I am in the process of modeling the medial half of the knee for a cartilage contact problem. I have designed idealized geometry of the medial size of the femoral cartilage above the tibial cartilage. As I am examining a steady-state loading plateau, I am neglecting viscoelasticity or any fluid flow, purely matrix stiffness. The cartilage is modeled as neo-hookean with a density of 1100kg/m^3, 3.3MPa stiffness, and 0.4 poissons. Both the femoral and tibial components are treated this way. On the articulating surfaces, I have an sliding elastic contact interface with a penalty of 1000, and frictional coefficient of 0.02. My boundary conditions no movement at all on the lower surface of the tibial cartilage (where it would contact bone) and z only movement on the opposite femoral cartilage side (again, where it wound contact bone.)
The model runs in a reasonable amount of time, but always ends with error termination as negative jacobians are detected for time points. When examining the results in postview, I notice the femoral component is deforming as expected, but the tibial cartilage exhibits no deformation _at all_. The femoral cartilage eventually pushes "through" the tibial cartilage and they wind up overlapping, hence the negative jacobian and resulting errors.
Does anyone have any insight on this? It isn't the boundary condition on the tibial cartilage- i've double and triple checked that its ONLY the underside surface, and nowhere else.
The model runs in a reasonable amount of time, but always ends with error termination as negative jacobians are detected for time points. When examining the results in postview, I notice the femoral component is deforming as expected, but the tibial cartilage exhibits no deformation _at all_. The femoral cartilage eventually pushes "through" the tibial cartilage and they wind up overlapping, hence the negative jacobian and resulting errors.
Does anyone have any insight on this? It isn't the boundary condition on the tibial cartilage- i've double and triple checked that its ONLY the underside surface, and nowhere else.
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