Deformation and fracture of rocks loaded with spherical indenters

The response of four rocks, namely granite, rhyolite, limestone and schist, to indentation by blunt cutting tools 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 and on indenter size. Blunt indenters promote elastic response; soft rocks tend to elastic-plastic response. In the elastic-plastic range, depression radius has a linear relationship to the indenter radius. Radial cracking occurs in soft rocks leading to strength degradation in the near surface regions. Sharp indenters promote radial cracking. The relationship between length of radial cracks and contact force is nonlinear. Lateral cracking occurs in soft rocks leading to material removal. Sharp indenters promote lateral fractures. Anisotropy and non-homogeneity affect the material response notably. The ratio between fracture toughness and hardness ('index of brittleness') is a promising parameter to evaluate the behaviour of rock materials in excavation, drilling and fragmentation processes.