Observations made on rocks during indentation testing with spherical indentation tools

The response of four rocks (granite, rhyolite, limestone, schist) to indentation by blunt indenters is discussed from a contact mechanics point of view. Contact forces between 0.1 kN and 2.45 kN are applied; indenter sizes are 1.0 and 5.0 mm, respectively. A transition from elastic to elastic-plastic response exists which depends on rock hardness. In the elastic-plastic range, depression radius has a linear relationship to the indenter radius. In soft rocks, radial cracking, accompanied by strength degradation, and lateral fractures, leading to material removal in the near surface regions, occur. Effects of anisotropy and non-homogeneity are discussed. The ratio between fracture toughness and hardness-frequently called 'index of brittleness'-is a promising parameter to evaluate the behaviour of rock materials.