Significant strain-softening of articular cartilage along its depth direction has been observed in several studies [1,2], where the free-swelling equilibrium state of the tissue was taken to be the reference configuration for subsequent deformation. Microstructural models have been proposed to interpret this softening as buckling of pre-stressed collagen fibers [3,4]. In this study, we propose that a constitutive model which accounts for the disparity in tensile and compressive moduli of cartilage as well as the osmotic swelling response, can explain this experimentally observed strain-softening mechanism. The strain-softening behavior of the tissue is investigated experimentally and theoretically along three mutually perpendicular directions: directions parallel and perpendicular to the split line direction (1- and 2-direction), and normal to the articular surface (3-direction). A conewise nonlinear elasticity model is incorporated into the triphasic theory [5] to interpret this strain-softening in the context of Donnan osmotic swelling [6].

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