Strength and failure characteristics of coal measures mudstone specimens containing a prefabricated flaw under true triaxial tests

In this study, a series of coal measures mudstone specimens containing a prefabricated flaw were subjected to true triaxial test (TTT), namely, specimens of the intermediate principal stress ( σ 2 ) parallel to prefabricated flaw (TTT-Flaw-2) and specimens of the minimum principal stress ( σ 3 ) parallel to prefabricated flaw (TTT-Flaw-3). The main objective of this study was to investigate the effects of the loading direction of σ 2 and the position of prefabricated flaw on the strength and failure modes of specimens. The results showed that the peak strength of intact and flawed specimens first increased and then decreased with increasing σ 2 , which could be fitted by the Mogi-Coulomb criterion. Under the same loading stresses, the strength of intact specimen was larger than that of flawed specimens, and specimens TTT-Flaw-2 had the lowest strength. The X-ray computerized tomography scanning results revealed that fractures were not always observed to form along the prefabricated flaw tips but were distributed randomly inside the specimen under conventional triaxial test conditions. Under TTT conditions, anti-wing cracks initiated from the vicinity of the prefabricated flaw tip and were observed in σ 2 -drrection for specimens TTT-Flaw-2. While for specimens TTT-Flaw-3, shear cracks appeared in σ 2 -direction, and few anti-wing cracks were observed in σ 3 -direction.