INDENTATION ANALYSIS OF BIPHASIC ARTICULAR-CARTILAGE - NONLINEAR PHENOMENA UNDER FINITE DEFORMATION

The nonlinear indentation response of hydrated articular cartilage at phsiologically relevant rates of mechanical loading is studied using a two-phase continuum model of the tissue based on the theory of mixtur es under finite deformation. The matrix equations corresponding to the governing mixture equations for this nonlinear problem are derived us ing a total Lagrangian penalty finite element method, and solved using a predictor-corrector iteration within a modified Newton-Raphson sche me. The stress relaxation indentation problem is examined using either a porous (free draining) indenter or solid (impermeable) indenter und er fast and slow compression rates. The creep indentation problem is s tudied using a porous indenter. We examine the finite deformation resp onse and compare with the response obtained using the linear infinites imal response. Differences between the finite deformation response and the linear response are shown to be significant when the compression rate is fast or when the indenter is impermeable. The finite deformati on model has a larger ratio of peak-to-equilibrium reaction force, and higher relaxation rate than the linear model during the early relaxat ion period, but a similar relaxation time. The finite deformation mode l predicts a slower creep rate than the linear model, as well as a sma ller equilibrium creep displacement. The pressure distribution below t he indenter, particularly near the loaded surface is also larger with the finite deformation model.