Calculating the elastic modulus from nanoindentation and microindentation reload curves

The Oliver–Pharr method was used to calculate the elastic modulus from the reloading curve and was compared to the traditional unloading curve method. Nanoindentation and microindentation testing instruments were used. This method was applied to load–unload–reload–unload, multistep, and cycle indentation testing procedures at various hold times and force rates. On unloading the reverse plasticity added to the elastic recovery which increased the apparent elastic modulus. During reloading there was mainly elastic deformation making it more reliable for the elastic modulus calculation. It was also found that the metals tested started yielding between 70% and 100% of the reload curve. The reload indentation elastic modulus for fused silica and several metals was equivalent to the tensile test elastic modulus from reference literature.