Search

A series of triaxial creep tests were carried out on red sandstone under the conditions of high temperature, high pore pressure, and high confining pressure, and the creep mechanism was analyzed in terms of the velocity and energy variations in the ultrasonic waves emitted during the creep test. Changes in porosity and pore distribution in the samples were investigated by nuclear magnetic resonance (NMR) for different creep stages, and the evolution of the internal microstructure of the samples was examined. The experimental results showed that shear microcracks were mainly initiated during the primary creep stage, these microcracks coalescing to form macroshear cracks in the accelerated creep stage. In addition, the changes in porosity exponentially increased with increased deviatoric stress, with small pores developing into medium pores, while the number of large pores was basically stable. [ABSTRACT FROM AUTHOR]

True triaxial experiment with a synchronous acoustic emission (AE) monitoring system is a popular measure for the assessment of strength and cracking behaviors of brittle flawed rocks under anisotropic stress circumstances. This measure was employed to investigate the laboratory-scale brittle failure of flawed sandstone containing two different en echelon flaw configurations. Quantitative interpretations and comparisons of acousto-mechanical properties and cracking behaviors in intact and flawed sandstone have become available. Observations first revealed that en echelon flaws considerably weakened rock strength and deformability, and strongly affected the damage and failure processes. Second, intact specimen showed a quasi-2D spallation failure, while the failure of flawed specimens was dominated by the induced cracks initiated from en echelon flaws. Third, the two fundamental failure types of rock bridge including tensile-dominated failure and shear-dominated failure were registered. Whatever the failure type, micro-shear failure mechanism co-existed with micro-tensile failure mechanism. Moreover, AE amplitude distributions at the stable and unstable cracking phases follow a power law. The current findings provide insights into the brittle failure mechanisms of flawed rocks and have significant implications at scales relevant to seismicity and engineering applications. Highlights: True triaxial experiment investigations of brittle failure in flawed sandstone Quantitative interpretations and comparisons of acousto-mechanical properties and cracking behaviors in intact and flawed sandstone Quasi-2D spallation failure occurs in intact specimen, while the shear cracks initiated from the flaw tips dominate the failure in flawed specimens Co-existences of micro-shear failure mechanism with micro-tensile failure mechanism at the triggering processes of rock fracture AE amplitude distributions at the stable and unstable cracking stages follow a power law [ABSTRACT FROM AUTHOR]

In this paper, the effects of materials and confining pressures on the mechanical properties of flawed coarse and fine sandstone is first analyzed. Then, the evolution processes of spatiotemporal acoustic emission (AE) events of coarse and fine sandstone specimens containing two flaws subjected to biaxial compression are investigated to character the damage characteristics and the fracture mechanism. Third, the ultimate failure modes of coarse sandstone specimens are compared with those of the fine sandstone specimens. Finally, the Electron Probe Microscope Analysis (EPEI) technique is conducted to observe the main macrofracture surfaces of coarse and fine sandstone specimens. Besides, the distribution characteristics of two AE indexes are also employed to qualitatively reveal the failure mechanism of tested specimens. The experimental results can provide new insight into the mechanical responses and the cracking behaviors of flawed rocks in complex stress loading conditions. Highlights: The damage characteristics and the fracture mechanism of coarse and fine sandstone are revealed.The distribution characteristics of two AE indexes are employed to qualitatively reveal the failure mechanism.Electron Probe Microscope Analysis technique is conducted to observe the main macrofracture surfaces of sandstone.The ultimate failure modes of coarse and fine sandstone specimens are investigated. [ABSTRACT FROM AUTHOR]

Creep behavior of red sandstone under different pore pressures with a high temperature and confining pressure condition is studied in this paper. Ultrasonic detection method is used to monitor the variation of S wave in the creep test. The attenuation coefficient and S‐wave velocity in the creep process are analyzed. Fast Fourier transform is used to analyze the variation of amplitude in peak frequency and four main frequencies. The experimental results show that the creep deformation increases with increasing pore pressure for a given deviatoric stress, and the S‐wave velocity and attenuation coefficient decrease with increasing pore pressure. The relationship between creep strain and damage of the sample is established. It is found that the increase of pore pressure accelerates the increased rate of sample damage. Frequency spectrum analysis shows that the generation and development rate of microcracks increases with increasing pore pressure. The research can provide a theoretical basis for mining of deep mineral resources and the structural stability analysis of deep underground engineering. Highlights: Creep behavior of red sandstone under different pore pressures with a high temperature and confining pressure condition is studied.Fast Fourier transform is used to analyze the variation of amplitude in peak frequency and four main frequencies.The S‐wave velocity and attenuation coefficient decrease with increasing pore pressure.The increase of pore pressure accelerates the increased rate of sample damage. [ABSTRACT FROM AUTHOR]

Water existing in micro-pores has tremendous influences on the mechanical and damage characteristics of rocks. This study aims to investigate the micro-pore water effects on the mechanical and damage characteristics of sandstone subjected to different loading configurations, in which four groups of sandstone specimens with different water contents were applied in the conventional uniaxial compression, cyclic loading and unloading experiments. X-ray CT imaging and 3D reconstruction techniques are applied to capture the cracking behaviors under uniaxial compression and cyclic loading–unloading. The digital microstructural variables are defined to describe the characteristics of microstructures and evolution of micro-pore water in rocks. Numerical simulation is applied to study the micro-pore effects on the mechanical properties of rocks. Results show that the specimens are mainly failed by the mixture modes of primary shear cracks and secondary tensile cracks. With increasing mass water content, the uniaxial compressive strength (UCS) decreases and the pore size first decreases, then increases, and finally decreases due to the infiltration of water phase in microstructures. The water content has a negative effect on the mechanical properties, in which the digital porosity and volumetric water content and the micro-pore water saturation have a negative effect on the UCS. In addition, the damage becomes easier near the pores with larger sizes. [ABSTRACT FROM AUTHOR]

Heterogeneity representations of rocks can bring out large errors due to inaccurate partitions of different phases. This study aims to deeply characterize the microscopic heterogeneity in rocks represented by different phases and to study their effects on damage properties of sandstone. X‐ray μ‐CT imaging with low‐high resolution is applied to capture microstructures information. Digital analysis with statistical models is employed to accurately characterize the heterogeneity. The effects of microscopic heterogeneity on the damage properties are analyzed using the numerical simulations and coupling multiple point statistic‐marching cube three‐dimensional reconstruction algorithm. Results show that the microscopic heterogeneity of sandstone can be effectively characterized by Lorentz distribution model, and model with high resolution shows better R2 values. With increasing segmentation accuracy of microscopic phases, micro‐grain‐2 phase plays decisive roles in determining the compressive strength of sandstone. The microscopic samples S1‐S2 are failed by the mixed modes of transgranular and intergranular failures. Plain Language Summary: Digital rock assessment techniques play significant roles in evaluating the petro‐physical properties of different rocks via X‐ray CT imaging. The simulations of the microscopic cracking behaviors are extremely dependent on the multiphase heterogeneity representations of rocks, which can bring out large errors due to inaccurate partitions of different phases. To settle this issue, the digital color difference segmentation algorithm and the statistical Lorentz distribution model for the microscopic element of rocks are applied to accurately represent the heterogeneity of rocks. The effects of microscopic heterogeneity on the damage properties are analyzed using the numerical simulations in sandstone with realistic geometries reconstructed by the coupling multiple point statistic‐marching cube three‐dimensional reconstruction algorithm. We demonstrate that transgranular and intergranular failure behaviors of rocks can be intuitively analyzed. The presented method provides new opportunities to study microscopic damage and mechanical properties of porous rocks. Key Points: Digital analysis with statistical models is employed to accurately characterize the heterogeneity of sandstoneMicroscopic heterogeneity effects on damage properties are analyzed by the numerical simulations and multiple point statistic‐marching cube three‐dimensional reconstruction algorithmMicroscopic transgranular and intergranular cracking behaviors can be intuitively analyzed based on the microscopic element of rocks [ABSTRACT FROM AUTHOR]

In this paper, the creep behavior of sandstone under the different confining pressures is investigated. The nuclear magnetic resonance (NMR) measurements are carried out to detect the micro-pore characteristics of sandstones before and after compression tests and creep tests. The variations of the parameters, including transverse relaxation time (T2) spectra distribution, percentage of the pore distribution and the changes of porosity are analyzed quantificationally. The experimental results show that creep behavior can be regarded as a process of gradual accumulation of damage to rocks. The confining pressure can limit the increase in porosity during the creep stage, and the effect of confining pressure on the incremental value of porosity is more obvious under high-stress conditions than under low-stress conditions. Moreover, the relationship between the creep stain and the incremental value of porosity is established to reveal the relationship between micro-crack expansion of rocks and macro-creep characteristics of sandstone. It is found that, as the loading ratio or confining pressure increases, the variation of the creep strain is similar to the variation of the incremental value of porosity. The larger the confining pressure, the smaller the effects of creep strain on the incremental value of porosity. However, the greater the loading ratio is, the faster the incremental value of porosity increases. The present study can provide a theoretical basis for the deep mining of mineral resources and the structural stability analysis of deep-buried underground engineering. [ABSTRACT FROM AUTHOR]

Summary: X‐ray computed tomography (CT) imaging and digital image correlation techniques are applied to study spatial cracking behaviors of sandstone under uniaxial compression, in which the angle between precracks is 45°, 90°, and 135° and the crack depth is 7.5 mm and 10 mm, respectively. Layered anisotropy damages and spatial cracking evolution are quantitatively analyzed by the defined digital layered anisotropy index and digital damage ratio, respectively. Three cases with different array of precracks evidence the depth effects of precracks on spatial crack propagation. Results show that the failure process of samples is first controlled by the coalescence of surface cracks in 2D space and then the samples are failed by the propagation of coalesced cracks (shear cracks with different shapes). The crack types for samples with precrack depth of 7.5 mm are all shear cracks for Cases 1‑3. Nevertheless, the crack types for samples with precrack depth of 10 mm are, respectively, the half X‐shape crack for Case 1, X‐shape crack for Case 2, and double shell crack for Case 3. The precrack has a significant promotion effect on the failure process when the angle between the two precracks is β = 90°, and the precrack has little to no effect on the failure process when the angle between the two precracks is β = 135°. As the depth of precrack increases to 10 mm, the crack types are changed in this study. The peak strength of sample subjected to uniaxial compression decreases with increasing depth of precracks, implying the decrease of the rock strength by the discontinuity. [ABSTRACT FROM AUTHOR]

In this paper, uniaxial compression tests are conducted on fissured red sandstone specimens to predict fracture damage (large‐scale events). The acoustic mission (AE) coupled with digital image correlation (DIC) technologies are used to monitor and record the real‐time cracking process of tested specimens. The AE characteristics are analysed during the cracking process. Moreover, three types of b‐value methods based on the AE parameters are adopted to predict the occurrence of large‐scale events (macro‐cracking). The results show that every macro‐cracking leads to a rapid decrease in three types of b value. When the fissured specimens reach to ultimate failure, all three types of b value reach to the minimum. The b value based on the AE parameters can be used as a predictor of large‐scale events during the cracking process of fissured rocks. [ABSTRACT FROM AUTHOR]

Summary: An integrated approach of improved pseudo color image enhancement (IPCE) method and three‐dimensional (3D) reconstruction technology is proposed. The evolution characteristics of two‐dimensional (2D) and 3D spatial cracks in X‐ray computed tomography (CT) images of sandstones subjected to triaxial compression with confining pressure of 10, 20, and 30 MPa are investigated. The equations of crack width, length, and dip angle are established based on the proposed digital damage ratio. Based on the pseudo color images, point cloud maps with roughness and spatial images of cracks are investigated. The numerical results show that this proposed method is effective to study and understand the fracturing properties of rocks. [ABSTRACT FROM AUTHOR]

The irregularity and multi-scale features are the most important and intrinsic characteristics for rocks, which are caused by the variety of microstructures. In this paper, a novel reconstruction method is proposed to reconstruct a porous rock model based on limited morphological information. The configurations of the pore structures in Xingluokeng sandstone are investigated by the X-ray Micro-CT observation. Then, a triaxial compressive test is conducted to obtain necessary mechanical properties. Compared with the traditional reconstruction method, the novel reconstruction method adopts the hierarchical annealing algorithm and gains considerable computational efficiency. Moreover, considering the irregularity and multi-scale features of rocks, the fractal function variables over time, and the local histogram function are introduced to describe these characteristics. The finite element-smooth particle dynamic coupling method is employed to simulate the mechanical responses and fractures of the reconstructed model under compression. The topological verification and comparison between the simulation results and experimental data show that the novel reconstruction method can predict the large scale properties of rocks with limited experimental results, which indicate the novel reconstruction method has a important application in the prediction of geological conditions for engineering project. • A novel hierarchical annealing algorithm is proposed to reconstruct the rock model. • A local histogram function and a fractal function considering time are introduced. • The irregularity and multi-scale features of rocks are characterized in the reconstruction model. [ABSTRACT FROM AUTHOR]

This paper investigates the effects of freeze‐thaw (FT) fatigue damage on the cracking behaviors of sandstone specimens containing two unparallel fissures under uniaxial compression. First, the effects of FT fatigue damage and fissure angle on the mechanical properties of sandstone specimens are analyzed. Second, the real‐time cracking process of sandstone specimens is captured by a high‐speed digital video camera system. Seven crack coalescence patterns are observed in this experiment. Local strong fatigue‐damaged zones, which are visualized as white zones, are observed in the specimens subjected to FT cycles during loading. Finally, scanning electron microscopy (SEM) observations show that the local strong fatigue‐damaged zones mainly consisted of microcracks and micropores induced by the FT fatigue damage. These experimental results are helpful for improving the understanding of the cracking process in cold‐region engineering. [ABSTRACT FROM AUTHOR]

Abstract Three-dimensional reconstruction technique plays a key role in understanding the characteristics of geomaterial pore structures through computed tomography images. In this paper, multiple-point statistics and a marching cube algorithm are integrated to reconstruct three-dimensional model to extract the pore structure, analyze connectivity and predict the permeability of the pore structure. This method is applicable not only to two-dimensional images to determine simple characteristics of geomaterials, but also to better understand inner pore structure via the reconstructed three-dimensional models. The results obtained by the proposed reconstruction method agree well with those obtained from the previous methods. Highlights • An integrate method for 3D reconstruction model of porous geomaterials is developed. • A struct array is proposed to store pattern library. • A compare study of simulating images based on struct array and tree structure is conducted. • The 3D model of sandstone is reconstructed, and the pore structure is extracted. • The permeability and coordination number are calculated and confirmed. [ABSTRACT FROM AUTHOR]

A novel reconstruction method is proposed to reconstruct porous rock model based on limited morphological information from experiments. X-ray Micro-CT observations and triaxial compressive tests are initially conducted to obtain the raw CT images and necessary mechanical parameters. Then, a hierarchical annealing algorithm is proposed to gain large profits in computational efficiency. Moreover, the fractal system control function with time variables and local histogram system control function are introduced to characterize the irregular and multi-scale features in rocks. Finally, the comparisons between the numerical results and experimental data show that the proposed novel reconstruction method has a powerful capacity to predict the properties of rocks for engineering purposes. [ABSTRACT FROM AUTHOR]

In this paper, a novel geometrical reconstruction model is proposed for numerical simulation of pore structure in rocks. The spatial configuration and geometrical information of the microstructure are firstly gathered from Chongqing sandstone by X-ray micro-CT observations, and experimental tests on Chongqing sandstone are applied to obtain the mechanical parameters. Then, on the basis of the gathered geometrical information, the surface geometrical model is established by using the modified Marching cubes algorithm. Next, the established surface geometrical model is extended to 3D tetrahedral model by using Delaunay refinement algorithm. Finally, the 3D tetrahedral model is transformed into finite element (FE) numerical model. Moreover, the relationship between the porosity, fractal dimension of pore structure and mechanical properties of Chongqing sandstone is also investigated. The comparison of the numerical results and the experimental data shows a good consistency. [ABSTRACT FROM AUTHOR]

This paper experimentally evaluates and quantitatively compares stress-induced damage progression and acoustic emission (AE) in granite and sandstone based on continuous acousto-optic-mechanical (AOM) observations. Experimental results reveal that the step-rise characteristics of the AE event rate preceding the entire macrofracturing process and the apparent white patching phenomena are the most significant acousto-optical evidences for process zone nucleation in granite but not in sandstone. During the unstable crack growth stage in granite, the robustly high AE event rate level has been mechanically correlated with the reactivation and intensification of fracture process zones (FPZs) by the stress build-up, and the relatively low AE event rate level has been revealed as a result of the progression of FPZs into macrocracks. It is the first time that the process zone nucleation in granite has been mechanistically related to the clustering of three types of grain-scale microcracks, i.e., grain boundary cracks, intragranular cracks and transgranular cracks, due to the grain interlocking effect. Comparatively, the clustering of microcracks is insignificant in the brittle failure of sandstone. Mechanistic correlations among the AOM characteristics in the two rock types are also investigated in detail. [ABSTRACT FROM AUTHOR]

• True triaxial compression tests are conducted on coarse and fine sandstone. • The coupled effects of the microstructure and the pre-existing flaws are revealed. • Mechanisms of micro-shear cracking and micro-tensile cracking are revealed. • The micro-shear cracking is seldom generated in fine sandstone. Failure characteristics of flawed rocks subjected to true triaxial stress are not studied, and the cracking mechanism is not fully understood. Especially, the damage and failure mechanisms incorporating the effect of material properties such as grain diameter are seldom revealed. In this study, true triaxial compression tests are conducted on flaws-contained specimens of coarse and fine sandstone, to explore the coupled effects of the microstructure and the pre-existing flaws on cracking characteristics and further to reveal the cracking mechanism. Acoustic emission (AE) data and the mechanical data are the main experimental output. It is found that cracking characteristics are collectively affected by the microstructure features and the geometrical features of specimen-sized pre-existing flaws. When the confining pressure is relatively large, the mechanical interaction between coarse grains inhibits the specimen's rupture and contributes to the increase of the strength of coarse sandstone. Due to the complex interaction between grain-scale microcracks, the micro-shear cracking mechanism and the micro-tensile cracking mechanism are presented in the physical process of damage progression in coarse sandstone. However, the microcrack clustering phenomenon in fine sandstone is insignificant, and the interaction between grain-scale microcracks is weak. Accordingly, the micro-shear cracking is seldom generated in fine sandstone. [ABSTRACT FROM AUTHOR]

• Acoustic emission (AE) and photographic technologies are employed. • The dominant frequency and amplitude characteristics of rocks are investigated. • Four modes of AE signals are found during the cracking process of flawed rocks. • The fracture mechanism of flawed rocks is discussed. To study the time-varying dominant frequency distribution characteristics during the cracking process of intact and flawed rocks, uniaxial compression tests are carried out on three types of red sandstone specimens. Both acoustic emission (AE) and photographic technologies are performed to monitor the real-time cracking process of tested specimens. The relationship between the AE rate curve, the axial stress-time curve and the cracking process is first analyzed, which can be employed to evaluate the microscopic and macroscopic deformation characteristics of rocks. The time-varying dominant frequency and amplitude distribution characteristics in different cracking stages of tested rocks are investigated. Four modes of AE signals, i.e., AE signals with low-frequency and high-amplitude (LF-HA), AE signals with low-frequency and low-amplitude (LF-LA), AE signals with intermediate-frequency and low-amplitude (IF-LA) and AE signals with high-frequency and low-amplitude (HF-LA), are found. The coupling application of four modes of AE signals extracted from unstable deformation stages can be viewed as early warning for unstable failure of rocks. The fracture mechanism of intact and flawed rocks is discussed based on the statistical characteristics of dominant frequency during the cracking process. Micro-shear cracking mainly dominants the ultimate failure of all of tested rocks. [ABSTRACT FROM AUTHOR]

Understanding the evolution of microstructures in freeze-thaw (FT) cycles is important to reveal the mechanisms of frost damage in porous rocks. This paper investigated the effect of microstructural evolution on the permeability of sandstone under FT cycles as well as the evolution pattern of microstructures. First, scanning electron microscopy (SEM) tests were carried out to investigate the evolution of sandstone microstructures at the same location under FT cycles. The experimental results showed that the FT cycles obviously caused the spalling of grains of the sample. Second, nuclear magnetic resonance (NMR) tests were conducted to investigate the evolution of the pore size distribution (PSD) and the permeability of sandstone samples under FT cycles. The experimental results showed that the FT cycles had obvious effects on pores with relaxation times varying from 1 ms to 100 ms. As the number of FT cycles increased, the percentage of nanoscale throats decreased, while the percentage of throats in the submicron and micron sizes increased. Third, the predictive model of permeability coefficient, dry density, porosity, surface porosity, surface fractal dimension, median pore size and median throat size was developed using a back propagation neural network (BPNN). Fourth, relative importance analysis of the above parameters was carried out, and the contribution of each parameter to the permeability coefficient of the sample was determined. Finally, the mechanism of FT damage was investigated. The present experimental results are helpful in improving the understanding of the leakage mechanisms of rock engineering in cold regions. • The evolution pattern of sandstone microstructures under freeze-thaw (FT) cycles is investigated. • The pore size distribution and permeability of sandstone samples under FT cycles are studied. • The relative importance analysis of parameters is carried out by Back Propagation Neural Network. • The effect of microstructural evolution on permeability of sandstone under FT cycles is discussed. [ABSTRACT FROM AUTHOR]

• The real-time fracture process of fissured red sandstone specimen is monitored. • Two indices of RA and AF are performed to classify the different fracture modes. • The adequacy of the fracture mode classification is evaluated by Kernel Density Estimation (KDE). • KDE can identify and visualise the high concentration regions of RA and AF values. • RA values can serve as an early warning for ultimate failure of rocks. To study the classification of fracture modes of rocks during the cracking process, uniaxial compression tests were conducted on intact and flawed red sandstone specimens. Meanwhile, both acoustic emission (AE) and digital image correlation (DIC) technologies were adopted to monitor and record the real-time cracking process of the specimens tested. In this study, the interevent time function F (τ) (AE events rate) was utilized to distinguish the transition from microcracking to macrocracking for the tested specimens. An AE parameter analysis method based on two indices of RA (rise time/amplitude) and AF (AE counts/duration) values was performed to classify the different cracking modes during the loading process. The classification results of the cracking modes coincide with the cracking type of macrocracks captured by the photographic system. Moreover, the adequacy of the crack classification was also evaluated by the kernel density estimation (KDE) function, a nonparametric density estimation method. KDE is used as a parametric model to overcome the randomness found in the data set generated by AE testing and can well identify and visualize the high concentration regions of RA and AF values. A continuous increase of RA values (more than 400 ms/v) can serve as an early warning for the ultimate failure of red sandstone. The results of this present investigation can be applied in the health monitoring of rock engineering. [ABSTRACT FROM AUTHOR]

• Analyze the effect of fissure inclinations on the macro-and meso-mechanical properties of sandstone under uniaxial compression. • Comparing the energy evolution characteristics of sandstone containing different fissure inclinations under uniaxial compression. • Investigate the failure process of fractured sandstones under uniaxial compression by the evolution of microcracks. For the rocky slope with potential sliding hazard, there are a larger number of flaws such as fractures and joints, which greatly affect the stability and safety of the slope. Thus, it is important and relevant to the behavior of fractured rock from a macroscopic and mesoscopic perspective by experiments and numerical simulation. In this paper, the mechanical properties and crack evolution behavior of grey sandstone containing two parallel fissures under the uniaxial compression tests were evaluated by experimental and bond-based Peridynamics (BB-PD) simulation. The results show that with the increase of fissure dip angle, the peak strength, elastic modulus and Poisson's ratio of pre-cracked sandstone samples under uniaxial compression condition gradually increase. At the same time, the total energy, dissipated energy and elastic peak energy of fractured sandstone also increase with the increase of fissure dip angle. In order to validate the experimental findings, a set of calibration parameters were used to model the intact samples using BB-PD. The numerical simulation reproduced the stress–strain curves, mechanical parameters and failure modes of double-fractured sandstone. Based on the results of indoor experiments and numerical simulations, it is concluded that the failure mode of double-fractured sandstone has three types under uniaxial compression: tensile failure, shear failure and conjugate shear failure. As the fissure dip angle increases, the main control factor for failure of sample gradually changes from rock bridge failure to secondary crack coalescence, and the stress distribution of fractured sandstone gradually changes from high stress concentration in the rock bridge zone to uniform stress distribution as a whole. [ABSTRACT FROM AUTHOR]