Rate effect on crack propagation measurement results with crack propagation gauge, digital image correlation, and visual methods

The rate-dependent measurement gaps among the crack propagation gauge (CPG), digital image correlation (DIC), and Visual methods for measuring dynamic rock crack propagation were determined using a case study. Dynamic notched semi-circular bending (NSCB) tests of a rock material, namely, red sandstone, were conducted using a modified split Hopkinson pressure bar system to form different crack propagating velocities (CPVs). From the viewpoint of the initial crack time, a normal strain threshold of 2% in the DIC was effective in obtaining a match between the CPG and DIC methods at a low CPV. However, the selected 2%-5% normal strains in the DIC were pivotal factors that could compromise the results from the CPG and Visual methods with a low CPV in terms of the peak CPV, whereas 2%-3% normal strains were recommended for cases with a high CPV. In addition, the equivalent crack-velocity-dependent strain thresholds (t(i)(DIC) equal to t(i)(CPG) and t(i)(DIC) equal to t(i)(Visual)) for the CPG, Visual, and DIC techniques displayed a linear fit with the CPV. The measurement gap between the DIC and Visual methods was narrower than the measurement gap between the DIC and CPG methods. The critical initial crack time of CPG for red sandstone needed the condition of a CPV of approximately 500 m/s. A potential explanation for these gaps was proposed in terms of the changes in the fracture process zone with CPV, and three suggestions were subsequently recommended based on the experimental results to aid in the interpretation of other experimental results.