Confined compression test

The specimen dimensions used for these tests are w=6.35 mm and h=1.78 mm. Material properties corresponding to a normal human cartilage were used, modulus of elasticity E=0.675 MPa, Poisson’s ratio ν=0.125, fluid fraction ratio θ^{f}=0.83, permeability coefficient κ=7.6×10^{15} m^{4}N^{1}s^{1}. Confined compression is a sample placed in a confining chamber and compressed with a permeable piston. The problem is one dimensional in y.


Unconfined compression test


The specimen dimensions and material constants used for these tests are the same as in case of confined compression test. Unconfined compression is a sample of material placed between two platens without side constraints. The platens may be perfectly lubricated or perfectly adhesive (Figure). In case of perfectly lubricated problem the deformation might be expected to be independent of y; thus the problem becomes one dimensional problem in x.

In both cases, the specimen is subjected to the ramp loading. The prescribed displacement is increased linearly in time to reach the prescribed deformation of 5% at instant t_{0}=500 s, i.e. ū=0.089 mm in the confined compression test, and ū=0.0445 mm in the unconfined compression test. The results were obtained with a regular mesh of 2x2 elements, in a single time step Δt=t_{max}=1000 s.


Indentation test


The specimen dimensions and material constants used for this test are the same as in previous cases. During this indentation test, the sample is compressed with a planeended impermeable indenter. The prescribed displacement is ū=0.089 mm. The loading conditions are the same as before. More refined mesh of 3x2 elements was used.

All the previous tests are socalled stressrelaxation tests. A creep test is also often performed in order to obtain material properties of a cartilage. At a time t_{0}, a solid phase of the specimen, the indenter is considered to be permeable, is loaded with a constant stress that is maintained for a sufficiently long time period (1000 s). Two different loading histories were applied. The prescribed value of the loading stress is increased linearly in time until t_{0}=500 s while in the other case the stress reaches its prescribed value τ=100 Pa suddenly at instant t_{0}=10 s. Same material constants, dimensions and mesh refinement as in previous indentation test were used.


Confined compression test:

Unconfined compression test with lubricated platens:

Unconfined compression test with adhesive platens:

Stressrelaxation indentation test:

Creep indentation test with linearly increased loading stress:

Creep indentation test with step constant loading stress:

Comments:
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