scholarly journals Energy Evolution and Acoustic Emission Characteristics of Sandstone Specimens under Unloading Confining Pressure

2019 ◽  
Vol 2019 ◽  
pp. 1-9
Author(s):  
Tao Qin ◽  
Yanwei Duan ◽  
Hongru Sun ◽  
Honglei Liu ◽  
Lei Wang
Keyword(s):  
Acoustic Emission ◽  
Elastic Strain ◽  
Strain Energy ◽  
High Stress ◽  
Confining Pressure ◽  
Emission Characteristics ◽  
Energy Evolution ◽  
Peak Point ◽  
Unloading Confining Pressure ◽  
Rock Specimens

The acoustic emission characteristics of rock specimens under different initial unloading confining pressures were tested to obtain the damage and rupture characteristics of the sandstone unloading confining pressure path. The CT scan and three-dimensional reconstruction of the fractured rock specimens were carried out to study the differences of energy evolution and acoustic emission characteristics during the failure of sandstone under different initial unloading pressures. The results show that the unloading confining pressure has a significant influence on the deformation and failure of the rock. There is a significant yielding platform for the circumferential strain and the bulk strain at the peak of the unloading pressure. The larger the initial unloading pressure is, the greater the axial absorption strain energy, the dissipative energy, and the elastic strain energy are at the peak point. After the stress peak point, the elastic strain can be quickly converted into the dissipative energy for rock damage. The elastic energy released from the moment of rock failure under high confining pressure is more concentrated. The acoustic emission ringing and b value characteristic parameters of the rock have a good correlation with the internal energy evolution of the rock, which better reflects the progressive damage of the rock under low stress and the sudden failure of high-stress unloading.

scholarly journals Study on the Meso-Energy Damage Evolution Mechanism of Single-Joint Sandstone under Uniaxial and Biaxial Compression

2021 ◽  
Vol 2021 ◽  
pp. 1-27
Author(s):  
Gui-Lin Wang ◽  
Tian-Ci Cao ◽  
Fan Sun ◽  
Xing-Xiang Wen ◽  
Liang Zhang
Keyword(s):  
Elastic Strain ◽  
Strain Energy ◽  
Elastic Strain Energy ◽  
Confining Pressure ◽  
Biaxial Compression ◽  
Level Energy ◽  
Energy Evolution ◽  
Peak Point ◽  
Joint Inclination ◽  
Joint Length

Energy conversion and release occur through the entire deformation and failure process in jointed rock masses, and the accumulation and dissipation of rock mass energy in engineering can reveal the entire process of deformation and instability. This study uses PFC2D to carry out numerical simulation tests on single-joint sandstone under uniaxial compression and biaxial compression, respectively, and analyse the influence of joint inclination, length, and confining pressure on the meso-energy conversion process and phase evolution of jointed sandstone. Through analysis, it is found that the input meso total strain energy is transformed into meso dissipated energy and meso-elastic strain energy. Macroscopic and microscopic joint sandstone law is consistent with the overall energy evolution; and the difference is reflected in two aspects: (1) the microlevel energy evolution has no initial compaction energy consumption section and (2) the linear energy storage section before the macroenergy evolution peak can be subdivided into two sections in the meso-level energy evolution. Under uniaxial compression, the energy values at the characteristic points of the meso-level energy evolution phases first asymmetrically decrease and then increase with the increase of the joint inclination. The initiation point of jointed sandstone is significantly affected by the length of the joint, and the degradation effect of the meso-energy at the damage point and peak point weakens with the increase of the joint length. Comparing the data obtained from the PFC numerical simulation with the experimental data, it is found that the error is small, which shows the feasibility of the numerical model in this paper. Under biaxial compression, the accumulation rate of meso-elastic strain at the peak point of the jointed sandstone first decreases and then increases with the joint inclination angle. After the peak of jointed sandstone, the rate of sudden change of meso-energy change decreases with the increase of joint length. The conditions of high confining pressure will promote the meso-accumulated damage degree of the jointed sandstone before the peak, while inhibiting the meso-energy and the mutation degree of the damage after the peak. The higher the confining pressure, the more obvious the joint length and inclination effect characteristics of the elastic strain energy at the peak point of the jointed sandstone.

scholarly journals Experimental Investigation of Energy Evolution in Sandstone Failure during Triaxial Unloading Confining Pressure Tests

2019 ◽  
Vol 2019 ◽  
pp. 1-11 ◽  
Author(s):  
Rui Yang ◽  
Depeng Ma ◽  
Yongjie Yang
Keyword(s):  
Elastic Energy ◽  
Triaxial Compression ◽  
High Stress ◽  
Confining Pressure ◽  
Energy Evolution ◽  
Compression Tests ◽  
Sandstone Sample ◽  
Pressure Tests ◽  
Unloading Confining Pressure ◽  
Sample Damage

The deformation and failure of sandstone samples are closely related to energy changes in the material. To explore the energy evolution during the process of sandstone sample damage, loading and unloading tests with different test paths were conducted. The results show that more energy is stored and consumed before the stress reaches its peak, while after the peak stress, more energy is released and consumed. Energy dissipation increases internal cracking, leads to sample damage and lithologic deterioration, and reduces the bearing capacity of the sample. During triaxial unloading of the confining pressure, the higher the initial unloading confining pressure, the more the elastic energy stored, and the more the energy released when the sandstone sample fails, resulting in more severe damage. Therefore, during the excavation of high-stress rock masses, large amounts of elastic energy stored in sandstone can be rapidly released, leading to rock burst disasters. Additionally, during triaxial unloading confining pressure tests, the damage in sandstone when the sample is close to failure increases more rapidly than that during conventional triaxial compression tests because of the unloading effect of the confining pressure. This phenomenon also illustrates that the failure of sandstone induced by unloading is more sudden than that induced by loading.

scholarly journals Experimental Investigation of Early-Warning Critical Energy for Strainbursts

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.

scholarly journals Experimental Investigation on Deformation and Strength Behavior of Marble with the Complex Loading-Unloading Stress Path

2020 ◽  
Vol 2020 ◽  
pp. 1-17
Author(s):  
Chen Chen ◽  
Lipeng Liu ◽  
Yu Cong
Keyword(s):  
Elastic Strain ◽  
Strain Energy ◽  
High Stress ◽  
Elastic Strain Energy ◽  
Complex Loading ◽  
Energy Criterion ◽  
Confining Pressure ◽  
Stress Path ◽  
Surrounding Rock ◽  
Strength Behavior

The excavation of deep tunnel in rock mass undergoes complex loading and unloading stress paths, resulting in rib spalling, flaking, and even severe rockburst disasters. Based on the variation law of the stress path of the surrounding rock, laboratory tests of rock mechanics are designed, and the deformation and strength behavior of marble with different initial confining pressure and unloading rates are systematically studied. By introducing strain increment, the characteristic stress, and the dilatancy index, the rock’s dilatancy and brittleness under different unloading conditions are quantitatively analyzed. During unloading, the energy transformation mechanism of rock is described, and the law of deformation and failure is discussed based on characteristic energy. The rock failure strength fitting formula is given by applying the Mogi–Coulomb criterion and elastic strain energy criterion. The advantages of the elastic strain energy criterion are theoretically explained. This study shows that comprehensive consideration of the complex stress paths, confining pressure levels, and the loading-unloading rates of surrounding rock is an effective way to accurately study unloading rock characteristics. The results can provide theoretical basis for stability analysis of high-stress underground engineering.

scholarly journals Hydraulic properties and energy dissipation of deep hard rock under H-M coupling and cycling loads

2019 ◽  
Vol 23 (Suppl. 3) ◽  
pp. 935-942 ◽  
Author(s):  
Cheng-Han Zhang ◽  
Shuang You ◽  
Hong-Guang Ji ◽  
Fei Li ◽  
Hong-Tao Wang
Keyword(s):  
Acoustic Emission ◽  
Osmotic Pressure ◽  
Hydraulic Properties ◽  
Coupling Effect ◽  
High Stress ◽  
Confining Pressure ◽  
In Situ Stress ◽  
Dissipated Energy ◽  
Underground Engineering ◽  
Loading And Unloading

The permeability of deep rock is closely related to the stability and safety of underground engineering. The rocks in deep stratum are mostly with high stress and high osmotic pressure. Therefore, it is necessary to consider the coupling effect between porewater pressure and in situ stress on rock mass. A series of triaxial cyclic loading and unloading experiments under hydraulic-mechanics coupling conditions are carried out to studied the mechanical and hydraulic properties of granite in the depth of 1300 m to 1500 m. Especially, the effect of the disturbance on the permeability of fractured rocks are investigated by unloaded the confining pressure. Tests results presented that the stress-strain curves of deep granite showed typical brittle characteristics. The principal stress of granite exhibited a linear relationship under the high confining pressure of 34-40 MPa and high osmotic pressure of 13-15 MPa. Dissipated energy of the rock decreased to a relatively low level after 2-3 loading cycles and then slowly increased. Permeability showed a decreasing trend as the loading and unloading cycles increase. Finally, acoustic emission technology was used to monitor the fracture evolution in rocks, the acoustic emission signal released as the fractures develop and energy dissipated. The results would provide basic data for the exploitation and excavation in the deep galleries.

scholarly journals The Effect of Confining Pressure and Water Content on Energy Evolution Characteristics of Sandstone under Stepwise Loading and Unloading

2018 ◽  
Vol 2018 ◽  
pp. 1-8 ◽  
Author(s):  
Shuren Wang ◽  
Paul Hagan ◽  
Yanhai Zhao ◽  
Xu Chang ◽  
Ki-Il Song ◽  
...  
Keyword(s):  
Energy Dissipation ◽  
Water Content ◽  
Elastic Energy ◽  
Strain Energy ◽  
Confining Pressure ◽  
Triaxial Tests ◽  
Energy Evolution ◽  
Evolution Characteristics ◽  
Loading And Unloading ◽  
Stepwise Loading

To investigate the mechanical properties and energy evolution characteristics of sandstone depending on the water contents and confining pressure, the uniaxial and triaxial tests were conducted. The test results show that the strain energy was stored in the sandstone samples at the prepeak stage, and that is suddenly released when the failure occurred, and energy dissipation is sharply increased at the postpeak stage. The damage and energy dissipation characteristics of the samples are observed clearly under the stepwise loading and unloading process. The critical strain energy and energy dissipation show a clear exponential relationship. The critical elastic energy decreases linearly as the water content increases. As the confining pressure increases, the critical elastic energy of the samples transforms from linear to exponential. The concept of energy enhancement factor is proposed to characterize the strengthening effect induced by the confining pressure on the energy storage capacity of the rock samples. The energy evolution of the sandstone samples is more sensitive to the confining pressure than that of the water content.

scholarly journals Study on Energy Evolution and Damage Constitutive Model of Siltstone

Crystals ◽  
2021 ◽  
Vol 11 (11) ◽  
pp. 1271
Author(s):  
Ruihe Zhou ◽  
Longhui Guo ◽  
Rongbao Hong
Keyword(s):  
Elastic Strain ◽  
Strain Energy ◽  
Yield Criterion ◽  
Axial Strain ◽  
Elastic Strain Energy ◽  
Confining Pressure ◽  
Energy Yield ◽  
Input Energy ◽  
Test Results ◽  
Damage Constitutive Model

In order to study the energy evolution characteristics and damage constitutive relationship of siltstone, the conventional triaxial compression tests of siltstone under different confining pressures are performed, and the evolution laws of input energy, elastic strain energy and dissipative energy of siltstone with axial strain and confining pressure are analyzed. According to the test results, the judgment criterion of the rock damage threshold is improved, and an improved three-shear energy yield criterion is proposed., The damage constitutive equation of siltstone is established based on the damage mechanics theory through the principle of minimum energy consumption and by considering the residual strength of rock, and lastly, the rationality of the model is verified by experimental data. The results reveal that (1) both the input energy and dissipative energy gradually increase with the increase of axial strain, and the elastic strain energy first increases and then decreases with the increase of axial strain, and reaches its maximum at the peak. (2) The input energy and dissipation energy increase exponentially with the increase of the confining pressure, and the elastic strain energy increases linearly with the increase of confining pressure. (3) According to the linear relationship between the sum of shear strain energy and hydrostatic pressure, an improved three-shear energy yield criterion is established. (4) The model curve can better describe the strain softening stage and the residual strength characteristics of siltstone. The relative standard deviation between the model results and the test results is only 4.35%, which verifies the rationality and feasibility of the statistical damage constitutive model that is established in this paper.

scholarly journals Numerical Research on Energy Evolution in Granite under Different Confining Pressures Using Otsu’s Digital Image Processing and PFC2D

Symmetry ◽  
2019 ◽  
Vol 11 (2) ◽  
pp. 131 ◽  
Author(s):  
Yubao Zhang ◽  
Tongbin Zhao ◽  
Yanchun Yin ◽  
Yunliang Tan ◽  
Yue Qiu
Keyword(s):  
Image Processing ◽  
Growth Rate ◽  
Digital Image Processing ◽  
Digital Image ◽  
Strain Energy ◽  
Confining Pressure ◽  
Energy Evolution ◽  
Energy Accumulation ◽  
Energy Growth ◽  
Confining Pressures

Research on energy accumulation and releasing in the rock plays a key role on revealing its failure mechanism. This paper establishes a microscopic structure model of granite using Otsu digital image processing (DIP) technology and particle flow code software (PFC2D). A series of numerical compression tests under different confining pressures were conducted to investigate the macro and micro characteristics of energy evolution in granite. The results showed that the energy evolution of granite is divided into three stages: stable accumulation, slow dissipation, and rapid release. With increasing confining pressure, the strain energy accumulation ratio decreased exponentially and the peak value of strain energy increased linearly. It was found that the energy accumulation speed in the pre-peak stage increased as a linear function, while the energy release speed in the post-peak stage decreased as an exponential function. In addition, the feldspar is the main microstructure which played a major part in accumulating energy in granite. However, the unit mineral energy of mica particles was bigger than that of feldspar and quartz. When subjected to increasing confining pressure, the feldspar’s total energy growth rate was fastest. Meanwhile, the mica’s unit energy growth rate was fastest.

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