ABSTRACT
Hardness, stiffness and toughness represent the resistance of solid materials to plastic and elastic deformations and crack propagation, respectively. The indentation hardness and constraint factor depend on the properties of the deformed material, the indenter geometry, and the coefficient of friction. For materials which exhibit significant work-hardening, the hardness is much larger than the yield stress. This is because hardness is conventionally measured at a strain of about 8%, where significant work-hardening can take place. Measurement and theory of hardness testing have been developed in the field of material testing, where it has been possible to carry out hardness tests on relatively large specimens. The measurement of hardness started with scratch testing of flat surfaces by various particles having varying degree of hardness, i.e. Mohs hardness test. Fracture toughness represents the resistance of solid materials to crack propagation.
