Experimental Technology and Management

[Objective] As mineral resource extraction in China continues to advance deeper into the Earth's crust, underground engineering faces formidable challenges such as high surrounding-rock stresses, severe mining-induced disturbances, and difficulty of fracturing hard rock formations. The solution to these problems is closely related to the mechanical properties of rock, and the fracture characteristics of rock are a research focus. Primary defects, such as joints and fractures, occur randomly within rock masses. The deformation and failure of rock structures are inevitably linked to the evolution of microcracks. Moreover, since the tensile strength of rock is much lower than its compressive strength, examining its tensile failure characteristics is critical. [Methods] In this study, Brazilian splitting tests were conducted on coal, sandy mudstone, and shale samples. The process was comprehensively monitored using both the JM3813 multifunctional static strain gauge and the DS5 series full-information acoustic emission(AE) signal system. The study examined the deformation characteristics, evolution of AE ringing counts and energy parameters, and failure modes during the splitting fracture processes of coal, sandy mudstone, and shale. Concurrently, computed tomography(CT) scanning technology and AVIZO 3D reconstruction software were utilized for an in-depth analysis of the pore and fracture characteristics and fracture morphology of the coal and rock after splitting. The analysis revealed the intrinsic fracture mechanisms within the coal and rock. [Results] After the shale reached the peak strength(8.26 MPa), the stress fell sharply in a “cliff-like” manner. The sharp drop was mainly attributed to the rapid propagation of internal microcracks in the shale under tensile stress, leading to macroscopic fracture and the typical brittle behavior. The sandy mudstone underwent a deformation process similar to that of the coal sample. Still, the peak stress of the coal sample(0.83 MPa) was significantly lower than that of the sandy mudstone(1.95 MPa) and was only one-tenth of the shale's strength. The relationship between strain values within the three deformation zones of the coal sample, sandy mudstone, and shale generally followed the pattern: Strain 3 > Strain 2 > Strain 1. AE ring counts, cumulative ring counts, energy, and cumulative energy increased with stress accumulation. Classification of failure modes based on the relationship between AF/RA and 1 revealed that shear fractures dominate over tensile fractures in coal samples. However, the maximum energy event occurs during tensile fractures. Sandy mudstone and shales have more tensile fractures than shear fractures. AVIZO 3D reconstruction revealed coal samples containing numerous large-scale fissure structures. Following shape filtering and binary conversion in image processing, the fracture surfaces in coal, sandy mudstone, and shale exhibited bold “Y”-shaped, conventional “T”-shaped, and finer “L”-shaped patterns, respectively, with areas of 0.002 95, 0.002 65, and 0.001 73 m², respectively. [Conclusions] Coal is a porous medium with a tensile strength significantly lower than that of shale and sandy mudstone. The splitting failure process of all three materials in the Brazilian splitting tests generally proceeds from the bottom to the top. AE parameters(i.e., ring count, cumulative ring count, energy, and cumulative energy) are closely correlated with the stress state of the material. The failure processes of coal, sandy mudstone, and shale all exhibit characteristics consistent with macroscopic tensile splitting failure. However, the fracture morphology of coal samples is more complex than that of sandy mudstone and shale.