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, Poissonís ratio ν=0.125, fluid fraction ratio θf=0.83, permeability coefficient κ=7.6◊10-15 m4N-1s-1. Confined compression is a sample placed in a confining chamber and compressed with a permeable piston. The problem is one dimensional in z.
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 z; thus the problem becomes one dimensional problem in r.
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 ū=0.0445 mm at instant t0=500 s. The results were obtained in a single time step Δt=tmax=1000 s. The confined and unconfined (lubricated) compression tests were modelled with a regular mesh of 4x1 elements. The unconfined (adhesive) compression test was modelled with a regular mesh of 6x1 elements.


Confined compression test:
Displacements: Solid ur≈0 Solid uz (3D) Fluid ur≈0 Fluid uz (3D) Vector form
Stresses: σrr (3D) σθθ (3D) σzz (3D) σrz≈0 Pressure (3D)
Unconfined compression test with lubricated platens:
Displacements: Solid ur (3D) Solid uz (3D) Fluid ur (3D) Fluid uz (3D) Vector form
Stresses: σrr (3D) σθθ (3D) σzz (3D) σrz≈0 Pressure (3D)
Unconfined compression test with adhesive platens:
Displacements: Solid ur (3D) Solid uz (3D) Fluid ur (3D) Fluid uz (3D) Vector form
Stresses: σrr (3D) σθθ (3D) σzz (3D) σrz (3D) Pressure (3D)

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