Fig. 4

The loading–unloading curve of −Z _max = 10, 20, 30 μm. The measured force F and calculated force F(Z, a) are plotted as a function of the measured contact radius a. The calculated forces is calculated by substituting the measurements of Z and a into F(Z, a) of Eq. (6); R = 0.2076 m, k = 10.5 kN/m, and E ^* = 2.67 MPa with an RMSE between F(Z, a) and F was 0.6 mN