Study on mechanical properties and damage evolution of carbonaceous shale under triaxial compression with acoustic emission

To explore the mechanical properties and damage evolution characteristics of carbonaceous shale with different confining pressures and water-bearing conditions, triaxial compression tests accompanied by simultaneous acoustic emission (AE) monitoring were conducted on carbonaceous shale rock specimens. The AE characteristics of carbonaceous shale were investigated, a damage assessment method based on Shannon entropy of AE was further proposed. The results suggest that the mechanical properties of carbonaceous shale intensify with increasing confining pressure and degrade with increasing water content. Moisture in rocks does not only weaken the cohesion but also reduce the internal friction angle of carbonaceous shale. It is observed that AE activities mainly occur in the post-peak stage and the strong AE activities of saturated carbonaceous shale specimens appear at a lower normalized stress level than that of natural-state specimens. The maximum AE counts and AE energy increase with water content while decrease with confining pressure. Both confining pressure and water content induce changes in the proportions of AE dominant frequency bands, but the changes caused by confining pressure are more significant than those caused by water content. The results also indicate that AE entropy can serve as an applicable index for rock damage assessment. The damage evolution process of carbonaceous shale can be divided into two main stages, including the stable damage development stage and the damage acceleration stage. The damage variable increases slowly accompanied by a few AE activities at the first stage, which is followed by a rapid growth along with intense acoustic emission activities at the damage acceleration stage. Moreover, there is a sharp rise in the damage evolution curve for the natural-state specimen at the damage acceleration stage, while the damage variable develops slowly for the saturated-state specimen.

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Damage evolution and recrystallization enhancement of frozen loess

International Journal of Damage Mechanics ◽

10.1177/1056789517731138 ◽

2017 ◽

Vol 27 (8) ◽

pp. 1131-1155 ◽

Cited By ~ 21

Author(s):

Zhiwei Zhou ◽

Wei Ma ◽

Shujuan Zhang ◽

Cong Cai ◽

Yanhu Mu ◽

...

Keyword(s):

Mechanical Properties ◽

Strain Rate ◽

Damage Evolution ◽

Deformation Process ◽

Triaxial Compression ◽

Confining Pressure ◽

Recrystallization Process ◽

Frozen Soils ◽

Time Deformation ◽

Pressure Melting

A series of multistage triaxial compression, creep, and stress relaxation tests were conducted on frozen loess at the temperature of −6℃ in order to study the damage evolution and recrystallization enhancement of mechanical properties during deformation process. The effect of strain rate, confining pressure, and hydrostatic stress history in the degradation laws of mechanical properties is investigated further. The strain rate has a significant influence on the stress–strain curve which dominates the evolution trend of mechanical properties. The mechanical behaviors (strength, stiffness, and viscosity) of frozen loess all exhibit evident response for the consolidation and pressure melting phenomenon caused by the confining pressure. The multistage loading tests under different hydrostatic stresses are capable of differentiating the development characteristics of mechanical properties during axial loading and hydrostatic compression process, respectively. The testing results indicated that the recrystallization of the ice particle in the frozen soils is an important microscopic factor for enhancement behaviors of mechanical parameters during the deformation process. This strengthening degree of mechanical properties is determined by temperature, duration time, deformation degree, and stress state during the recrystallization process. The phase transformation led by pressure melting and ice recrystallization is a nonnegligible changing pattern of frozen soils microstructure, which has apparent role in the damage evolution of mechanical properties.

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Mechanical Properties and Acoustic Emission Characteristics of Karst Limestone under Uniaxial Compression

Advances in Materials Science and Engineering ◽

10.1155/2018/2404256 ◽

2018 ◽

Vol 2018 ◽

pp. 1-14 ◽

Cited By ~ 3

Author(s):

Jianxun Chen ◽

Qingsong Wang ◽

Jiaqi Guo ◽

Yanbin Luo ◽

Yao Li ◽

...

Keyword(s):

Mechanical Properties ◽

Acoustic Emission ◽

Water Content ◽

Uniaxial Compression ◽

Wave Velocity ◽

Brittle Failure ◽

Shear Failure ◽

Testing Machine ◽

Natural State ◽

Saturated State

Firstly, I-RPT ultrasonic detector was used to test the wave velocity of karst limestone with different initial microstructure and water content. Then, RMT-150B rock testing machine and DS2-16B acoustic emission system were used to test the acoustic emission (AE) under uniaxial compression. Mechanical properties and AE characteristics were obtained during rock failure. The detailed relationship between stress-strain and AE characteristics was studied in this paper. Research results indicated the following: (1) For samples with many primary fissures and defects, wave velocity in dry state was larger than that in its natural state. From natural state to saturated state, the wave velocity tended to increase. For samples with good integrity, wave velocity increased with increasing of water content. (2) In the dry state, the samples presented tension failure. In saturated state, the samples presented tension-shear failure. For samples with cracks and good integrity, samples showed brittle failure. For samples with many corrosion pores which showed ductile damage under natural and saturated state, the spalling phenomenon was enhanced under saturated state. (3) With increasing of water content, the peak stress and AE peak reduced dramatically. In brittle failure, AE peak could be considered a sign of failure. In ductile failure, AE activity decreased gradually with the decrease of stress. (4) The mechanical properties and AE characteristics corresponding to four main fracture propagation types were also discussed.

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Mechanical Properties of Bentonite-Sand Mixtures Under Triaxial Stress Condition

MRS Proceedings ◽

10.1557/proc-353-307 ◽

1994 ◽

Vol 353 ◽

Cited By ~ 1

Author(s):

M. Umedera ◽

A. Fujiwara ◽

N. Yasufuku ◽

M. Hyodo ◽

H. Murata

Keyword(s):

Mechanical Properties ◽

Shear Strength ◽

Water Content ◽

Triaxial Compression ◽

Confining Pressure ◽

Strength Properties ◽

Compression Tests ◽

Optimum Water Content ◽

Triaxial Compression Tests ◽

Optimum Water

AbstractA series of triaxial compression tests is being conducted under the drained condition on bentonite and sand mixtures, known as buffer, in saturated and optimum water content states to clarify the mechanical properties of the buffer.It was found that the mechanical properties of bentonite and sand mixtures are strongly influenced by water and bentonite contents: shear strength in a saturated state is less than that in an optimum water content state; shear strength decreases rapidly with increasing bentonite content. Strength properties are much dependent on confining pressure.

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Study on Mechanical Properties and Damage Characteristics of Red Sandstone under Freeze-thaw and Load

Advances in Civil Engineering ◽

10.1155/2021/8867489 ◽

2021 ◽

Vol 2021 ◽

pp. 1-13

Author(s):

Lei Shi ◽

Yang Liu ◽

Xiangzhen Meng ◽

Huimei Zhang

Keyword(s):

Mechanical Properties ◽

Triaxial Compression ◽

Confining Pressure ◽

Damage Variable ◽

Mechanical Parameters ◽

Compression Tests ◽

Internal Damage ◽

Red Sandstone ◽

Freeze Thaw ◽

Total Damage

To analyze the effects of freeze-thaw cycles and confining pressure on the mechanical properties of red sandstone, through freeze-thaw cycles and triaxial compression tests, full stress-strain curves of different freeze-thaw cycles and different confining pressures were obtained. The degradation degree of red sandstone was quantitatively considered from different mechanical parameters of ultimate stress, elastic modulus, and Poisson’s ratio. Based on summarizing the characteristics of rock under freeze-thaw and load, the total damage variable of rock was determined by the reasonable measurement of freeze-thaw damage variable and load damage variable, and a damage constitutive model under freeze-thaw and load was established. The research showed that the freeze-thaw cycles aggravate the degree of rock damage deterioration, the rock stiffness and strength were reduced, and the characteristics of plastic deformation and ductile failure were more obvious. The confining pressure inhibited red sandstone internal damage, and with the increase of confining pressure, the stiffness and strength and the plastic characteristics were increased. In the overall trend, the mechanical parameters had different sensitivity to the degradation effect of freeze-thaw cycles and confining pressure. Regardless of the increase in the number of freeze-thaw cycles or confining pressure, the strain softening modulus tended to decrease gradually, and red sandstone plastic damage became more obvious after the stress peak. The total damage evolution path of red sandstone reflected the nonlinear influence of freeze-thaw and load on the total damage propagation. The research results provide theoretical support for the improvement of the technology of the effluent coal rock in Balasu Coal Mine.

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Damage and fractal evolution trends of sandstones under constant-amplitude and tiered cyclic loading and unloading based on acoustic emission

International Journal of Distributed Sensor Networks ◽

10.1177/1550147719861020 ◽

2019 ◽

Vol 15 (7) ◽

pp. 155014771986102

Author(s):

Dongxu Liang ◽

Nong Zhang ◽

Lixiang Xie ◽

Guangming Zhao ◽

Deyu Qian

Keyword(s):

Acoustic Emission ◽

Cyclic Loading ◽

Damage Evolution ◽

Damage Accumulation ◽

Fractal Dimensions ◽

Damage Variable ◽

Constant Amplitude ◽

Kaiser Effect ◽

Cyclic Loading And Unloading ◽

Loading And Unloading

It is of significance to study the damage and destruction of rock under cyclic loading in geotechnical engineering. We determined the trends in damage evolution of sandstone under constant-amplitude and tiered cyclic loading and unloading under uniaxial compression. The results of the study show that (1) the variation of acoustic-emission events was consistent with the stress curves and 89% of all acoustic-emission events occurred during the cycling stages. The observed Kaiser effect was more notable in tiered cycling. (2) The damage variable increased sharply in the cycling stages and its increment was 0.07 higher for tiered cycling than constant-amplitude cycling. Sandstone exhibited greater damage under tiered cyclic loading and unloading. (3) Equations for the evolution of the damage variable under the two cycle modes were obtained by fitting of experimental data. (4) The fractal dimensions of the constant-amplitude cycle were larger than those of the tiered cycle. The process of damage and destruction presents a trend of reducing fractal dimension. The damage accumulation of sandstone under tiered cycling was faster than under constant-amplitude cycling. These results provide references for damage and early warning of rock under both constant-amplitude and tiered cyclic loading and unloading.

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Cracking Behaviors and Mechanical Properties of Rock-Like Specimens with Two Unparallel Flaws under Conventional Triaxial Compression

Advances in Civil Engineering ◽

10.1155/2019/5849703 ◽

2019 ◽

Vol 2019 ◽

pp. 1-15 ◽

Cited By ~ 1

Author(s):

Huilin Le ◽

Shaorui Sun ◽

Chenghua Xu ◽

Liuyang Li ◽

Yong Liu

Keyword(s):

Mechanical Properties ◽

Inclination Angle ◽

Failure Modes ◽

Shear Failure ◽

Triaxial Compression ◽

Confining Pressure ◽

Critical Value ◽

Tensile Failure ◽

Rock Bridge ◽

Bridge Length

Flaws existing in rock masses are generally unparallel and under three-dimensional stress; however, the mechanical and cracking behaviors of the specimens with two unparallel flaws under triaxial compression have been rarely studied. Therefore, this study conducted comprehensive research on the cracking and coalescence behavior and mechanical properties of specimens with two unparallel flaws under triaxial compression. Triaxial compressive tests were conducted under different confining pressures on rock-like specimens with two preexisting flaws but varying flaw geometries (with respect to the inclination angle of the two unparallel flaws, rock bridge length, and rock bridge inclination angle). Six crack types and eleven coalescence types in the bridge region were observed, and three types of failure modes (tensile failure, shear failure, and tensile-shear failure) were observed in experiments. Test results show that bridge length and bridge inclination angle have an effect on the coalescence pattern, but the influence of bridge inclination angle is larger than that of the bridge length. When the confining pressure is low, coalescence patterns and failure modes of the specimens are greatly affected by flaw geometry, but when confining pressure rose to a certain level, the influence of confining pressure is larger than the effect of flaw geometry. The peak strength of the specimens is affected by flaw geometry and confining pressure. There is a critical value for the bridge length. If the bridge length is larger than the critical value, peak strengths of the samples almost keep constant as the bridge length increases. In addition, as the bridge inclination angle increases, there is an increase in the probability of tensile cracks occurring, and with an increase in the confining pressure, the probability of the occurrence of shear cracks increases.

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Experimental Investigation and Micromechanical Modeling of Elastoplastic Damage Behavior of Sandstone

Materials ◽

10.3390/ma13153414 ◽

2020 ◽

Vol 13 (15) ◽

pp. 3414

Author(s):

Chaojun Jia ◽

Qiang Zhang ◽

Susheng Wang

Keyword(s):

Damage Evolution ◽

Triaxial Compression ◽

Damage Model ◽

Confining Pressure ◽

Micromechanical Modeling ◽

Compression Tests ◽

Integration Algorithm ◽

Damage Behavior ◽

Plastic Strain Increment ◽

Elastoplastic Damage

The mechanical behavior of the sandstone at the dam site is important to the stability of the hydropower station to be built in Southwest China. A series of triaxial compression tests under different confining pressures were conducted in the laboratory. The critical stresses were determined and the relationship between the critical stress and confining pressure were analyzed. The Young’s modulus increases non-linearly with the confining pressure while the plastic strain increment Nϕ and the dilation angle ϕ showed a negative response. Scanning electron microscope (SEM) tests showed that the failure of the sandstone under compression is a coupled process of crack growth and frictional sliding. Based on the experimental results, a coupled elastoplastic damage model was proposed within the irreversible thermodynamic framework. The plastic deformation and damage evolution were described by using the micromechanical homogenization method. The plastic flow is inherently driven by the damage evolution. Furthermore, a numerical integration algorithm was developed to simulate the coupled elastoplastic damage behavior of sandstone. The main inelastic properties of the sandstone were well captured. The model will be implemented into the finite element method (FEM) to estimate the excavation damaged zones (EDZs) which can provide a reference for the design and construction of such a huge hydropower project.

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Study on Damage Model and Damage Evolution Characteristics of Backfill with Prefabricated Fracture under Seepage-Stress Coupling

Advances in Materials Science and Engineering ◽

10.1155/2020/3642356 ◽

2020 ◽

Vol 2020 ◽

pp. 1-11

Author(s):

Jifeng Hou ◽

Zhongping Guo ◽

Weizhen Liu ◽

Hengze Yang ◽

WenWu Xie

Keyword(s):

Mechanical Properties ◽

Damage Evolution ◽

Coupling Effect ◽

Water Pressure ◽

Damage Model ◽

Confining Pressure ◽

Seepage Water ◽

Confining Pressures ◽

Initial Fracture ◽

Total Damage

Aiming at the backfill with prefabricated fracture under seepage-stress coupling, the concepts of fracture macrodamage, loaded mesodamage, seepage mesodamage, and total damage of backfill were proposed. Based on the macroscopic statistical damage model, the coupling effect of seepage, stress, and initial fracture was considered comprehensively and the damage model of backfill with prefabricated fracture under seepage-stress coupling was established. The mechanical properties of backfill with prefabricated fracture under different seepage water pressures and confining pressures were tested and the rationality of the model was verified. The research shows that the mechanical properties of backfill with prefabricated fracture under the seepage-stress coupling are determined by the seepage water pressure, the load, the initial fracture, and the coupling effect. Fracture and seepage have significant effects on the damage of the backfill. When the seepage water pressure is low, the fracture damage dominates; however, when the seepage water pressure is high, the seepage damage dominates; the total damage under the coupling action is more serious than the single factor. The development laws of the total damage evolution curves under different seepage water pressures and confining pressures are basically the same, and they show the S-shaped distribution law with the increase of the axial strain. With the increase of confining pressure, the damage effect of fracture and seepage on the backfill is weakened, indicating that the confining pressure has a certain inhibitory effect on the damage evolution of the backfill. The research results can provide a theoretical basis for the study of the stability of backfill with geological defects such as joints and fractures in deep high-stress and high-seepage water pressure coal mines.

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Experimental Analysis and Discussion on the Damage Variable of Frozen Loess

Advances in Materials Science and Engineering ◽

10.1155/2017/1689251 ◽

2017 ◽

Vol 2017 ◽

pp. 1-13

Author(s):

Cong Cai ◽

Wei Ma ◽

Shuping Zhao ◽

Yanhu Mu

Keyword(s):

Damage Evolution ◽

Deformation Process ◽

Triaxial Compression ◽

Engineering Application ◽

Damage Variable ◽

Dissipated Energy ◽

Strain Rates ◽

Damage Factor ◽

Energy Increase ◽

Advantages And Disadvantages

The damage variable is very important to study damage evolution of material. Taking frozen loess as an example, a series of triaxial compression and triaxial loading-unloading tests are performed under five strain rates of 5.0 × 10−6–1.3 × 10−2/s at a temperature of −6°C. A damage criterion of frozen loess is defined and a damage factorDcis introduced to satisfy the requirements of the engineering application. The damage variable of frozen loess is investigated using the following four methods: the stiffness degradation method, the deformation increase method, the dissipated energy increase method, and the constitutive model deducing method during deformation process. In addition, the advantages and disadvantages of the four methods are discussed when they are used for frozen loess material. According to the discussion, the plastic strain may be the most appropriate variable to characterize the damage evolution of frozen loess during the deformation process based on the material properties and the nature of the material service.

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Experimental Investigation on the Influence of Water Content on the Mechanical Properties of Coal under Conventional Triaxial Compression

Shock and Vibration ◽

10.1155/2020/8872438 ◽

2020 ◽

Vol 2020 ◽

pp. 1-11

Author(s):

Meichang Zhang ◽

Rongshan Nie

Keyword(s):

Mechanical Properties ◽

Elastic Modulus ◽

Internal Friction ◽

Water Content ◽

Triaxial Compression ◽

Friction Angle ◽

Conventional Triaxial Compression ◽

Influence Of Water ◽

Confining Pressures ◽

Water Contents

The presence of water is one of the most important factors in coal mining, and it has a dual influence on the mechanical behavior of rock. To study the influence of water content on the mechanical properties of coal under complicated stress conditions, dry coal specimens and wet coal specimens with water contents of 1.8% and 3.6% were conducted by uniaxial and conventional triaxial compression tests. The relations between the uniaxial compressive strength, deformation, and water content were observed. The reductions in the strength and elastic modulus under different confining pressures were obtained. The mechanical properties of coal specimens with different water contents under triaxial compression were studied. The influences of water content on the microstructure, clay minerals, internal friction angle, and cohesive force of coal were discussed. The results show that the strengths and elastic moduli of wet specimens are clearly lower than those of dry specimens under different confining pressures. The water content has a significant influence on the postfailure mechanical behavior of coal. The loss rates of strength and elastic modulus decrease with increasing confining pressure. The water content has almost no effect on the internal friction angle, while the cohesive force of the saturated specimens is 36.5% lower than that of the dry specimens. The results can provide a reference for inhibiting the occurrence of disasters during coal mining and exploiting coal efficiently.

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