Uniaxial experimental study of the acoustic emission and deformation behavior of composite rock based on 3D digital image correlation(DIC)

In this paper, uniaxial compression tests were carried out on a series of composite rock specimens with different dip angles, which were made from two types of rock-like material with different strength. The acoustic emission technique was used to monitor the acoustic signal characteristics of composite rock specimens during the entire loading process. At the same time, an optical non-contact 3 D digital image correlation technique was used to study the evolution of axial strain field and the maximal strain field before and after the peak strength at different stress levels during the loading process. The effect of bedding plane inclination on the deformation and strength during uniaxial loading was analyzed. The methods of solving the elastic constants of hard and weak rock were described. The damage evolution process, deformation and failure mechanism, and failure mode during uniaxial loading were fully determined. The experimental results show that the θ = 0?–45?specimens had obvious plastic deformation during loading, and the brittleness of the θ = 60?–90?specimens gradually increased during the loading process. When the anisotropic angle θincreased from 0?to 90?, the peak strength, peak strain,and apparent elastic modulus all decreased initially and then increased. The failure mode of the composite rock specimen during uniaxial loading can be divided into three categories:tensile fracture across the discontinuities(θ = 0?–30?), slid-ing failure along the discontinuities(θ = 45?–75?), and tensile-split along the discontinuities(θ = 90?). The axial strain of the weak and hard rock layers in the composite rock specimen during the loading process was significantly different from that of the θ = 0?–45?specimens and was almost the same as that of the θ = 60?–90?specimens. As for the strain localization highlighted in the maximum principal strain field, the θ = 0?–30?specimens appeared in the rock matrix approximately parallel to the loading direction,while in the θ = 45?–90?specimens it appeared at the hard and weak rock layer interface.

Particle flow study on strength and meso-mechanism of Brazilian splitting test for jointed rock mass

A discrete element method （DEM） called particle flow code （PFC2D） was used to construct a model for Brazilian disc splitting test in the present study. Based on the experimental results of intact Brazilian disc of rock-like material, a set of micro-parameters in PFC2D that reflected the macro-mechanical behavior of rock-like materials were obtained. And then PFC2D was used to simulate Brazilian splitting test for jointed rock mass specimens and specimen containing a central straight notch. The effect of joint angle and notch angle on the tensile strength and failure mode of jointed rock specimens was detailed analyzed. In order to reveal the meso-mechanical mechanism of crack coalescence, displacement trend lines were applied to analyze the displacement evolution during the crack initiation and propagation. The investigated conclusions can be described as follows. （1） The tensile strength of jointed rock mass disc specimen is dependent to the joint angle. As the joint angle increases, the tensile strength of jointed rock specimen takes on a nonlinear variance. （2） The tensile strength of jointed rock mass disc specimen containing a central straight notch distributes as a function of both joint angle and notch angle. （3） Three major failure modes, i.e., pure tensile failure, shear failure and mixed tension and shear failure mode are observed in jointed rock mass disc specimens under Brazilian test. （4） The notch angle roles on crack initiation and and joint angle play important propagation characteristics of jointed rock mass disc specimen containing a central straight notch under Brazilian test.

Discrete element modeling on the crack evolution behavior of brittle sandstone containing three fissures under uniaxial compression

Based on experimental restilts of brittle, intact sandstone under uniaxial compression, the micro-parameters were firstly confirmed by adopting particle flow code （PFC2D）. Then, the validation of the simulated models were cross checked with the experimental results of brittle sandstone containing three parallel fissures under uniaxial compression. The simulated results agreed very well with the experimental results, including the peak strength, peak axial strain, and ultimate failure mode. Using the same micro- parameters, the numerical models containing a new geometry of three fissures are constructed to investigate the fissure angle on the fracture mechanical behavior of brittle sandstone under uniaxial compression. The strength and deformation parameters of brittle sandstone containing new three fissures are dependent to the fissure angle. With the increase of the fis- sure angle, the elastic modulus, the crack damage threshold, and the peak strength of brittle sandstone containing three fissures firstly increase and secondly decrease. But the peak axial strain is nonlinearly related to the fissure angle. In the entire process of deformation, the crack initiation and propagation behavior of brittle sandstone containing three fissures under uniaxial compression are investigated with respect to the fissure angle. Six different crack coalescence modes are identified for brittle sandstone containing three fissures under uniaxial compression. The influence of the fissure angle on the length of crack propagation and crack coalescence stress is evaluated. These investigated conclusions are very important for ensuring the stability and safety of rock engineering with intermittent structures.

Macro and micro mechanics behavior of granite after heat treatment by cluster model in particle flow code

In this paper, a cluster model in particle flow code was used to simulate granite specimens after heat treatment under uniaxial compression. The results demonstrated that micro-cracks are randomly distributed in the specimen when the temperature is below 300?C, and have partial coalescence when the temperature is up to 450?C, then form macro-cracks when the temperature is above 600?C. There is more inter-granular cracking than intra-granular cracking, and their ratio increases with increasing temperature.The micro-cracks are almost constant when the temperature decreases from 900?C to room temperature, except for quartz α–β phase transition temperature(573?C). The fracture evolution process is obviously affected by these cracks, especially at 600–900?C. Elevated temperature leads to easily developed displacement between the grains, and the capacity to store strain energy becomes weaker, corresponding to the plasticity of granite after heat treatment.

Study on the damage-softening constitutive model of rock and experimental verification

A damage-softening model is presented to describe the stress-strain curve of rock. By comparing the Hoek-Brown (H-B) and Mohr-Coulomb (M-C) yield criterion, the equivalent M-C yield criterion is selected as the strength criterion in this model. To better characterize the rock damage and failure processes with considering the relationship between damage and deformation, the concept of yield stress ratio is introduced to describe the yield strengthening deformation before rock peak stress. Damage events are described by two cumulative damage evolution laws. The evolution equations of tensile and shear damage are presented based on the equivalent plastic strains and the maximum value between tensile and shear damage represents the total damage for rock. Considering that rock cannot bear tensile load after tensile failure but still has a certain shear strength, its tensile and shear strengths are small after shear failure. The elastic modulus is affected by tensile damage, whereas the angle of internal friction, the cohesion, and dilation angles are influenced by shear damage. The proposed damage-softening model describes the strain-softening, brittle stress-drop, and residual strength of rock after peak stress, and finally the model is implemented in FLAC3D. Comparing the test and the numerical calculation results, the damage-softening model better describes the total stress-strain curve of rock.

Effect of damage on gas seepage behavior of sandstone specimens

In underground engineering,such as geological CO2 sequestration,unconventional oil and gas exploration,and radioactive waste storage,permeability of rock is important to evaluate the potential CO2 storage capacity,improve oil and gas production,and prevent leakage of radioactive waste.In this study,hydrostatic stress tests and triaxial compression tests with gas permeability measurements were carried out on intact and damaged sandstone specimens.Three series of experiment were designed to evaluate the permeability evolution laws of sandstone under different testing conditions.They included triaxial seepage tests on intact specimens under different confining pressures,triaxial seepage tests on damaged specimens with different extents of damage,and hydrostatic seepage tests on damaged specimens under increasing and decreasing gas pressures.Based on the experimental results,the effects of effective confining pressure,extent of damage and increasing and decreasing gas pressure on permeability of sandstone were investigated.It shows that the permeability of the intact sandstone specimens first decreased and then increased,followed by a constant value with increase in axial strain.The permeability of the sandstone specimens was observed to decrease with increase in effective confining pressure.The extent of damage affects the permeability evolution,but does not influence the failure patterns of damaged sandstone.As the gas pressure increased,the permeability of the damaged sandstone specimen increased.Under the same gas pressure condition,the permeability during the decreasing process is generally higher than that during the increasing process.These experiments are expected to enhance our understanding of seepage behavior in underground rock masses.

An experimental study on seepage behavior of sandstone material with different gas pressures

The seepage evolution characteristic of brittle rock materials is very significant for the stability and safety of rock engineering. In this research, a series of conventional triaxial compression and gas seepage tests were carded out on sandstone specimens with a rock mechanics servo-controlled testing system. Based on the experimental results, the relationship between permeability and deformation is firstly analyzed in detail. The results show that the permeabilityaxial strain curve can be divided into the following five phases： the phase of micro-defects closure, the phase of linear elastic deformation, the phase of nonlinear deformation, the phase of post-peak stress softening and the phase of residual strength. The seepage evolution characteristic is also closely correlated with the volumetric deformation according to the relationship between permeability and volumetric strain. It is found that the gas seepage pressure has a great effect on the permeability evolution, i.e. permeability coefficients increase with increasing gas seepage pressures. Finally, the influence of gas seepage pressures on the failure behavior of brittle sandstone specimens is discussed.

An experimental study on fracture mechanical behavior of rock-like materials containing two unparallel fissures under uniaxial compression

Strength and deformability characteristics of rock with pre-existing fissures are governed by cracking behavior. To further research the effects of pre-existing fissures on the mechanical properties and crack coalescence process, a series of uniaxial compression tests were carried out for rock-like material with two unparallel fissures.In the present study, cement, quartz sand, and water were used to fabricate a kind of brittle rock-like material cylindrical model specimen. The mechanical properties of rock-like material specimen used in this research were all in good agreement with the brittle rock materials. Two unparallel fissures（a horizontal fissure and an inclined fissure） were created by inserting steel during molding the model specimen.Then all the pre-fissured rock-like specimens were tested under uniaxial compression by a rock mechanics servocontrolled testing system. The peak strength and Young＇s modulus of pre-fissured specimen all first decreased and then increased when the fissure angle increased from 0？to 75？.In order to investigate the crack initiation, propagation and coalescence process, photographic monitoring was adopted to capture images during the entire deformation process.Moreover, acoustic emission（AE） monitoring technique was also used to obtain the AE evolution characteristic of prefissured specimen. The relationship between axial stress, AE events, and the crack coalescence process was set up： when a new crack was initiated or a crack coalescence occurred, thecorresponding axial stress dropped in the axial stress–time curve and a big AE event could be observed simultaneously.Finally, the mechanism of crack propagation under microscopic observation was discussed. These experimental results are expected to increase the understanding of the strength failure behavior and the cracking mechanism of rock containing unparallel fissures.

Influence of stress and high temperature treatment on the permeability evolution behavior of sandstone

Permeability is an important property of rock in gas and oil exploration engineering, environment temperature and geo-stress have great influence on it. This paper aims to analyze the influence of thermal treatment on the permeability of sandstone under triaxial compression. Based on the gas seepage tests on a sandstone specimen after different thermal treatment temperatures with different gas pressures, hydrostatic stresses and deviatoric stresses, the thermal effect on physical property of sandstone is firstly analyzed. The results show that the mass of the sandstone specimen decreases with the increase of temperature, some spalling damage and tensile crack occur on the lateral surface of the specimen at 400℃. According to the seepage test results with various gas pressures, an exponential relationship has been found between the permeability coefficient and temperature. The permeability coefficient is approximately 100 times as large as the initial value when the temperature increases from 20℃ to 800℃. The permeability evolution of the heated sandstone under hydrostatic and deviatoric compression has also been analyzed. A simplified double pore texture model is proposed which can well describe the permeability evolution of sandstone under compression with hydrostatic stress and deviatoric stress, it can be helpful to estimate the permeability of thermal treated sandstone under elastic triaxial compression.

Correction to: Uniaxial experimental study of the acoustic emission and deformation behavior of composite rock based on 3D digital image correlation (DIC)

In the original publication of this article,Table 2 is incorrectly published due to the negligence of the author's proofreading.The correct version of Table 2 is provided below.

Fracture behavior of granite specimen containing a single fissure under uniaxial compression by grain-based model 3D

The understanding of the mechanical behavior of granite containing fissures is of great significance for geothermal energy exploitation and high-level nuclear waste disposal repository.To consider the grain interlocking and rotational resistance behavior in granite specimens,the grain-based model 3D(GBM3D)was constructed to investigate the fracture behavior of granite specimens containing a single fissure.The numerical result indicates that this GBM3D can be used to investigate the mechanical behavior of granite constituted by different minerals,and it can provide more information to understand the mechanical behavior of specimens under loading process.The inter-granular cracks initiate in the first place,but the initiation of intra-granular cracks mainly concentrates after the peak strength.The tensile cracks between different minerals are more than that between the same minerals,and the intra-granular cracks in muscovite initiate firstly,and the number of them is more than that in other minerals.The wing crack is mainly constituted by the inter-granular cracks,and the shape of it is different in different depths,whereas the anti-wing is similar in different depths.

Three-dimensional failure behavior and cracking mechanism of rectangular solid sandstone containing a single fissure under triaxial compression

In actual rock engineering,fissures play an important role in determining the mechanical parameters of rock mass,whereas it is very difficult to construct fissures in cylindrical specimens.Therefore,the pre-fissured rectangular rock specimens were constructed innovatively.Moreover,a series of triaxial compression experimental results on the failure mechanical behavior of rectangular solid sandstone specimens containing a single fissure were reported.The lateral strain in different directions was monitored and the experimental results show that elastic modulus and axial strain increase non-linearly with confining pressure,and the average Poisson’s ratio parallel to fissure(μ2)is larger than that vertical to fissure(μ3).The cohesion,Hoek-Brown parameters of peak strength show similar trends with that of crack damage threshold to the fissure angle(α),and the parameters of the peak strength are larger than those of crack damage threshold.However,the internal friction angles of the peak strength and crack damage threshold are almost equal.Based on the geometries and properties of cracks,ten typical crack types are identified.Cracks vertical to pre-existing fissures occur in specimens under uniaxial compression,whereas cracks parallel to pre-existing fissures occur under triaxial compression.Finally,X-ray micro-computed tomography(CT)observations are conducted to analyze the internal damage mechanism of sandstone specimens with respect to various fissure angles.Reconstructed 3-D CT images indicate obvious effects of confining pressure and fissure angle on the crack system of sandstone specimens.This research elucidates the fundamental nature of rock failure under triaxial compression.