Fracture Analysis in Brittle Sandstone by Digital Imaging and AE Techniques: Role of Flaw Length Ratio

To investigate the roles of nonisometric flaws in rock failure behaviors, uniaxial compression tests with the combined monitoring of acoustic emission (AE) and digital imaging were conducted on sandstone specimens containing two preexisting flaws with a variety of flaw length ratios. The AE rate-process theory was used to analyze fracture-related AE event rate characteristics. Acoustical observations suggest that soon before macrofracturing, the AE event rate rapidly increases, while the AE event rate rapidly decreases after macrocracks initiate, propagate, and coalesce with each other. Optical inspections imply that the strain localizes around the flaw tips prior to the formation of macrocracks, the strain releases at the moment of the macrofracturing, and the first crack initiation is always induced by tensile cracking, while the ultimate failure is triggered by shear fracturing. Combined acousto-optic-mechanical (AOM) inspections imply that the cracking processes of sandstone specimens with relatively large flaw length ratios are more progressive than those with small flaw length ratios. For isometric flaw configurations, the stress fields interact with a stress buildup pattern, while for nonisometric flaw configurations, the stress buildup pattern and the stress shadow pattern coexist. Finally, the microscopic tensile and shear failure mechanisms are revealed by parametric analysis of the AE signals.