ABSTRACT
Hydrostatic compression produces volume changes, while axial compression produces a combination of volume change and shape change. For gases and liquids there is only one kind of compression—hydrostatic—caused by pressure. For solids there are two types: hydrostatic and axial compression. Chemical hardness densities are a measure of stability which is why it is of interest for compression-induced transformation. Compression-induced structural changes, in the language of chemistry, are monomolecular reactions. Therefore, Pearson’s rule should apply to them, and there should be a connection between chemical hardness and critical transformation pressures. For small shear deformations, the volume does not change but finite shear deformations do cause volume changes so the analysis of the driving forces that cause phase transitions in solids is not simple. Analysis of the strains associated with phase changes themselves is even more complex. A phase change rarely occurs at a single flat interface.
