Most metallic materials do not show tension-compression asymmetry in plastic deformation. In these cases, the true stress-strain relationship is independent of the hydrostatic component of the stress state, i.e. there is no volume change, and the true stress-true strain response of the material is the same in tension and compression. The conversion into a nominal stress-strain relationship can then be easily achieved using standard analytical equations [1].
During a PIP test, the material is modelled using an empirical true stress-strain relationship (the Voce law) and is assumed to conform to this true stress-strain relationship under both tension and compression. This is valid for the vast majority of metallic materials. However, in some cases, particularly metals containing significant porosity or strongly textured hcp metals, tension-compression asymmetry may be observed. An example of this is shown in the figure below, which shows the true stress-strain curves for a magnesium sample derived under uniaxial tension and uniaxial compression.
During a PIP test, most of the probed material is under compression. As a result, in cases where tension-compression asymmetry is significant, the results derived from PIP testing tend to give better agreement with compressive uniaxial tests.
[1] Clyne, T.W., Campbell, J.E., Burley, M. and Dean, J., 2021. Profilometry‐based inverse finite element method indentation plastometry. Advanced Engineering Materials, 23(9), p.2100437.