Ratcheting behavior of articular cartilage under cyclic unconfined compression.

The ratcheting deformation of articular cartilage can produce due to the repeated accumulations of compressive strain in cartilage. The aim of this study was to investigate the ratcheting behavior of articular cartilage under cyclic compression. A series of uniaxial cyclic compression tests were conducted for online soaked and unsoaked cartilage samples and the effects of stress variation and stress rate on ratcheting behavior of cartilage were investigated. It is found that the ratcheting strains of online soaked and unsoaked cartilage samples increase rapidly at initial stage and then show the slower increase with cyclic compression going on. On the contrary, the ratcheting strain rate decreases quickly at first and then exhibits a relatively stable and small value. Both the ratcheting strain and ratcheting strain rate increase with stress variation increasing or with stress rate decreasing. Simultaneously, the optimized digital image correlation (DIC) technique was applied to study the ratcheting behavior and Young's modulus of different layers for cartilage under cyclic compression. It is found that the ratcheting behavior of cartilage is dependent on its depth. The ratcheting strain and its rate decrease through the depth of cartilage from surface to deep, whereas the Young's modulus increases.