Systematic detection and evaluation of cracking behavior of flawed brittle sandstones with AE and 3D-DIC techniques

Determination of the cracking behavior during crack propagation helps to better understand damage and fracture processes in brittle rocks. The paper studies the cracking behavior of rocks on three scales: macro-deformation (or macro-cracking), internal micro-fracture, and surface crack coalescence. Under uniaxial compression, the cracking behavior of two types of sandstone specimens having single flaws was experimentally and systematically investigated. Acoustic emission (AE) and three-dimensional digital image correlation (3D-DIC) techniques were utilized to continuously monitor the acoustic shock signals generated by micro-fracture events inside the specimen and the specimen surface cracking process. The experimental results show that at the crack initiation stage, many micro-tensile fractures within the rock are initiated and coalesced, and small strain localized zones (SLZs) appear on the specimen surface. In the crack propagation stage, micro-fractures coalesce into macro-fractures that propagate in tensile mode to form surface cracks, which finally break in tension or slide against each other in shear mode. The formation of SLZs is related to the dip angle of pre-existing flaws, which determines the direction and mode of crack propagation. In conclusion, the strong acoustic-optical evidence accompanying different cracking behaviors is discussed in detail. From both acoustic and optical perspectives, it reveals and explains how flaws and material properties affect the strength and cracking mechanisms of brittle rocks. The study aids comprehension of the potential relation between internal micro-fracture and surface cracking in the process of engineering rock mass failure.