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Journal ArticleDOI: 10.1016/J.ENGGEO.2021.105992

Microcracking behavior transition in thermally treated granite under mode I loading

05 Mar 2021-Engineering Geology (Elsevier)-Vol. 282, pp 105992
Abstract: An in-depth understanding of the thermomechanical properties of rocks is fundamentally important in many fields of geotechnical engineering. However, the microcracking mechanisms of thermally treated granite under mode I loading are very complex. To investigate the effect of thermal treatment on the microcracking behavior, we perform mode I three-point bending tests on a set of pre-notched semi-circular specimens. The specimens are pre-heated to different target temperatures (i.e. 50 °C, 100 °C, 150 °C, 200 °C, 400 °C, and 600 °C), which are then naturally cooled down to room temperature. Acoustic emissions are monitored during the loading tests to provide clues on the microcracking processes. The fracture process zone (FPZ) development features before the initiation of macrofracture are interpreted through analysis of the spatial-temporal evolution of AE events. Based on the AE signatures, we identify the microcracking behavior transition phenomenon as the thermal treatment temperature increases from 150 °C to 200 °C, which is related to the development of thermal microfractures. As the transition occurs, we observe (1) relatively lower load levels at the beginning of the rapid FPZ development phase; (2) longer rapid FPZ development duration; (3) larger size and maximum event density for fully-developed FPZs. Taking the microscopic observation into consideration, we propose an extended conceptual model to describe the FPZ evolution before the unstable fracture propagation in crystalline rocks. The present findings provide useful insights into the microcracking behavior in geoengineering practices where the host rocks are subjected to temperature changes.

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5 results found


Journal ArticleDOI: 10.1016/J.ENGGEO.2021.106287
Fanzhen Meng1, Song Jie1, Louis Ngai Yuen Wong2, Zaiquan Wang1  +1 moreInstitutions (3)
Abstract: Despite a large number of studies examining the effects of high temperature on the physical and mechanical properties of various rock types, few studies have addressed the effects of high temperature on the frictional behavior of rough fractures. As a benchmark of high temperature effects on rock integrity, we measure the uniaxial compression strength (UCS) and p-wave velocity (Vp) of granite samples after they are heated to various temperatures as high as 500 °C and allowed to cool to room temperature. We then investigate changes in the roughness and shear behavior of thermally treated rough granite fractures using direct shear tests. The results indicate that the UCS increases by 37% at 200 °C compared with that of granite without thermal treatment, and then decreases with temperature, while the Vp decreases almost linearly with increasing treatment temperature. The strengthening of rock upon heating is very likely associated with the thermal expansion of minerals, water evaporation and microcrack blunting, while the decrease in the UCS is closely related to the increase in thermal cracks. The degradation of the fracture roughness increases with normal stress, and it also tends to be greater for higher treatment temperatures. The peak shear strength, post-peak stress drop and stick-slip magnitude, which are not significantly influenced by the treatment temperature between room temperature and 300 °C, decrease with treatment temperatures above 300 °C. In particular, no post-peak stress drop occurs for the thermally treated fractures sheared under low normal stress, and the post-peak stress drop is inhibited in fractures with much higher temperature treatments and shearing under high normal stress. Our new findings contribute to a better understanding of fault slip mechanics at high temperatures in geothermal recovery and high-level radioactive waste repositories.

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Topics: Shear strength (discontinuity) (55%), Direct shear test (54%), Shearing (physics) (54%) ... show more

2 Citations


Open accessJournal ArticleDOI: 10.1155/2021/9018462
Yanan Gao1, Yanan Gao2, Yunlong Wang1, Taiping Lu3  +3 moreInstitutions (4)
Abstract: With the further development of deep rock mechanics engineering, such as the exploitation and utilization of geothermal resources, the exploitation of deep mineral resources, and the safe disposal of nuclear waste, the study of mechanical properties of deep high-temperature rock is gaining the attention of the researchers. However, not only the high temperature but also the cooling condition/method that will be used in the construction such as drilling cooling will also greatly affect the mechanical properties of the rock. In this paper, the mechanical behaviour and the evolution of the mechanical properties of the high-temperature (600°C–1,000°C) granite under different cooling methods are studied. The following conclusions can be obtained: (1) The peak stress of the granite decreases with the heating temperature. Compared with natural cooling, water cooling has a more significant effect on strength degradation. (2) The increase of the heating temperature increases the maximum axial strain of the granite. The water cooling method more greatly induces the maximum axial strain of granite than the natural cooling. The maximum axial strain of the specimen under the water cooling reaches 117.3% of that under natural cooling (800°C). (3) The elastic modulus of the granite decreases with the heating temperature. Water cooling will have a stronger effect on the reduction of the elastic modulus than natural cooling. The maximum difference value (2.02 GPa) of the elastic modulus under the different cooling methods occurs at the temperature of 800°C. (4) Poisson’s ratio of the granite increases with heating temperature, and the cooling method does not have an evident effect on it. The relationship between Poisson’s ratio and the heating temperature under different cooling methods can be described using the linear model. (5) According to the influence of the temperature on the peak stress, the elastic modulus, and Poisson’s ratio, the heating temperature domain can be divided into the unapparent zone, the significant zone, and the mitigation zone. (6) The thermal stress due to the nonuniform temperature field and the different thermal expansion coefficients is incompatible. Such incompatibility stresses the essences of the degradation of the mechanical properties of the granite.

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Topics: Water cooling (63%), Elastic modulus (53%)

2 Citations


Journal ArticleDOI: 10.1016/J.ENGGEO.2021.106268
Abstract: Thermally induced microcracks alter the mechanical properties of rocks. Mode I loading prevails in nature, while rocks are typically weak in tension. Understanding the effects of pre-existing thermal microcracks on the microcracking mechanisms of granite under mode I loading is of both scientific and practical value. We conducted three-point bending tests on pre-notched semi-circular bend Hong Kong granite specimens, in which the acoustic emissions (AEs) were monitored. To introduce the thermal microcracks, the specimens have been thermally treated to different temperature levels (50 °C, 100 °C, 150 °C, 200 °C, and 400 °C) before the loading tests. The microcracking mechanisms are analyzed by deciphering the AE signals recorded from the start of loading until the macrocrack initiation. We find that the presence of the thermal microcracks aggravates the microcrack damage. Our detailed AE analysis reveals that the thermal microcracks do not significantly affect the proportions of three types of AE events (i.e. tensile, shear and mixed-mode) as well as their energy distributions. The tensile events dominate and the proportions of the three types are independent of event magnitude. The temporal evolution features of the event-type ratio are different between the specimens with few thermal microcracks and those with substantial thermal microcracks. For the former, the tensile event ratio rapidly increases to its peak, while the shear and mixed-mode event ratio decreases, which becomes a signature precursor prior to the initiation of a macrocrack. In contrast, this precursor is absent for the latter.

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1 Citations


Journal ArticleDOI: 10.1016/J.IJRMMS.2021.104852
Tian Yang Guo1, Louis Ngai Yuen Wong1Institutions (1)
Abstract: Cracks at various scales in rock masses are often subjected to mixed-mode I-II loadings in Earth's dynamic processes and geotechnical engineering works. Insightful understanding of the macroscopic and microscopic cracking mechanisms of rocks under this loading condition is of far-reaching significance. To investigate the effect of the loading condition and granite heterogeneity on the cracking mechanism, we conduct three-point bending tests on pre-notched semi-circular medium-grained granite specimens. For comparison, identical tests are also conducted on polymethyl methacrylate (PMMA) specimens. The results show that the initiation angles of the macrocracks in granite are less influenced by the loading condition as compared with the more homogeneous PMMA. Based on the monitoring of the acoustic emissions (AEs), we find that the microcracks under mixed-mode I-II loading do not nucleate as easily as compared with that in mode I case. For the AE event density contours characterizing the fully-developed fracture process zones, their shape is sub-circular and symmetric with respect to the pre-existing notch under mode I loading, while those under mixed-mode I-II loadings are irregular and asymmetric. Most of the macrocrack paths traverse the high AE event density region. As the mode II stress intensity factor ( KII ) increases, the AE events with higher energy tend to concentrate beside the crack paths in front of the notch tips . The moment tensor inversion suggests that the presence of mode II loading component alters the AE source mechanisms in terms of the temporal evolution characteristics of event-type ratios preceding the unstable propagation of macrocracks.

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1 Citations


Journal ArticleDOI: 10.1016/J.ENGFRACMECH.2021.107818
You Wu1, Tubing Yin1, Xiaosong Tan1, Dengdeng Zhuang1Institutions (1)
Abstract: Previous researches mainly concentrate on investigating the mixed mode I/II fracture characteristics of heat-treated rocks through laboratory tests. This paper makes an effort to obtain the mixed mode fracture characteristics of heat-treated granite by means of the extended finite element method (XFEM), which is based on the cohesive zone model (CZM). The experimentally obtained unstable fracture toughness, tensile strength and tensile elastic modulus of the granite after being subjected to different high temperatures are integrated into the numerical simulation models to estimate the fracture behaviours of the cracked straight-through notch Brazilian disc (CSTBD) specimens under mixed mode loading. Findings and observations from this study indicate that by applying the unstable fracture toughness as the mode independent cohesive zone fracture parameter into the numerical models, although some limitations still exist, the numerical models can successfully predict the fracture loads and the crack propagation paths of the CSTBD specimens under mixed mode loading within the temperature range of 25–600 °C. This paper provides a numerical approach to investigate the mixed mode I/II fracture characteristics of thermally treated rocks.

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Topics: Fracture mechanics (66%), Cohesive zone model (66%), Fracture (geology) (58%) ... show more
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64 results found


Journal ArticleDOI: 10.1002/NME.1151
Timon Rabczuk1, Ted Belytschko1Institutions (1)
Abstract: A new approach for modelling discrete cracks in meshfree methods is described. In this method, the crack can be arbitrarily oriented, but its growth is represented discretely by activation of crack surfaces at individual particles, so no representation of the crack's topology is needed. The crack is modelled by a local enrichment of the test and trial functions with a sign function (a variant of the Heaviside step function), so that the discontinuities are along the direction of the crack. The discontinuity consists of cylindrical planes centred at the particles in three dimensions, lines centred at the particles in two dimensions. The model is applied to several 2D problems and compared to experimental data. Copyright © 2004 John Wiley & Sons, Ltd.

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Topics: Meshfree methods (58%), Heaviside step function (53%), Discontinuity (linguistics) (52%) ... show more

1,002 Citations


Journal ArticleDOI: 10.1016/0040-1951(83)90198-1
Robert L. Kranz1Institutions (1)
01 Dec 1983-Tectonophysics
Abstract: In the past decade the number of studies about microcracks in rocks has rapidly increased. This review of recent work concentrates on microcracks in rock as separate entities, emphasizing microcrack morphogenesis, kinematics, dynamics, population statistics and observational techniques. Cracks are produced when the local stress exceeds the local strength. The local stress may be augmented by twin lamellae interactions, kink bands and deformation lamellae, stress concentrations at grain boundary contacts and around intracrystalline cavities. Local strength may be reduced along cleavage planes, along grain boundaries and along any internal surface as a result of corrosion by chemically active fluids. Dislocations appear not to be a significant factor for crack nucleation below about 500°C in silicates. Spatial and temporal changes in temperature can also induce microcracking as a result of differential thermal expansion between grains with different thermoelastic moduli and thermal conductivities. The amount of quartz in the rock has a significant effect on thermally induced microcracks because of its large and variable thermal expansivity. The application of hydrostatic pressure between 100 and 200 MPa effectively closes most cracks, but the closure may not be uniform if crack wall asperities exist. Hydrostatic pressure appears to stabilize cracks and make crack growth more difficult. The number and average size of mechanically induced microcracks is greater in rock deformed at higher pressures. The application of a deviatoric stress field on the boundaries of a rock mass results, on a microscopic scale, in a very complex stress system which greatly affects nucleation and propagation paths. The relative amount of intragranular and intergranular cracking appears to depend upon mineralogy, rock type and stress state. The vast majority of stress-induced microcracks in rocks appear to be extensional. Statistically, they are predominantly oriented within 30° of the macroscopic maximum stress direction. Crack densities increase as macroscopic deviatoric stress increases above a threshold level. Crack size distributions may be either lognormal or exponential. Fracture in rock under compressive boundary loads is a result of the coalescence of many microcracks, not the growth of a single crack. Some crack configurations are more favorable for coalescence than others. As deviatoric stress increases and rock failure is approached, the microcrack population changes spatially from random to locally intense zones of cracking. Away from the fault, the crack density dies off rapidly to the background level a few grains away. Under lesser deviatoric stresses, slow, subcritical microcrack growth can occur as a result of stress-aided corrosion at the crack tip. The rate governing mechanism may be either the chemical reaction rate or the rate at which water can get to the crack tip. Important details still remain to be worked out.

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Topics: Crack closure (65%), Stress concentration (62%), Stress field (56%) ... show more

713 Citations


Journal ArticleDOI: 10.1016/0148-9062(93)90041-B
David A. Lockner1Institutions (1)
Abstract: The development of faults and shear fracture systems over a broad range of temperature and pressure and for a variety of rock types involves the growth and interaction of microcracks. Acoustic emission (AE), which is produced by rapid microcrack growth, is a ubiquitous phenomenon associated with brittle fracture and has provided a wealth of information regarding the failure process in rock. This paper reviews the successes and limitations of AE studies as applied to the fracture process in rock with emphasis on our ability to predict rock failure. Application of laboratory AE studies to larger scale problems related to the understanding of earthquake processes is also discussed. In this context, laboratory studies can be divided into the following categories. 1) Simple counting of the number of AE events prior to sample failure shows a correlation between AE rate and inelastic strain rate. Additional sorting of events by amplitude has shown that AE events obey the power law frequency-magnitude relation observed for earthquakes. These cumulative event count techniques are being used in conjunction with damage mechanics models to determine how damage accumulates during loading and to predict failure. 2) A second area of research involves the location of hypocenters of AE source events. This technique requires precise arrival time data of AE signals recorded over an array of sensors that are essentially a miniature seismic net. Analysis of the spatial and temporal variation of event hypocenters has improved our understanding of the progression of microcrack growth and clustering leading to rock failure. Recently, fracture nucleation and growth have been studied under conditions of quasi-static fault propagation by controlling stress to maintain constant AE rate. 3) A third area of study involves the analysis of full waveform data as recorded at receiver sites. One aspect of this research has been to determine fault plane solutions of AE source events from first motion data. These studies show that in addition to pure tensile and double couple events, a significant number of more complex event types occur in the period leading to fault nucleation. 4) P and S wave velocities (including spatial variations) and attenuation have been obtained by artificially generating acoustic pulses which are modified during passage through the sample.

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Topics: Acoustic emission (59%), Fracture (geology) (57%)

704 Citations


Open access
01 Dec 1993-
Abstract: The development of faults and shear fracture systems over a broad range of temperature and pressure and for a variety of rock types involves the growth and interaction of microcracks. Acoustic emission (AE), which is produced by rapid microcrack growth, is a ubiquitous phenomenon associated with brittle fracture and has provided a wealth of information regarding the failure process in rock. This paper reviews the successes and limitations of AE studies as applied to the fracture process in rock with emphasis on our ability to predict rock failure. Application of laboratory AE studies to larger scale problems related to the understanding of earthquake processes is also discussed. In this context, laboratory studies can be divided into the following categories. 1) Simple counting of the number of AE events prior to sample failure shows a correlation between AE rate and inelastic strain rate. Additional sorting of events by amplitude has shown that AE events obey the power law frequency-magnitude relation observed for earthquakes. These cumulative event count techniques are being used in conjunction with damage mechanics models to determine how damage accumulates during loading and to predict failure. 2) A second area of research involves the location of hypocenters of AE source events. This technique requires precise arrival time data of AE signals recorded over an array of sensors that are essentially a miniature seismic net. Analysis of the spatial and temporal variation of event hypocenters has improved our understanding of the progression of microcrack growth and clustering leading to rock failure. Recently, fracture nucleation and growth have been studied under conditions of quasi-static fault propagation by controlling stress to maintain constant AE rate. 3) A third area of study involves the analysis of full waveform data as recorded at receiver sites. One aspect of this research has been to determine fault plane solutions of AE source events from first motion data. These studies show that in addition to pure tensile and double couple events, a significant number of more complex event types occur in the period leading to fault nucleation. 4) P and S wave velocities (including spatial variations) and attenuation have been obtained by artificially generating acoustic pulses which are modified during passage through the sample. (A) This paper was presented at the 34th U.S. Symposium on rock mechanics, 27-30 June 1993, University of Wisconsin-Madison. For the covering abstract see IRRD 863389.

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Topics: Acoustic emission (59%), Rock mechanics (56%)

696 Citations


Open accessBook
01 Jan 1995-
Abstract: Hydraulically Induced Fractures in the Petroleum and Related Industries. Linear Elasticity, Fracture Shapes and Induced Stresses. Stresses in Formations. Fracture Geometry. Rheology and Laminar Flow. Non-Laminar Flow and Solids Transport. Advanced Topics of Rheology and Fluid Mechanics. Material Balance. Coupling of Elasticity, Flow and Material Balance. Fracture Propagation. Fracture Height Growth (3D and P-3D Geometries). Appendix. References. Index.

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Topics: Fracture mechanics (64%), Fracture (geology) (59%), Linear elasticity (53%) ... show more

411 Citations