Influence of different concealment conditions of parallel double flaws on mechanical properties and failure characteristics of brittle rock under uniaxial compression

Previous 2D studies and tests have mainly focused on the discontinuity of fracture structures along the flaw-inclination direction, and there has been little research into the discontinuity or concealment characteristics of flaws along the flaw-strike direction. To this end, hard brittle rock-like gypsum specimens containing two parallel flaws with three types of flaw-concealment conditions (with proportions of 3D internal flaws of 0%, 50%, and 100%) were prepared. Uniaxial compression tests, accompanied by digital image correlation and acoustic emission monitoring, revealed the following. 1) The concealment conditions of the parallel flaws mainly influenced the uniaxial compression strength of the samples, while the existence of fracture structures was the main factor affecting their elastic modulus. 2) The flaw-concealment conditions had a significant effect on the distributions of the external and internal secondary failure structures. 3) When the proportion of 3D internal flaws was 50%, local failures occurred around the internal flaws and remained hidden, never showing on the outside surface of the specimen; when the proportion of 3D internal flaws was 100%, this kind of 3D sample had greater strength, a faster failure process, more severe damage behaviors, and a more concealed process of destruction than the corresponding 2D sample. 4) The higher strength and concealed failure process of samples containing 3D internal flaws explains why the conclusions of previous 2D research have been found to be not applicable to real 3D conditions.