Measuring the depth-dependent fracture toughness of articular cartilage based on CTOD method

Studying the anisotropic mechanical properties of cartilage induced by inhomogeneous structure is significant to prevent cartilage damage induced by overloading or fatigue and to develop biomaterials for tissue engineering. The depth-dependent fracture behavior of knee articular cartilage was studied by conducting the fracture tests on the clamped single-edge notched tensile (SENT) samples. The articular cartilage exhibited excellent load bearing capacity and ductility due to its unique depth-dependent matrix structure. The necking length of cartilage decreases through the depth of cartilage, while the peak tensile load first increases, then decreases in depth. The grow of crack in full-thickness cartilage starts in superficial layer at about peak load and the cartilage fails gradually once the crack begins to grow. The fibril network experiences irreversible damage with large area of collagen fibrils yielding before the fracture of full-thickness cartilage. Three fracture parameters, namely the stress intensity factor (K), the critical total work (CTW) and the crack tip opening displacement (CTOD) are calculated to study the fracture properties of cartilage, and CTODΔa=0.2 is employed to evaluate the depth-dependent initiation fracture toughness of cartilage. The fracture toughness of cartilage decreases along depth direction and the fracture toughness of full-thickness cartilage is close to that of deep layer. In addition, the superficial layer with external layer exhibits larger ductility and larger crack-tip blunting degree than the middle and deep layer of cartilage, and plays a vital role in fracture of cartilage. In addition, the fracture toughness of cartilage increases slightly with the increase of loading rate. The study could provide mechanical reference for repairing cartilage defects with artificial cartilage produced by tissue engineering.