Articular cartilage represents a complex inhomogeneous, multiphase material. Due to its sophisticated structure articular cartilage distinguishes itself by exceptional load-bearing properties under a wide variety of loading conditions.
Currently, it is generally recognized that articular cartilage is subjected to large strains under in vivo mechanical conditions, which can be described as a first approximation with nonlinear elastic material models. Furthermore, rate dependent phenomena and osmotic effects caused by the presence of fixed charges can be observed.
We propose to model structural effects of articular cartilage load response within the context of a phenomenological biphasic material approach resulting from an overlay concept (cf. [
]). The basic idea of this superposition methodology is the additive decomposition of the stress tensor according to specific mechanical properties whose underlying physics can partly be illustrated on rheological models adapted to large strain conditions (see [
] and others).
Recent investigations have shown that due to the varying orientation of the collagen fibers the solid matrix of the cartilage tissue can be considered as an anisotropic hyperelastic material with tensioncompression nonlinearities. Frequently, the rate-dependency of a biphasic material behavior is exclusively attributed to the fluid flow through the solid matrix. However, recent observations demonstrate that viscoelastic properties of the solid phase must not be neglected. Therefore, viscous overstresses have been considered by the authors as well as an osmotic pressure model to simulate swelling states.
The theoretical background and the numerical algorithms of all the parts of a suitable material model are presented. This model has been implemented into a commercially available FE-code. Some numerical examples showing several structural effects are discussed within the context of experimental results.