scale parameters and associated strain gradient terms which modify the response of the standard elastic macroscopic continuum: one internal length related to the longitudinal fibres and the other related to the transversal Haversian systems. Thus, anisotropic material behaviour is not only included in the anisotropy of the elastic effective stiffness properties, but also in the anisotropic sources of heterogeneity. The model is validated numerically in tests with bone fractures in the longitudinal and the transversal directions. It was found that the dominant length scale effects are those that coincide with the direction of fracture, as defined by the orientation of a preexisting crack.

This paper details a procedure to determine lower bounds on the size of representative volume elements (RVEs) by which the size of the RVE can be quantified objectively for random heterogeneous materials. Here, attention is focused on granular materials with various distributions of inclusion size and volume fraction of inclusions. An extensive analysis of the RVE size dependence on the various parameters is performed. Both deterministic and stochastic parameters are analysed. Also, the effects of loading mode and the parameter of interest are studied. As the RVE size is a function of the material, some material properties such as Young's modulus and Poisson's ratio are analysed as factors that influence the RVE size. The lower bound of RVE size is found as a function of the stochastically distributed volume fraction of inclusions; thus the stochastic stability of the obtained results is assessed. To this end a newly defined concept of stochastic stability (DH-stability) is introduced by which stochastic effects can be included in the stability considerations. DH-stability can be seen as an extension of classical Lyapunov stability. As is shown, DH-stability provides an objective tool to establish the lower bound nature of RVEs for fluctuations in stochastic parameters.