Evolutionary Analysis of Heterogeneous Granite Microcracks Based on Digital Image Processing in Grain-Block Model

Abstract

Rocks are natural materials with a heterogeneous microstructure, and the heterogeneity of the microstructure plays a crucial role in the evolution of microcracks during the compression process. A numerical model of a rock with a heterogeneous structure under compression is developed by digital image processing techniques and the discrete element method. On the grain scale, the damage mechanism and microcrack characteristics of a heterogeneous Biotite granite under compression fracture are investigated. First, the process of constructing a digital image-based heterogeneous grain model is described. The microscopic characteristics of geometric heterogeneity, elastic heterogeneity, and contact heterogeneity are all considered in the numerical model. Then, the model is calibrated according to the macroscopic properties of biotite granite obtained in the laboratory, and the numerically simulated microcrack cracking processes and damage modes are obtained with a high degree of agreement compared to the experiments. Numerical simulations have shown the following: (1) Microcracking occurs first at the weak side of the grain boundaries, and the appearance of intragranular shear cracks indicates that the rock has reached its peak strength. (2) The stress concentration caused by the heterogeneity of the microstructure is an essential factor that causes rock cracks and induces rupture. Intragranular cracks occur successively in quartz, feldspar (plagioclase), and biotite, with far more intragranular cracks in quartz and feldspar (plagioclase) than in biotite. (3) Microcracking in quartz occurs as clusters, fork and fracture features, and in feldspar (plagioclase) it tends to cause penetration microcracking, which usually surrounds or terminates at the biotite. (4) As the confining pressure increases, the tensile break between the grains is suppressed and the number of shear cracks increases. At the macro level, the rock failure mode of the numerical model changes from split damage to shear destruction, which is consistent with the law shown in laboratory experiments.

Characteristics of the Fracture Process Zone for Reservoir Rock with Various Heterogeneity

“…Natural rocks usually contain various microscopic structures, such as mineral grain boundaries, cleavages, interfaces, pores, and cracks [2][3][4]. Compared to mineral crystal, these structures are local weak zones in rocks.…”

Evaluation of Thermophysical and Mechanical Properties of Sandstone Due to High-Temperature

“…The combined effect of the structural stress associated with the gasification chamber formation and the thermal stress induced by high temperature will damage the rock. When the damage reaches a certain level, the rock will experience the expansion of original fractures and the generation of new fractures, eventually leading to rock fracturing [ 12 , 13 , 14 , 15 , 16 ]. Furthermore, it may cause roof caving, excessive movement, fracturing damage, and surface subsidence of rock strata above the gasification chamber at worst.…”

Fracture and Damage Evolution of Multiple-Fractured Rock-like Material Subjected to Compression

“…The cracks in the rock mass not only decrease its strength and stiffness, but also bring a source of initiation of new cracks, which, in turn, may propagate and link with other cracks. Understanding the failure process of fractured rock mass subjected to compression is, therefore, essential in rock engineering, mining engineering, deep underground structures, and geotechnical engineering practice [6][7][8][9][10]. In order to study the physical properties, deformation behavior and mechanical mechanism of rock-like materials during the fracturing process, laboratory loading tests have been extensively employed [11][12][13], and comprehensive research has been conducted to develop crack propagation theories [14][15][16][17] or calculate the stress intensity factors at the crack tips [18][19][20][21], as well as quantifying the relationship between micro damage and the macro-mechanical properties of rock mass [22][23][24][25].…”