THE EFFECT OF THE STRAIN RATE ON THE SHORT TERM SHEAR RESPONSE OF ARTICULAR CARTILAGE

The shear stress relaxation of cylindrical, osteochondra1 plugs was measured by means of ramp displacements at strain rates ranging approximately from 0.06 s-1 to 13 S-1. The plugs were prepared from patellar grooves of fresh bovine femora. For each specimen separately, the data was fitted t o a five-parameter quasi- linear viscoelastic model. The different loading rates were pooled together to obtain a solution that would conform as well as possible to all time-dependent force functions measured with a particular specimen. This method makes it possible to analyze stress functions caused by arbitrary strain histories. From strain rate 10 3-1 to rate 0.1 s-1 the shear modulus dropped in average by a factor of 1 . 5 2 and the relaxation was roughly logarithmic. At rate 10 s-1, the average ,shear modulus was 4 . 8 5 MPa. All the moduli were assessed a t a fixed, 4% strain to avoid errors due to nonlinearity of the material. The measured stresses were well within human physiological limits, and the strain rates used in the experiment correspond to the time-scale of human knee joint loading during walking or running