Here is a list of a few terms associated with compression testing of materials:
σ=Eδ
Stress - σ, The amount of force applied to the load divided by the cross sectional area, in units of Pa
Modulus of Elasticity- E, A material property that is used to relate stress to strain, in units of Pa
Strain- δ, The change in position over the initial height of the sample (commonly calculated in %)
Toughness - The ability of a material to absorb energy and plastically deform without fracturing. Computed as the integral under the stress strain curve.
We are using three separate tests (and a combination of all three) in order to simulate the compressive forces that a knee implant will experience. The general schemes of these tests are seen below:
Creep - This test steadily increases the strain on the sample until the desired stress, or load, is reached. This load is then maintained over a certain amount of time, and the change of strain (Δ δ) is monitored.
This entails dynamic programming for the load cell in the CNC machine. Basic commands include the change in position over time. However, to maintain a certain load, the program must use the real-time data to compute the position change needed to provide a constant load on the sample.
SRL - This test changes the position of the load cell in order to reach a
desired strain on the sample. The load cell reports the force required to create that strain, and monitors the change in that stress as time progresses. A simulation of the SRL test may be found
here.
Dynamic - This test oscillates the load cell at a given frequency and monitors the load on the sample. This simulates the constant wear and tear on a sample such as a knee replacement.
