Sample Porosity

In the largely compressive stress state during indentation, pores will tend to reduce in volume, leading to an overall decrease in volume which invalidates some of the assumptions made when carrying out FE (Finite Element) analysis of an indent.

As volume is not conserved, the material piles up less during indentation (the pile-up is the material pushed out by the indenter to above the original surface level). This means that when carrying out inverse FE on the profile, the inferred tensile properties tend to show a lower yield stress and higher level of work hardening than the true tensile properties. Notable deviations from the true tensile properties begin to occur when sample porosity is larger than 1-2%. Large pores can also lead to premature fracture (i.e., before the necking point) in tension by initiating cracks.