Experimental study on the influence of prefabricated fissure size on the directional propagation law of rock type-I crack

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Abstract

To investigate the influence of prefabricated fissure size on the directional propagation law of rock type-I cracks, a simple device for simulating rock crack directional propagation was developed. Subsequently, loading tests for white sandstone samples with different prefabricated fissure lengths and widths were performed. The evolution laws of the deformation field and acoustic emission (AE) were analyzed during the entire crack propagation process. Finally, the influence of fissure size on the crack directional propagation mechanism was elucidated. The obtained results indicate that: The initial crack angle was mainly influenced by the prefabricated fissure width. The initial crack angle increased with the increase in the prefabricated fissure width. The crack directional propagation process can be divided into four stages: local strain-band initiation, local strain-band development, crack initiation, and crack coalescence. The crack maximum relative strain increases with an increase in the prefabricated fissure length but decreases with an increase in the prefabricated fissure width. This phenomenon indicates that increasing the prefabricated fissure length is advantageous to crack propagation at the fissure tip. The AE evolution process includes quiet, slowly increasing, booming, and decreasing periods. When the prefabricated fissure length increased, the durations of the quiet and slowly increasing periods decreased, but the durations of booming and decreasing periods increased. However, the durations of quiet, slowly increasing, and booming periods decreased as the prefabricated fissure width increased. In other words, increasing the prefabricated fissure length can enhance the stability of crack directional propagation, but increasing the prefabricated fissure width leads to the instability of main crack propagation path. In underground engineering practice, increasing the prefabricated fissure length and decreasing the prefabricated fissure width may be excellent approaches to realizing the crack directional propagation of rock or coal fracturing.

Introduction

Long-term geological processes result in numerous defects, such as joints and fissures in the rock, which reduce the overall strength of the rock. When disturbed by mining, the stress on the surrounding rock changes, which triggers the initiation, propagation, and coalescence of primary cracks in the rock; in addition, numerous cracks converge to form a macro fracture surface, eventually leading to the instability and failure of the coal and rock mass.1, 2, 3 The instability failure of coal and rock mass is the main factor that causes engineering geological disasters such as rock bursts, large deformations of roadway surrounding rock, and slope landslides.4, 5, 6, 7, 8 Therefore, studying crack propagation in coal and rock mass, including its influencing factors, has an important theoretical guiding significance for predicting the instability of coal and rock mass and preventing engineering geological disasters.

Researchers, home and abroad, have conducted numerous discussions on the crack initiation and propagation law of rock materials containing prefabricated fissures for several years. Subsequently, many static loading tests on fractured rocks have been conducted. The basic mechanical properties and crack propagation mechanism of fractured rocks under different loading conditions have been studied.9, 10, 11, 12 Accordingly, it has been deduced that the fissure shape, geometric size, number, and inclination angle13, 14, 15, 16, 17, 18, 19, 20 of the prefabricated fissure significantly influence the crack evolution process. In practical engineering, the rock mass is subjected to dynamic loads such as impact, earthquakes, and explosions under the action of large-scale excavation.21,22 Hence, it is of practical significance to study crack propagation and coalescence mechanisms under impact loads. Therefore, some researchers have studied the effects of hollow hole23 and prefabricated fissure length24 on the crack coalescence phenomena and mechanical properties of the sample under impact loading, while others have studied the influence of geometric configuration25, 26, 27, 28, 29, 30 on the crack propagation behavior of fractured rock under dynamic loads on the crack propagation behavior of fractured rocks under dynamic load. Based on the fracture mechanics theory, Irwin31 classified cracks into three types: type-I opening, type-II sliding, and type-III anti-plane shear modes. Among these, type-I cracks, which are manifested as cracks subjected to tensile stresses perpendicular to the crack surfaces, produce displacements perpendicular to the crack surfaces. Type-II and III cracks are in-plane and out-of-plane shear cracks in which the surface slip directions are parallel and perpendicular to the crack directions, respectively. In general, the failure of an engineering rock mass is primarily caused by the tensile stress exceeding its ultimate tensile strength to form a tensile fracture surface. Researchers have performed basic mechanical tests such as direct tensile, Brazilian splitting, and three-point bending, to analyze the type-I crack coalescence failure mode under tensile stress.32, 33, 34, 35, 36 AE, digital speckle, infrared imaging, and other monitoring methods37, 38, 39, 40, 41, 42, 43, 44 have been adopted to obtain information on the entire process of crack propagation simultaneously.

Although the aforementioned research experiments mostly analyze the crack initiation and propagation behavior from a qualitative perspective, there are few quantitative studies on the entire process of crack propagation by precisely controlling the direction of crack propagation. A simple device and method for simulating rock crack directional propagation was developed, and the influence of fissure size on the type-I crack directional expansion of sandstone samples was studied. Loading tests of white sandstone samples with different prefabricated fissure lengths and widths were performed, during which the deformation field and AE evolution law throughout the crack propagation were monitored and analyzed by AE and digital image technology, and the effect of prefabricated fissure size on the directional propagation of type I cracks was revealed.

Section snippets

Test method

The white sandstone samples used in the experiments were obtained from a mine in Zizhong County, Neijiang City, Sichuan Province, China. The sampling depth was 80 m at the excavation face, and the sampling direction was perpendicular to the rock deposition direction. The obtained rock samples were grayish white (Fig. 1a). Fig. 1b shows the X-ray diffraction result of a white sandstone sample. The white sandstone sample is mainly composed of elements such as silicon, sodium, aluminum, potassium,

Load curve and failure mode

The load-time curves of the samples with different lengths of prefabricated fissures are presented in Fig. 4. The change rate of the load curve initially increased, and then decreased with an increase in load. When the load increases to the peak load, the tensile stress in the prefabricated fissure area reaches the limit value, which controls the main crack formation, and the sample fracture leads to a rapid reduction in the load. Meanwhile, the peak load of the sample decreases with an

Load curve and failure mode

The load–time curves of the samples with different widths of prefabricated fissures are presented in Fig. 11. The rate of change of the load curve initially increases, and then decreases as the load increases. When the load increases to the peak load, the formation of the main crack triggers a decrease in the bearing capacity of the sample and a rapid decrease in the load. At the post-peak stage, the rate of the curve gradually decreases from rapid to steady. Meanwhile, the peak load and the

Discussion on mechanism and engineering guidance of rock crack directional propagation under the influence of fissure size

Fig. 17 presents a diagram of crack directional propagation. When the maximum bending stress at the fissure tip exceeds the tensile strength of the rock, microcracks began to appear at the prefabricated fissure tip, and the microcracks aggregate to form the fracture process zone (FPZ). Because some cohesion still exist between the two crack surfaces, in equivalent elastic fracture mechanics, FPZ is considered a crack with closed cohesion, and it is regarded as an effective crack Le together

Conclusion

To investigate the influence of prefabricated fissure size on the directional propagation law of rock type-I cracks, a simple device for simulating rock crack directional propagation was developed. Subsequently, loading tests for white sandstone samples with different prefabricated fissure lengths and widths were performed. The evolution laws of the deformation field and AE were analyzed during the entire crack propagation process. Finally, the influence of fissure size on the crack directional

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The research described in this paper was financially supported by National Natural Science Foundation of China (No. 52274086, No. 51904165), Major Program of Shandong Provincial Natural Science Foundation(No. ZR2019ZD13).

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