Size of process zone in fracture testing of rock

Fracture of rock involves the formation of a localized region of damage called the process zone, which influences size effects on strength and stability. To study the development of the process zone in rock, physical and numerical experiments are conducted using a three point bending test on beams without a notch. A discrete element model with softening of normal (tensile) bonds is used to capture the quasi-brittle behavior. In the simulations, the normal bond is gradually reduced by increasing the relative normal displacement at the contact point of two particles. The slope of the softening line is assumed to be a material property. It is shown that this property can affect the dimensions of the process zone. The numerical results are compared with some physical experiments with acoustic emission monitoring. Both experimental and numerical results suggest that as the material becomes less brittle, which means a larger process zone, the size of process zone becomes more dependent on the specimen size. Fracture characteristics are size independent only for brittle materials (a small process zone).