scholarly journals Experimental Study on Mechanical and Acoustic Emission Characteristics of Rock Samples under Different Stress Paths

2018 ◽  
Vol 2018 ◽  
pp. 1-9 ◽  
Author(s):  
Tao Qin ◽  
Hongru Sun ◽  
Heng Liu ◽  
Junwen Zhang ◽  
Tao Li ◽  
...  
Keyword(s):  
Acoustic Emission ◽  
Failure Mode ◽  
Shear Failure ◽  
Rock Specimen ◽  
Axial Strain ◽  
Failure Process ◽  
Confining Pressure ◽  
Stress Path ◽  
Initial Value ◽  
Unloading Confining Pressure

A series of tests on characteristics of acoustic emission have been performed on sandstone under uniaxial, conventional, and triaxial conditions and the unloading confining pressure path. The failure mode of rock specimen has been scanned by CT and a three-dimensional reconstruction was made. The differences on characteristics of AE, mechanics, and the failure mode of sandstone during the failure process under three paths are studied. The results show that the deformation of rock specimen is bigger, and axial strain and circumferential strain have a deformation platform at peak point of stress under the unloading confining pressure path. Characteristics of AE ringing are significantly affected by the confining pressure and stress path. AE ringing counts peak value, and accumulative ringing on the breaking moment as well as cumulative release energy is higher, which indicates that the rock failure is more violent under the unloading confining pressure path. The failure mode of rock specimen was dominated by shear failure under the conventional triaxial stress path. The tension failure is the main form at a lower initial value of unloading confining pressure, and the shear failure is more prominent at a higher initial value of unloading confining pressure.

scholarly journals Strength and Failure Characteristics of Saturated Sandstone Under Loading and Unloading

2021 ◽  
Author(s):  
Zhenhua Zhang ◽  
Huayan Yao ◽  
Hongguo Li ◽  
Hanbin Bian ◽  
Dayong Zhu
Keyword(s):  
Failure Modes ◽  
Shear Failure ◽  
Water Saturation ◽  
Triaxial Compression ◽  
Axial Direction ◽  
Confining Pressure ◽  
Stress Path ◽  
Failure Characteristics ◽  
Dry Conditions ◽  
Unloading Confining Pressure

Abstract Water has effects on the strength and failure characteristics of the sandstone in natural environment. Conventional triaxial compressive or unloading confining pressure experiments were conducted on sandstone specimens. Experimental results indicate that the compressive strength of sandstone decreases significantly under saturated conditions in comparison with dry conditions, the strength parameters of saturated specimens under unloading confining pressure are also lower than those of dry rock samples; for the sandstone with the same water content, the strengths under triaxial unloading confining pressure is slightly higher than those under triaxial compressive condition; compared with the stress path of triaxial compression, the stress path of unloading confining pressure makes cracks propagate more easily along the axial direction, and the angle between fracture surface and axial direction is smaller. Under triaxial unloading confining pressure, there failure modes of dry sandstone are tension failure and shear failure, while that of saturated sandstone is mainly shear failure. In the process of water saturation, the bond and friction characteristics between grain particles are degraded due to water weakening the cementation between the grain particles and softening grains boundary, and the expansion of clay minerals in the sandstone, which leads to the decrease of macroscopic mechanical strengths.

scholarly journals Deformation and Acoustic Emission Characteristics of Cracked Granite during Creep

2020 ◽  
Vol 2020 ◽  
pp. 1-11
Author(s):  
Chunping Wang ◽  
Jingli Xie ◽  
Jian Liu
Keyword(s):  
Acoustic Emission ◽  
Axial Strain ◽  
Failure Process ◽  
Deformation Characteristic ◽  
Temporal Distribution ◽  
Confining Pressure ◽  
High Energy ◽  
Lateral Strain ◽  
Creep Tests ◽  
Confining Pressures

A series of multistage creep tests under different confining pressures with acoustic emission monitoring have been performed to investigate the deformation characteristic and failure process of cracked granite during creep. The critical axial strain of cracked sample showed an increasing tendency with the increase of confining pressure. In contrast, critical lateral strain experienced a process of descending first at low confinement and then remaining nearly constant at high confinement. Compared with loading-cracked specimen, smaller critical axial strain, greater critical lateral strain, and higher lateral creep strain rate were found for unloading-cracked specimen. Based on the spatial and temporal distribution of acoustic emission events, the cracking process during creep was analysed. The AE events with high energy are mainly concentrated at the final fracture area of the specimen. The higher the confining pressure, the more the AE events with low energy. Compared with the loading-cracked specimen, the percentage of AE events with high energy is relatively small for the unloading-cracked specimen.

scholarly journals Effect of Variable Confining Pressure on Cyclic Triaxial Behavior of K0-consolidated Soft Marine Clay

2018 ◽  
Vol 4 (4) ◽  
pp. 755
Author(s):  
Lei Sun
Keyword(s):  
Pore Water ◽  
Pore Water Pressure ◽  
Axial Strain ◽  
Water Pressure ◽  
Confining Pressure ◽  
Stress Path ◽  
Cyclic Stress ◽  
Marine Clay ◽  
Stress Ratios ◽  
Variable Confining Pressure

The effect of variable confining pressure (VCP) on the cyclic deformation and cyclic pore water pressure in K0-consolidated saturated soft marine clay were investigated with the help of the cyclic stress-controlled advanced dynamic triaxial test in undrained condition. The testing program encompassed three cyclic deviator stress ratios, CSR=0.189, 0.284 and 0.379 and three stress path inclinations ηampl=3,1 and 0.64. All tests with constant confining pressure (CCP) and variable confining pressure (VCP) have identical initial stress and average stress. The results were analyzed in terms of the accumulative normalized excess pore water pressure rqu recorded at the end of each stress cycle and permanent axial strain, as well as resilient modulus. Limited data suggest that these behavior are significantly affected by both of the VCP and CSR. For a given value of VCP, both of the pore water pressure rqu and permanent axial strains are consistently increase with the increasing values of CSR. However, for a given value of CSR, the extent of the influence of VCP and the trend is substantially depend on the CSR.

scholarly journals The meso-fracturing mechanism of marble under unloading confining pressure paths and constant axial stress

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Linna Sun ◽  
Liming Zhang ◽  
Yu Cong ◽  
Yaduo Song ◽  
Keqiang He
Keyword(s):  
Shear Failure ◽  
Axial Stress ◽  
Confining Pressure ◽  
Particle Flow Code ◽  
Shear Cracks ◽  
Confining Pressures ◽  
Tension Cracks ◽  
Trial And Error Method ◽  
Unloading Confining Pressure ◽  
Fracturing Mechanism

AbstractFailure tests on marble during unloading confining-pressure under constant axial stress and simulations with the particle flow code were performed. The influence mechanism of the unloading rate of the confining pressure, initial unloading stress, and confining pressure on the failure characteristics of, and crack propagation in, marble was studied. By using the trial-and-error method, the conversion relationship between the unloading rates of confining pressures in laboratory tests and numerical simulations was ascertained. Micro-cracks formed in the unloading process of confining pressure are dominated by tension cracks, accompanied by shear cracks. The propagation of shear cracks lags that of tension cracks. As the confining pressure is increased, more cracks occur upon failure of the samples. The proportion of shear cracks increases while that of tension cracks decreases. The failure mode of samples undergoes a transition from shear-dominated failure to conjugated shear failure.

scholarly journals Numerical Simulation of Particle Breakage of Granular Assemblies in Discrete Element Analyses

2019 ◽  
Vol 2019 ◽  
pp. 1-17 ◽  
Author(s):  
Tao Zhang ◽  
Chi Zhang
Keyword(s):  
Total Energy ◽  
Energy Input ◽  
Axial Strain ◽  
Failure Process ◽  
Computational Cost ◽  
Confining Pressure ◽  
Discrete Element ◽  
Particle Breakage ◽  
Biaxial Tests ◽  
Total Energy Input

This paper presents numerical simulations of high-pressure biaxial tests on breakable granular soils with the discrete element method. The 2D setting is more economic in terms of computational cost, which allows simulation with a larger number of particles with a wider size distribution. The results of breakable and unbreakable agglomerates show that particle breakage has a significant influence on the macro- and micromechanical behaviors of the assembly. Higher confining pressure and larger axial strain result in the variation of particle grading and agglomerate numbers. The evolution of bond breakage during shearing makes it possible to trace the failure process and breakage mechanism at the microlevel. The breakage energy is found to account for a small fraction of total energy input compared with friction energy. A hyperbolic correlation between relative particle breakage and total energy input per unit volume was established regardless of the influence of confining pressure.

scholarly journals Deformation, Permeability and Acoustic Emission Characteristics of Coal Masses under Mining-Induced Stress Paths

Energies ◽  
2018 ◽  
Vol 11 (9) ◽  
pp. 2233 ◽  
Author(s):  
Yi Xue ◽  
Faning Dang ◽  
Zhengzheng Cao ◽  
Feng Du ◽  
Jie Ren ◽  
...  
Keyword(s):  
Acoustic Emission ◽  
Coal Seam ◽  
Coal Mining ◽  
Confining Pressure ◽  
Stress Path ◽  
Coal Mass ◽  
Emission Characteristics ◽  
Induced Stress ◽  
High Confining Pressure ◽  
Pillar Mining

The geomechanical and seepage evolution characteristics of coal masses during mining are the key factors that affect the drainage of coalbed methane and the safety of coal mining. Nevertheless, the influence of mining paths on coal seam permeability is rarely investigated given the complexity of mining-induced stress experiments. To study the effect of mining-induced stress on coal mining, the mechanical properties, acoustic emission characteristics and energy evolution of coal masses were experimentally evaluated through mining-induced stress experiments. Experimental results indicated that at peak intensity, the deviatoric stress and axial strain of coal samples under the stress path of protective coal-seam mining are lower than those of coal samples under the non-pillar stress path. The unloading ratio of confining pressure is large under a stress path of non-pillar mining, and the elastic energy, the absorbed energy, and the dissipated energy of coal mass are low during destruction. The effect of high confining pressure on AE events is pronounced under the non-pillar mining path. The overall b value under high confining pressure is smaller than that under low confining pressure, and AE events generally have high energy. The fracture structure of coal mass is complex, and the fractal size of coal is large under high unloading rates of confining pressure, which induce the increase of permeability after coal destruction.

Acoustic Emission Characteristics of Sericite Schist Coarse Aggregates under Consolidated Undrained Triaxial Tests

2013 ◽  
Vol 423-426 ◽  
pp. 909-913
Author(s):  
Lan Qiang Yang ◽  
Shang Lin Qin ◽  
Hui Gao ◽  
Shan Xiong Chen
Keyword(s):  
Acoustic Emission ◽  
Large Scale ◽  
Sliding Friction ◽  
Axial Strain ◽  
Confining Pressure ◽  
Rolling Friction ◽  
Triaxial Tests ◽  
Emission Characteristics ◽  
Coarse Aggregates ◽  
Ae Signals

In order to study the acoustic emission characteristics of coarse aggregates, improved of large-scale triaxial apparatus is used to do the consolidated undrained triaxial tests of sericite schist coarse aggregates, with the acoustic emission signals monitoring. The test results show that a large number of AE signals of sericite schist coarse aggregates are generated in the initial stage. Before the axial strain reach 10%, AE counts are relatively quiet. But after 10%, they become anomalous and emerge obvious leap values. When the confining pressure up to 200kPa, AE signals are mainly generated by sliding friction. With confining pressure increasing, the proporation of rolling friction and particle breakage is more and more obvious.

scholarly journals Experimental study on permeability characteristics of gas-containing raw coal under different stress conditions

2018 ◽  
Vol 5 (7) ◽  
pp. 180558 ◽  
Author(s):  
Dongming Zhang ◽  
Yushun Yang ◽  
Hao Wang ◽  
Xin Bai ◽  
Chen Ye ◽  
...  
Keyword(s):  
Flow Rate ◽  
Coal Sample ◽  
Axial Stress ◽  
Axial Strain ◽  
Confining Pressure ◽  
Gas Pressure ◽  
Permeability Characteristics ◽  
Loading And Unloading ◽  
Unloading Confining Pressure ◽  
Function Relation

The present experimental study on permeability characteristics for raw coal under different stress states is implemented by applying the triaxial self-made ‘THM coupled with servo-controlled seepage apparatus for gas-containing coal’; the result indicates that the flow rate of gas in the coal sample gradually decreases with the nonlinear loading of axial pressure and increases with the nonlinear unloading of axial stress and confining pressure. The flow rate, axial stress and confining pressure curves all satisfy the negative exponential function relation. When the sample reaches the peak intensity, the sample will be destroyed and the stress will drop rapidly; then the flow rate of the sample will increase rapidly. At this stage, the flow rate and axial strain show an oblique ‘v' pattern. The flow rate of the coal sample increases nonlinearly with the increase of gas pressure; the relation curve between flow rate and gas pressure satisfies the power function relation. Under the same confining pressure and gas pressure conditions, the larger the axial stress, the smaller the flow rate of the coal sample. Under the same axial stress and gas pressure conditions, the flow rate of the coal sample will first decrease, but then increase as the confining pressure decreases. During the post-peak loading and unloading process, the flow rate of the coal sample will decrease with the loading of confining pressure but increase with the unloading of confining pressure, and there will be an increase in wave shape with the increase in axial strain. The flow rate of each loading and unloading confining pressure is higher than that of the previous loading and unloading confining pressure. At the post-peak stage, the relation curve between the flow rate of the coal sample and the confining pressure satisfies the power function relation in the process of loading and unloading confining pressure.

scholarly journals Investigation on Pore-Fracture of Coal and Its Influence Mechanism on Tensile Failure Behavior of Coals with Bursting Proneness

Geofluids ◽  
2021 ◽  
Vol 2021 ◽  
pp. 1-16
Author(s):  
Yutao Li ◽  
Qingwei Guo ◽  
Xunchen Liu ◽  
Yaodong Jiang ◽  
Bo Zhang ◽  
...  
Keyword(s):  
Acoustic Emission ◽  
Spatial Distribution ◽  
Failure Mode ◽  
Failure Process ◽  
Elastic Strain Energy ◽  
Tensile Failure ◽  
Failure Behavior ◽  
Secondary Fracture ◽  
Deformation And Failure ◽  
Influence Mechanism

Both computed tomography (CT) and notched semicircular bend (NSCB) tests are performed for coals with high and medium bursting proneness to extract the scientific expression of pore-fracture and its influence mechanism on the tensile failure behavior. The acoustic emission (AE) parameters in the sample during loading and failure are monitored, and the influence mechanism of pore-fracture on tensile failure behavior of coal is analyzed. The result illustrates that the spatial distribution feature of the pore-fracture in coals with high and medium bursting proneness is extremely different. The deformation and failure mode of the coals are affected by many factors, loading mode, notch depth and width, mechanical properties of matrix and minal, spatial distribution feature of pore-fracture, etc. The influence of primary pore-fracture in the coal on the extension and penetration of the secondary fracture could be divided into two types: bifurcation and promotion, which would cause different local damage in the sample and affect the final failure mode. The feature of acoustic emission parameters indicates that the deformation and failure process of a sample under loading could be divided into four stages: compaction stage, elastic deformation stage, displacement plastic growth stage, and post peak failure stage, which is the result of comprehensive action of many factors. The evolution process of secondary fracture is accompanied by the dissipation of elastic strain energy and the intensification of internal damage of coal, which reflects the failure process of coal.

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