Sandra Ilic

Application of the multiscale FEM to the modeling of cancellous bone

This paper considers the application of multiscale finite element method (FEM) to the modeling of cancellous bone as an alternative for Biot’s model, the main intention of which is to decrease the extent of the necessary laboratory tests. At the beginning, the paper gives a brief explanation of the multiscale concept and thereafter focuses on the modeling of the representative volume element and on the calculation of the effective material parameters, including an analysis of their change with respect to increasing porosity. The latter part of the paper concentrates on the macroscopic calculations, which is illustrated by the simulation of ultrasonic testing and a study of the attenuation dependency on material parameters and excitation frequency. The results endorse conclusions drawn from the experiments: increasing excitation frequency and material density cause increasing attenuation.

APPLICATION OF A BIPHASIC REPRESENTATIVE VOLUME ELEMENT TO THE SIMULATION OF WAVE PROPAGATION THROUGH CANCELLOUS BONE

This paper deals with the application of the multiscale finite element method for simulating the cancellous bone. For this purpose, two types of biphasic representative volume elements are proposed. In the first one, the solid frame consists of thin walls simulated by shell elements. On the other hand, the solid phase of the second model is made up of columns consisting of eight-node brick elements. This choice of representative volume elements is motivated by experimental investigations reporting on the existence of plate-like and rode-like types of cancellous bone and possible conversions between them. The proposed representative volume elements are used to calculate effective material tensors and parameters and to investigate their change in terms of increasing porosity, which is typical for osteoporosis. As a first example, changes in the geometry of the representative volume elements are used to explore material anisotropy. In the end, the final example considers wave propagation through the bone treated as a homogenized medium.

Homogenization Theories and Inverse Problems

Various approaches are presented for modelling the acoustic response of cancellous bone to ultrasound interrogation. As the characteristic pore size in cancellous bone is much smaller than a typical bone sample, there is a clear scale separation (micro versus macro). Thus, our modelling methods are mainly based on homogenization techniques and numerical upscaling. First, we consider the so-called direct problems and present models for both periodically perforated domain and a domain with random distribution of pores, as well as nonlinear model with a shear-thinning viscoelastic material emulating the blood-marrow mixture. A numerical procedure is given for the upscaling of a diphasic mixture using different trabeculae thicknesses and various frequencies for the ultrasound excitation. Finally, the results of a quite accurate two-dimensional inversion for the Biot parameters are presented. Further details for these different problems are amply described in the literature cited in the bibliography.

Solution-Precipitation Creep – Modeling and Extended FE Implementation

The topic of this contribution is the mechanical modeling of solutionprecipitation creep, a process occurring in polycrystalline and granular structures under specific temperature and pressure conditions. The model presented has a variational structure and is based on a novel proposal for the dissipation while the elastic energy is kept in the standard form. The assumed dissipation term depends on two kinds of velocities characteristic for the process: velocity of material transfer and velocity of inelastic deformations, both manifesting themselves on the boundaries of the grains. For the numerical implementation, the standard finite element program FEAP together with the pre- and postprocessing software package GID are used. The simulations are illustrated by two examples, a polycrystal with regular hexagonal microstructure and a polycrystal with random microstructure.