Rock brittleness evaluation based on energy dissipation under triaxial compression

The energy dissipation usually occurs during rock failure, which can demonstrate the meso failure process of rock in a relatively accurate way. Based on the results of conventional triaxial compression experiments on the Jinping marble, a numerical biaxial compression model was established by PFC2D to observe the development of the micro-cracks and energy evolution during the test, and then the laws of crack propagation, energy dissipation and damage evolution were analyzed. The numerical simulation results indicate that both the crack number and the total energy dissipated during the loading process increase with confining pressures, which is basically consistent with the experiment results. Two damage variables were presented in terms of the density from other researchers’ results and energy dissipation from numerical simulation, respectively. The energy-based damage variable varies with axial strain in the shape of “S,” and approaches one more closely than that based on density at the final failure period. The research in the rock failure from the perspective of energy may further understand the mechanical behavior of rocks.

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An Evaluation Method of Rock Brittleness Based on the Prepeak Crack Initiation and Postpeak Stress Drop Characteristics

Mathematical Problems in Engineering ◽

10.1155/2021/5639649 ◽

2021 ◽

Vol 2021 ◽

pp. 1-9

Author(s):

Meiben Gao ◽

Tianbin Li ◽

Lubo Meng

Keyword(s):

Crack Initiation ◽

Stress Drop ◽

Evaluation Method ◽

Triaxial Compression ◽

Experimental Tests ◽

Confining Pressure ◽

Brittleness Index ◽

Comparison Results ◽

Confining Pressures ◽

Rock Brittleness

Recent research shows that the brittleness of rock is closely related to the initiation and propagation of internal microcracks, but there are few brittleness evaluation indices considering the characteristics of rock initiation. Based on the theoretical analysis of brittleness and the characteristics of rock initiation, this study proposes an evaluation method of rock brittleness based on the prepeak crack initiation and postpeak stress drop characteristics. First, based on the description and definition of brittleness by George Tarasov and Potvin et al., the feasibility of an evaluation method based on the prepeak crack initiation and postpeak stress drop is theoretically analyzed. Second, the component Bi representing the prepeak brittleness of rock and component Bii representing the prepeak brittleness of rock are constructed, and the product of the two is the brittleness index BI, representing the prepeak crack initiation and postpeak stress drop. Finally, experimental tests of granite and marble were conducted to evaluate the new index, and the brittleness indices of different methods are calculated and compared. The results show that, like other brittleness indices (B1∼B5), the brittleness index BI can effectively reflect the effects of different confining pressures and loading modes on rock brittleness. The brittleness of marble decreases with increasing confining pressure from 5 MPa to 35 MPa. At a confining pressure of 5 MPa, the brittleness of granite during a triaxial unloading test is greater than that during a triaxial compression test. The calculated results are consistent with the experimental results. By tests and comparison results, the reliability of this evaluation method was verified, which provides a way to evaluate rock brittleness from the perspective of crack initiation and is helpful to enrich the analysis and evaluation of rock brittleness in the laboratory.

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Effect of Bedding Angle and Confining Pressure on the Brittleness of Geomaterials: A Case Study on Slate

Advances in Materials Science and Engineering ◽

10.1155/2019/1650170 ◽

2019 ◽

Vol 2019 ◽

pp. 1-17

Author(s):

Xianjie Hao ◽

Quansheng Xu ◽

Dequan Yang ◽

Shaohua Wang ◽

Yingnan Wei

Keyword(s):

Energy Release ◽

Triaxial Compression ◽

Great Influence ◽

Confining Pressure ◽

Brittleness Index ◽

Compression Tests ◽

External Energy ◽

Whole Process ◽

Bedding Angle ◽

Rock Brittleness

Brittleness is one of the most significant properties of geomaterials. However, very few studies have been conducted on factors influencing the rock brittleness indices. In this paper, conventional triaxial compression tests were carried out to investigate the effects of confining pressure and bedding angle on the brittleness of slate. From the perspective of energy, brittleness is an index that could reflect the release rate of energy that accumulated in the slate under the effect of external energy after reaching peak strength. Therefore, a new brittleness index of slate based on postpeak energy release is proposed herein. The applicability of this index is illustrated by comparing with other five existing brittleness indices. The following results can be obtained. (1) The confining pressure exerts a great influence on the brittleness of slate. With the increase of confining pressure, the brittleness of slate decreases significantly. The dispersion of brittleness values of slate declines with increasing confining pressure. (2) There is a parabolic relationship between slate brittleness and bedding angle. As bedding angle increases, the brittleness is intensified and reaches its maximum at a bedding angle of about 45° and then decreases gradually. (3) In contrast to the previous indices, the brittleness index proposed in this paper can describe the whole process of the postpeak stage through an index of the energy release, which makes this measure more suitable for rock that has the characteristics of step-drop or bench-drop at the postpeak stage.

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Energy Dissipation and Release During Coal Failure Under Conventional Triaxial Compression

Rock Mechanics and Rock Engineering ◽

10.1007/s00603-014-0602-0 ◽

2014 ◽

Vol 48 (2) ◽

pp. 509-526 ◽

Cited By ~ 110

Author(s):

Ruidong Peng ◽

Yang Ju ◽

J. G. Wang ◽

Heping Xie ◽

Feng Gao ◽

...

Keyword(s):

Energy Dissipation ◽

Triaxial Compression ◽

Conventional Triaxial Compression

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Experimental Investigation of Early-Warning Critical Energy for Strainbursts

10.21203/rs.3.rs-437380/v1 ◽

2021 ◽

Author(s):

Wang Ling ◽

Ruyu Yan ◽

Zhang zhi ◽

Xie Lei ◽

Huang chuhui

Keyword(s):

Acoustic Emission ◽

Energy Dissipation ◽

Early Warning ◽

Elastic Strain ◽

Strain Energy ◽

Triaxial Compression ◽

Elastic Strain Energy ◽

Critical Energy ◽

Input Energy ◽

Energy Evolution

Abstract This research aimed to establish an early-warning critical energy for coal instability based on the energy theory and acoustic emission characteristics of coal under triaxial compression. To obtain an early-warning critical strain energy indicating the increase in the risk of coal instability, conventional triaxial compression and acoustic emission (AE) tests were carried out on coal specimens taken from a 980-m-deep mine with initial confining pressures of 10, 15, 20, 25, 30 and 35 MPa. Stress-strain relations, AE features, and energy evolution characteristics during triaxial compression were analyzed. It was found that the energy evolution and AE event count changes across different loading stages. With increasing axial stress, most of the input energy stored in the coal specimens was in the form of elastic strain energy and the AE event count was close to zero, indicating that the coal grains reach a state of balance. After the elastic deformation stage, a portion of the input energy was consumed by inelastic deformation. Once the stress level exceeded the volumetric compressibility–dilatancy transition stress, the AE event entered a period of relative quiet, and the rate of energy dissipation abruptly accelerated, indicating that the coal grains achieved another state of balance before THE instability or failure. The balance of the rock grains is broken again (AE event count and the rate of energy dissipation both increased dramatically), coal achieved the peak strength and instability soon. The point at which the dissipated energy ratio α increased rapidly or the starting point of a quiet period, indicates an increase in the risk of coal instability. The corresponding elastic strain energy accumulated within the coal can be regarded as a precursor to instability or strainburst. Accordingly, a fitting formula is presented to predict the early-warning critical energy for brittle coal subject to different minimum principal stress. The analysis results in this paper can be helpful in the assessment of coal instability risk.

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A New Rock Brittleness Evaluation Method Based on the Complete Stress-Strain Curve

Lithosphere ◽

10.2113/2021/4029886 ◽

2021 ◽

Vol 2021 (Special 4) ◽

Author(s):

Chenyang Liu ◽

Lizhi Du ◽

Xiaopei Zhang ◽

Yong Wang ◽

Xinmin Hu ◽

...

Keyword(s):

Stress Drop ◽

Evaluation Method ◽

Triaxial Compression ◽

Strain Curve ◽

Confining Pressure ◽

Brittleness Index ◽

Peak Strain ◽

Stress Strain ◽

Rock Brittleness ◽

Drop Rate

Abstract Brittleness is a crucial parameter of rock mass and the key indicator in rock engineerings, such as rockburst prediction, tunnelling machine borehole drilling, and hydraulic fracturing. To solve the problem of using present brittleness indexes, the existing rock brittleness indexes were firstly summarised in this paper. Then, a brittleness index (BL), which considers the ratio of stress drop rate and stress increase rate and the peak stress, was proposed. This new index has the advantage of simplifying the acquisition of key parameters and avoiding dimensional problems, as well as taking the complete stress-strain curves into account. While applying the BL, the peak strain is used to describe the difficulty of brittle failure before the peak point, and the ratio of stress drop to strain increase can reflect the stress drop rate without dimension problem. In order to verify the applicability of BL, through the PFC2D, the microparameters and confining pressure were changed to model different types of rock numerical specimens and different stress condition. The results show that the BL can well reflect and classify the brittleness characteristics of different rock types and characterise the constraint of confining pressure on rock brittleness. Moreover, the influence of microparameter on macroparameter was studied. In order to further verify the reliability of the brittleness index (BL), this study conducted uniaxial and triaxial compression tests (30 MPa) on marble, sandstone, limestone, and granite under different confining pressure.

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