Functional mechanical behavior of cartilage can be investigated by testing excised tissue in confined, unconfined or indentation geometries using creep, stress relaxation and dynamic sinusoidal tests. Potential nonlinear behavior of cartilage has been mostly characterized in confined compression [1,2]. The extent to which the nonlinearities are intrinsic to the tissue or depend on the specificities of the testing configuration of confined compression is not known. We have therefore performed experimental and analytical tests of articular cartilage in unconfined compression to reveal linear and nonlinear behavior. We found equilibrium stress responses to behave linearly, but transient or dynamic stress responses to be nonlinear. Transient compressive responses stiffened nonlinearly when increasing the static offset compression present at the beginning of the step. On the contrary, dynamic stiffness decreased (weakened) nonlinearly when the amplitude of sinusoidal displacement, imposed on a static offset, was increased. We also found that the articular cartilage nonlinearly maintained a compressive stress when a release displacement was applied from a 10% static offset compression, suggesting possible physiological roles of these nonlinear behaviors.

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