Cancellous bone is a two-component structure consisting of the bone frame and interstitial blood marrow. In the scope of this presentation, the multiscale finite element method is used for its modeling. This method results from a combination of homogenization theory and the theory of finite elements and is based on the calculation of effective material parameters by investigating representative volume elements (RVEs). For the particular kind of material considered here, a cubic two-phase RVE is assumed where the dry skeleton is modeled in different ways. Apart from the variations of the geometry, the influence of the usage of different types of finite elements is studied in this context. Note that the presence of a liquid phase requires dynamic investigation including the viscous phenomena. To this end, acoustic excitation and an analysis in the complex domain are chosen. The method permits calculation of the effective material parameters such as Young's modulus, bulk modulus and Poisson's ratio and furthermore the simulation of the behaviour of the complete bone or of its parts.
Multiscale conceptAlready our former works [1,2] elaborate the topic of the application of the multiscale FEM to the modeling of cancellous bone. This method belongs to the group of the homogenization methods where the simulation of the macroscopic body requires investigation of a so called representative volume element (RVE). The connection of the scales is based on the equality of the macrowork with the volume average of the microworkwhich is known as Hill-Mandel macrohomogeneity condition. Here the overbar symbol denotes the quantities related to the macroscopic body and averaging is carried over the RVE Ω of the volume V . The notation typical for the theory of small deformations is applied: σ denotes stress and strain tensor. Moreover relation (1) is used for the derivation of possible boundary conditions for the RVE. Among them the periodic boundary conditions [3] are the most convenient for modeling of cancellous bone. In such a case the microdeformation u depends on macrostrain tensor¯ and microfluctuationsũ (2. a) which additionally have to be periodic on the periodic boundary of the RVE (2. b,c)Hereũ + andũ − are microfluctuations on the boundary parts with oppositely oriented normal vectors N + and N − . The calculated microfluctuations permit also the calculation of the microstrains (3. a,b) and furthermore of the microstresses σ whose volume average represents sought counterpart on the macroscaleσ (3. c)The numerical interpretation of the definition of the elasticity tensor (3 .d) is the final output from the microscale. Note also that the activation of viscous phenomena in the interstitial marrow requires a dynamic interrogation of the RVE and that harmonic behaviour in time is assumed for all quantities. In such a case, the load p(x, t) and the initiated displacements u(x, t) read p(x, t) = p(x)e iωt , u(x, t) = u(x)e iωtwhere p(x) and u(x) may belong to the complex domain, i is imaginary unity and ω is excitation freque...