Crack Evolution and Failure Modes of Shale Containing a Pre-Existing Fissure under Compression

Abstract

To investigate the crack evolution of Longmaxi shales with a single prefabricated fissure, a CCD (charge coupled device) camera and AE (acoustic emission) monitoring equipment were employed. On the basis of real-time CCD photographs and AE events, a real-time crack evolution process in fissured shale specimens under uniaxial compression was investigated. The crack initiation angle and extension angle were calculated, the relationship between the crack initiation stress, strength, and crack angle was compared, and the proportion of tensile and shear cracks at different stages of the whole compression process was briefly analyzed. The results demonstrate that, with the increase in fissure angle (α), the weakening ability of the prefabricated fissure to uniaxial compressive strength and crack initiation stress was reduced. The initial cracks and secondary cracks always appeared at the tip of the pre-existing fissure in the form of tensile cracks for α = 30–90°. The crack initiation angle and expansion angle increased first and then decreased rapidly with α increasing. Furthermore, the ultimate failure modes were mixed tensile and shear failure when α = 0–90°. The crack evolution of the fissured shale was progressive, but the final failure of the fissured specimen occurred rapidly. Furthermore, the appearance of the cracks, stress drops, and AE counts had good consistency in time.

Study on the global stress field of the rock mass under compression and shear considering three kinds of crack parameters

“…Combining Equations ( 11), (18), and z = a + re iθ near the crack tip, so the Taylor expansion of r is performed, where the singular and constant terms are retained, then the stress components near the crack tip can be written as…”

“…Therefore, the theoretical predictions are irrelevant to the mechanical properties of the cracked rock mass, which is obviously unreasonable. For example, there are significant differences in the wing crack initiation angle and fracture mechanism of cracked specimens made of rocks or rock-like materials such as marble, 15 granite, 16,17 shale, 18 sandstone, 19,20 and cement 21 under compression. But the previous studies cannot explain the difference well.…”

Crack Evolution Behaviors and Bursting Liability of Sandstone with Different Sizes: An Experimental Study

“…Also, based on CT images and slices, Duan et al 8 qualitatively studied the crack evolution of Longmaxi shale in the horizontal and vertical direction, and the distributions patterns of crack areas during loading were revealed. In another related study, Li et al 9 examined the real-time crack evolution processes of fractured shale samples under uniaxial compression. The results demonstrated that the crack evolution of fissured shale was progressive.…”

Multifractal Analysis for the Acoustic Emission Energy Dissipation of Shale after Long-Term Immersion in Fracturing Fluids with Various pH

“…AE technology has the advantages of noncontact and real-time monitoring; the AE method can identify the propagation of fractures and describe the inhomogeneity and irregularity of rocks, making it well suited for the investigation of the rock fracture process and characteristics . Scholars have studied the deformation–destruction process of shale through uniaxial compression, triaxial compression, , and Brazilian splitting tests . The AE characteristics of shale can be understood by describing the time- and frequency-domain characteristics of the AE waveform .…”