Initiation of tensile and mixed-mode fracture in sandstone

Initiation of failure under three-point bending was observed in sandstone specimens with and without a stress concentrator. Acoustic emission (AE) and electronic speckle pattern interferometry (ESPI) were used to quantify the fracture phenomena through AE locations and ESPI images of an intrinsic damage zone. To investigate mixed mode fracture, beam tests with off-center notches were studied as well. Before about 95% of peak load, the locations of AE were somewhat random for beams with a smooth boundary (no notch). From 96-100% of peak, microcrack hypocenters indicated localization in the form of an intrinsic zone, which was also identified by the high resolution technique of ESPI. The discrete element technique was used to mimic the failure process. A synthetic rock was composed of rigid circular particles that interact through normal and shear springs, and the particles were glued to each other using normal and shear bonds. A microcrack forming in tension or shear was simulated by breakage of the bond between two particles in contact. The synthetic rock was calibrated for elastic properties, unconfined compressive strength, and bending tensile strength. The beam strength with and without notches and the fracture path were studied in the numerical experiments, and the results compared favorably with those obtained from the laboratory experiments.