The use of Knoop indentation for the assessment of the plastic properties of mortars and natural stones

The present work investigates the Knoop hardness of rocks and other pressure sensitive materials. We tested ten different types of rocks, a cement paste, and PMMA. We also analyzed existing experimental results for Mg-PSZ (magnesia-partially stabilized zirconia) and Pb40Ni40P20 metal glass. We performed finite element indentation analysis for three types of material models: Mohr-Coulomb (associative), Drucker Prager (associative and non-associative), and foam-type. The material models assumed isotropic elasticity (with low elastic strains) and no strain-hardening effects. We compared our analytical results with our experimental results using material data from the literature. We found that different materials can be best described by either the Mohr-Coulomb or Drucker-Prager models. Highly porous materials showed unreasonably high Knoop hardness, raising questions on how their surface can be prepared for indentation tests. Materials with known model types such as cement, PMMA, Mg-PSZ and metal glass, verified our analytical results.