Cartilage is poroelastic, not viscoelastic (including and exact theorem about strain energy and viscous loss, and an order of magnitude relation for equilibration time)

Cartilage is often called viscoelastic, yet when strain lags stress in cartilage it is not primarily because of effects within the material of the cartilage skeleton itself. It is because the cartilage skeleton is bathed in fluid. Except in pure shear deformation, attaining equilibrium strain requires that pore fluid flow within the cartilage. Viscous forces retard this flow. This behavior is known as poroelastic. The equilibrium time is of the order L2(Yσ) , where Y is the Young's modulus, σ the permeability of the cartilage, and L is the length of the path along which liquid flows during equilibration. I show that this is true for any consolidation experiment, whatever the direction of consolidation and the direction of liquid flow. In the course of this demonstration I prove that if load is applied abruptly to a Hookean material and is thereafter held constant, the strain energy at equilibrium equals the energy dissipated in the material during equilibration.