Abstract
The purpose of this paper is to study the crack initiation, propagation, and coalescence of the sandy mudstone sample with two sets of prefabricated cross-flaws under uniaxial compression. This study is different from previous studies on single or multiple parallel prefabricated flaws. The prefabricated cross-flaws are characterized by the dip of the rock bridge with the direction of the main flaw ( β ) and the angle between the direction of main and minor flaws ( γ ). The effects of these two parameters on crack initiation, propagation, coalescence, crack initiation stress, and coalescence stress are analyzed. Moreover, numerical simulation of the uniaxial compression experiments is performed using PFC2D with a flat-joint model, and the simulation results are in good agreement with those from the experiments. The results demonstrate that the dip angle of the rock bridge with the direction of the main flaw ( β ) has strong effects on the crack initiation and coalescence stresses. The larger the angle between the direction of main and minor flaws γ , the greater the crack initiation and coalescence stresses. The crack initiation stress is reduced for the case with cross-flaws compared with that with non-cross-flaws. Meanwhile, the connection type of main flaws and the width of the crack coalescence zone are difficult to observe through the experiments and are discovered from the numerical simulation.
Highlights
Faults and joints in rocks affect the mechanical properties of rocks. erefore, it is of great significance to study the initiation and propagation modes of cracks in rocks for the failure mechanism of rocks [1]. e study of the rock failure process mainly includes three aspects: experiment, theory, and numerical simulation
For β 90°, the types of cracks generated are most complicated, while the patterns of crack coalescence are diverse and the total number of cracks is high. is indicates that crack propagation and coalescence in rocks are greatly affected by the dip of rock bridge, whereas the positional relationship of flaw distribution plays a considerable role in rock failure
Uniaxial compression tests were performed on sandy mudstone specimens containing two sets of crossflaws. e experimental processes were simulated by the Flat Joint Model (FJM) model based on PFC2D numerical simulation. en, the effects of two geometric parameters on crack propagation, initiation stress, and coalescence stress were analyzed. e
Summary
Faults and joints in rocks affect the mechanical properties of rocks. erefore, it is of great significance to study the initiation and propagation modes of cracks in rocks for the failure mechanism of rocks [1]. e study of the rock failure process mainly includes three aspects: experiment, theory, and numerical simulation.In order to study the crack evolution mode during the failure of crystalline rock materials, many researchers have conducted laboratory experiments with rock materials such as resin, plexiglass, or gypsum and analyzed the initiation, propagation, and coalescence of wing cracks and secondary cracks which lead to the macroscopic failure of brittle materials [2]. ese prismatic samples of rock-like materials which are usually prefabricated with one, two, or more cracks with different geometric parameters and are subjected to uniaxial compression test crack [3, 4], crack coalescence categories [4,5,6], crack initiation criteria [7, 8], and crack propagation patterns in different rock-like materials [9] were studied from different perspectives, which is helpful to the identification of rock cracks and the study of failure mechanism. ree types of cracks are observed during the rock failure process [4]: tensile cracks, mixed tensile-shear cracks, and shear cracks (Figure 1). e types and forms of crack coalescence are summarized through experiments and have been classified into nine categories [10,11,12]. Erefore, it is of great significance to study the initiation and propagation modes of cracks in rocks for the failure mechanism of rocks [1]. In addition to the uniaxial compression test, the Brazilian split test of precracked specimens was carried out to study crack propagation and coalescence [18,19,20,21,22], by focusing on the effects of inherent rock properties. Experimental studies on single ellipse defects show that the extension of tensile cracks around the pores of marble specimens is mainly affected by the nucleation and propagation of local strain zones [25]. Real-time dynamic monitoring has extremely high requirements on the size and accuracy of the sensor and the method of CT scanning while loading is too expensive, so the available experimental methods are still not enough to explore the relevant micromechanisms
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