This tool allows for the computing and visualizing of the shear modulus (MPa) and the Young’s modulus (MPa) from an indentation test performed using a spherical indenter (Figure 1). This model is well adapted for the mechanical characterization of samples (with critical diameter (width) at least 5 times the indenter diameter) bonded on a flat rigid support (at least 10 times stiffer than the sample), for example: any sheet of material lying on a rigid chamber base (limited sliding at the interface can be assumed) or a cartilage layer attached to a thick sub-chondral bone. A user license is required to obtain the results of the elastic model in indentation (Software Add-On, contact Biomomentum to obtain your license).
Results analysis is performed considering the indentation mechanics of an infinitely wide elastic layer bonded to a rigid half-space which has been developed as a model for the layered geometry of cartilage and subchondral bone (Hayes et al., 1972). (Figure 2) shows a schematic representing the modeled situation.
Many parameters need to be specified in order to calculate the different material properties.
1. Indenter Shape is:
- Spherical
- Plane-Ended Cylindrical
2. Indenter radius in mm.
3. Poisson ratio can be selected from a drop-down list (Figure 3)
4. Position @ contact is – defines the position of the left cursor:
- to be curvefit – will automatically find the position contact which will give the smallest mean squared error (mse)
- the value at t0 – will automatically take the position contact at the start of the curve
- the following value (mm) – the user need to specify the value in the corresponding field next to the drop-down list
- cursor position – the user can drag the left cursor to the wanted position contact
5. Sample thickness is the:
- position @ contact – will automatically find the position at contact (position of the left cursor)
- following value (mm) – the user need to specify the value in the corresponding field next to the drop-down list
6. Curvefit is up to – defines the position of the right cursor:
- peak load – will automatically take the position of the peak load
- delta load (unit) – the user need to specify the value of the desired delta load in the corresponding field next to the drop-down list
- delta position (mm) – the user need to specify the value of the desired delta position in the corresponding field next to the drop-down list
The error on the estimation of the best curve representing the stress relaxation curve is calculated using the root of the mean squares (RMS) method and is displayed under “RMSE”. For example, (Figure 4) shows parameters used for the “Normal Indentation” (Mach-1 Motion) function in an indentation testing protocol on ovine articular surfaces. Corresponding results are shown in (Figure 1).
