Fracture Processes in Granite Blocks Under Blast Loading

The fracturing of six granite cubes (400x400x400mm(3)) in response to blast loading was investigated using a combination of data collected from strain gauges and generated by digital image correlation (DIC) of pictures captured using a high-speed camera. This instrumentation permits the observation of the crack initiation, crack opening velocity, fracture pattern, full-field strain variation, and fragment movement at the cube's surface. In each experiment, an explosive charge was positioned at the center of the block in a vertical drill hole. Two charge weights, 6g and 12g, of pentaerythritol tetranitrate (PETN) were used. Using the high-speed camera, dominant vertical cracks were found to initiate on the surface of the cubes within 250 mu s of the charge detonation. Two or three dominant vertical cracks appeared in specimens with a 12g charge, while a single dominant vertical crack came into view in specimens with a 6g charge. In addition, a single dominant horizontal crack was observed in all specimens, irrespective of charge weight. The maximum concentration of strain obtained from a DIC analysis agreed well with the dominant cracks and fracture patterns observed in the specimens. By combining the results from the strain gauges and the results from the DIC analysis for the specimen with a 12g charge, the first crack initiation was found to occur at 67 mu s after detonation. The crack opening velocities were determined using a boundary identification method and ranged from 5.0 to 7.6ms(-1), whereas the observed in-plane fragment velocities were slightly less. These experiments may contribute to a better understanding of the fundamental mechanisms of the rock fracture by blasting.