| description abstract | Abstract. Cartilage damage under loading is significantly affected by viscoelasticity. This study validates a finite deformation, nonlinear viscoelastic constitutive model for the collagen matrix of immature bovine articular cartilage, using reactive viscoelasticity. To examine the flow-independent viscoelasticity of cartilage collagen, tissue samples underwent proteoglycan (PG) digestion, losing more than 98% of their initial PG content to increase their hydraulic permeability. To verify that this PG-digestion eliminated flow-dependent viscoelasticity, PG-depleted samples were subjected to a gravitational permeation experiment, demonstrating that their hydraulic permeability, k=268 ± 152 mm4/N⋅s (n = 8), was five orders of magnitude greater than reported for untreated cartilage, confirming negligible flow-dependent viscoelasticity. Digested cartilage plugs were then subjected to unconfined compression stress relaxation (four consecutive ramp-hold profiles, each increasing the compressive strain by 10%) to fit the load response and extract material properties (RMSEfit=1.86 ± 0.61 kPa, n = 8). Successful curve-fitting served as a necessary condition for validating the model. Then, a separate unconfined compression stress-relaxation test was performed on the same samples, to 40% compressive strain at the same ramp rate. The model was able to faithfully predict this experimental response using fitted material properties (RMSEpred=3.95 ± 1.33 kPa, with 0≤ stresses ≤ 155 ± 37 kPa), providing a sufficient condition for validation in unconfined compression stress-relaxation. A computational model then showed that flow-independent viscoelasticity of cartilage collagen can enhance the stress response by ∼15% at fast strain rates, over flow-dependent effects. However, we estimate from prior studies that flow-independent viscoelasticity may enhance the stress response of cartilage by up to 200%, implying that PGs probably contribute significantly to the tissue's flow-independent viscoelasticity. | |
