Search

In this work, the color difference phase separation (CDPS) approach is proposed to segment the solid and pore phases. Pore‐scale variables are defined to describe the microstructural characteristics. Novel relations to quickly determine the deformation moduli and thermal conductivity are established. Digital–virtual modeling to investigate the mechanical and thermal properties is addressed. The correlation between pore‐scale variables and the mechanical and thermal properties is investigated. Results show that the calculated shear and Young's moduli decrease and the computed Poisson's ratio increases as porosity increases. The deformation moduli and thermal conductivity increase with increasing pore radius for different types of rocks. Excellent consistencies are found between the digital–virtual and realistic experimental results. The proposed method provides useful tools to fast and accurately determine the deformation moduli and thermal conductivity, which is helpful for the underground space and deep energy resource explorations. Highlights: Multiphase structure is segmented without overestimation or underestimation.Micro‐deformation moduli and thermal conductivity of rocks are fast determined.Thermo‐mechanical properties are effectively validated in microstructures of rocks. [ABSTRACT FROM AUTHOR]

Understanding the 3D spatial cracking behavior plays significant role in evaluating the mechanical properties of rocks. This work studied the spatial propagations of 3D cracks and its effects on the mechanical properties during the failure process of sandstone under uniaxial compression using X-ray CT imaging and 3D reconstruction techniques. Results show that five cracking stages can be identified including the initial flaw compaction stage, linear elastic deformation stage, microcrack propagation and coalescence to macrocrack formation stage, stable propagation stage of macrocracks and the sample failure stage. As cracking direction increases from 0° to 90°, peak stress decreases, microcrack initiation stress increases for sample S1 and decreases for samples S2–S3, which is opposite to macrocrack initiation stress of cracks. However, these variations of microcrack initiation stress, macrocrack initiation stress and peak stress are opposite as cracking direction increases from 180° to 270°. The proposed relation between the stress and normalized damage volume is effective to evaluate the stress during the failure process of sandstone. Highlights: Digital framework is proposed to characterize real-time 3D spatial crack characteristics. 3D cracking behavior is studied by digital damage ratio and normalized damage volume. A stress-damage volume relation is established to evaluate the stress during the failure process of rocks. Effects of spatial cracking direction on the failure process and mechanical properties are analyzed. [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]

Understanding the mechanical properties plays pivotal roles in rock engineering. This work aims to establish novel relations linking the porosity, ultrasonic wave and fluid saturation to estimate the uniaxial compressive strength (UCS) fast and simply. The uniaxial compressive coupled with ultrasonic wave tests on sandstone samples are carried out to obtain the datasets of the UCS, P-wave and S-wave velocities. X-ray CT imaging technique is employed to capture the microstructure information. The color difference phase separation approach to segment the pore, water and solid phases is proposed, and pore-scale variables to describe the microstructure characteristics are defined. Novel relations to determine the micro velocities of P-wave and S-wave are established, and the modulus of deformation and the physical properties of rocks are evaluated. Novel relation to determine the UCS is established and validated by the real and virtual experiment datasets. Results show that the UCS, P-wave and S-wave velocities computed by the proposed method decrease with increasing fluid saturation. The errors between the calculated and experimental UCS, P-wave and S-wave velocities are all < 5%, showing excellent consistency with each other. The proposed method is effective to estimate the mechanical properties fast and accurately, simplifying the estimation of the UCS in rock 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]

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]

Cracking is an important phenomenon in the failure of geomaterials. The prediction of crack paths is difficult and challenging because of the randomness and uncertainty in the cracking behaviors of geomaterials. In this paper, to predict crack paths based on 2D X‐ray computed tomography (CT) images, a digital energy grade‐based approach is proposed, and the corresponding energy principle is established. Excellent consistency of crack paths is found between the predicted crack path and the real crack path of the specimen. The numerical results indicate that the proposed approach provides a useful way to predict cracking paths in geomaterials. Meanwhile, the stress and displacement fields before and after the specimen fails can be obtained by combining the proposed method with finite element (FE) analysis. In total, this proposed method can be applied not only to monitoring the health of but also to the stability analysis of engineering structures during engineering activities. [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]

Characterizing and predicting 3D microscopic fractures are rarely reported due to the challengeable difficulty to determine its spatial trajectories. In this study, a double threshold segmentation algorithm is proposed to obtain 3D microstructure models of porous geomaterials by X-ray computing tomography (CT) images. A novel voxel-particle energy (VPE) approach is proposed to predict 3D fracture characteristics, and fracture permeability modeling is conducted. Results show that excellent agreements are found between the experimental and numerical results, showing the accuracy of proposed VPE method. As dynamic viscosity increases, microscopic fracture permeability increases. Microscopic fracture permeability decreases with increasing input pressure, and it increases with increasing voxel scale. The proposed VPE method provides an effective tool to predict 3D cracks and its spatial trajectories, which is helpful for the deep resource and underground space development. • A digital analysis framework is proposed to predict 3D fracture geometry. • Voxel-particle energy method can automatically extract 3D fracture morphology. • Voxel scale effects on 3D fracture morphology prediction are analyzed. • Microscopic fracture permeability modeling is analyzed. [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 continuous reconstruction method is proposed to generate a numerical porous rock model that is statistically equivalent to natural rock. First, the spatial and mechanical properties of pore microstructures in rocks are obtained by computed tomography (CT) and triaxial compression tests. Then, the reconstructed rock models are generated based on the novel continuous reconstruction method. The generated rock models preserve the interest characteristics of the microstructures in rock. Compared with the exchange of discrete and isolated elements in our previous contributions, the microstructures are directly continuously reconstructed to form the reconstruction model. With the continuous approach, the natural rock sample can be efficiently represented by the reconstruction model on a large scale. We also consider reconstructing the anisotropic microstructures by different spatial correlation functions, such as the two-point probability function (TPPF), the lineal-path function (LPF), the fractal function (FF), and the two-point cluster function (TPCF). In addition, we introduce the percolating cluster volume (PCV) to improve the characterization capabilities of the proposed reconstruction method. The comparisons among the reference model, reconstructed model and rock samples show that the proposed continuous reconstruction method is able to reconstruct porous rocks while preserving mechanical and topological properties. [ABSTRACT FROM AUTHOR]