Fracture initiation in cavity expansion of rock

•Initiation of fracture is monitored from a cylindrical opening.•Digital image correlation provided details of process zone prior to peak pressure.•A discrete element model showed size dependency of peak pressure.•Size effects are governed by both material and structural length scales.

Initiation of fracture from a cylindrical opening in blocks of Berea sandstone under plane strain was monitored with a cavity expansion apparatus. Using the particle tracking technique called digital image correlation, a discontinuity in the displacement field near the cavity boundary was identified at 80% of the peak internal pressure, which meant that a process zone developed prior to peak. To investigate the effect of scaling on fracture, a two dimensional bonded particle model was validated, and several specimens sizes with different borehole and tension softening characteristics were tested numerically. It is demonstrated that with increase in specimen size or with increase in borehole size, the peak pressure decreases, and these size effects are governed by both material and structural length scales.