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.
  • Basic commands will tell the CNC machine what position change to make.
• 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.