Fracture evolution in mudstone specimens containing a pre-existing flaw under true triaxial compression

In this study, a novel true triaxial loading device that can be paired with an X-ray computerized tomography (CT) scanning system was developed. Using this test equipment, a series of in-situ CT scans of mudstone specimens containing a pre-existing flaw were performed under two true triaxial stress states, namely, the intermediate principal stress parallel to the pre-existing flaw (IPSPPF) and the minimum principal stress parallel to the pre-existing flaw (MPSPPF). The results showed that the difference between the two lateral deformations of the specimen under the IPSPPF condition was more significant than that under the MPSPPF condition and that the stress level of the specimen entering the damage stage under the IPSPPF condition was less than that under the MPSPPF condition. Under the IPSPPF condition, two wing cracks formed in the damage stage first, and an anti-wing crack and a far-field crack then formed successively in the post-peak stage. The surfaces of these four cracks were all parallel to the front edge of the pre-existing flaw. After failure, the pre-existing flaw in the specimen remained fully open. Conversely, under the MPSPPF condition, a far-field crack appeared first in the damage stage, which stopped propagating after intersecting the pre-existing flaw. Concurrently, a wing-like crack and an anti-wing-like crack formed at the tips of the pre-existing flaw in the damage stage, and a new far-field crack formed in the post-peak stage. The surfaces of these four cracks were all perpendicular to the front edge of the pre-existing flaw. The pre-existing flaw was cut off at multiple locations by these cracks and eventually collapsed in large areas. Under both conditions, the inclination angles of all of the cracks in the cross sections of the specimens with respect to the direction of the intermediate principal stress were approximately 0°.