Initiation of fracture from a circular cavity was monitored with a cavity expansion apparatus for two rock materials: Berea sandstone and Indiana limestone. Fracture characteristic from cavity expansion tests were compared to three-point-bending (3PB) tests on the same rock.
Using a particle tracking technique called digital image correlation (DIC), a discontinuity in the displacement field near the cavity boundary was identified at 80% and 60% peak internal pressure for Berea sandstone and Indiana limestone, respectively. The critical crack opening displacement (CCOD) was assumed to be associated with unstable crack growth from the 3PB tests, which occurs at peak loading. The CCOD measurements from three-point-bend tests were 51-58μm for Berea sandstone and 15-16μm for Indiana limestone. The measured COD from the cavity expansion test at peak pressure was 57-90μm for Berea sandstone and 20-24μm for Indiana limestone. The results show the CCOD from cavity expansion were on average 35% higher than three-point-bend test for Berea sandstone and 40% higher for Indiana limestone, suggesting stable growth prior to peak pressure.