Observed and simulated fracture pattern in diametrically loaded discs of rock material

To investigate the influence of a tensile stress gradient on fracture initiation and fracture growth in rock material, a configuration, consisting of a diametrically loaded disc with a hole on the diameter perpendicular to the loaded diameter, is used. The maximum local tensile stresses the material is able to withstand increase as the stress gradient increases. Depending on the diameter and the eccentricity of the hole, the disc splits along the loaded diameter or macro-fracturing starts at the hole. However, the tensile stresses at the top and the bottom of the hole are for nearly all cases considerably higher than the stress along the loaded diameter and than the macroscopic tensile strength of the material, determined by conventional Brazilian tests. To better understand this particular fracturing behaviour, numerical simulations of the experiments are conducted using the boundary element code DIGS, which allows the incorporation of weak elements (flaws), representing defects and weaknesses in the rock material. It is shown that the influence of the stress gradient on the stress concentration at the tip of mobilised defects lies at the origin of the particular fracturing behaviour in the diametrically loaded disc with a hole. The study of the new configuration leads also to a number of conclusions with regard to the failure in diametrically loaded discs in general. Based on the flaw model, fracture initiation in the Brazilian test has to be attributed to fracture growth of a mobilised defect, situated in the area close to one of the platens.