ABSTRACT
This chapter represents only an introduction to time effects in soft tissues. It adopts the rule of time translation invariance and addresses essentially linear viscoelasticity of non aging tissues while real tissues display both intrinsic time effects and nonlinearity at large strains. Overall incompressibility of tissues can be a good approximation only for short term responses. In fact, a clear distinction should be made between overall, or apparent, properties of the whole tissue, and intrinsic properties of the solid skeleton. Actually, collagen viscoelasticity is thought to account for most of the stress relaxation in tension while poroelastic effects associated with fluid diffusion dominates the relaxation effects in compression, Li et al. Wilson et al. develop a finite element model of osteoarthritis using a poro-viscoelastic fibril-reinforced model, accounting for the directional non random distribution of fibrils and their spatial inhomogeneities across the depth.
