Narrow Results

An in-depth understanding of the deterioration characteristics of porous rock materials in freeze–thaw (F–T) environments is very important for rock mass engineering in cold regions. However, quantitative descriptions of key rock indicators such as porosity, permeability, and anisotropy are lacking. In this paper, computed tomography (CT) was used to study saturated intact sandstone, saturated fractured sandstone, and ice-filled fractured sandstone under various F–T cycles and stress states. Meso-structural parameters were obtained by reconstructing the three-dimensional fracture networks from CT images. Then, based on fractal geometry theory, the fractal dimension ($D_F)$, tortuosity fractal dimension ($D_T)$, and anisotropic two-dimensional fractal dimension ($D_A)$ of the sandstone samples were analyzed quantitatively. The $D_F$ gradually increased during the F–T process, while $D_T$ gradually decreased. Compared with $D_F$, $D_T$ was found to describe changes in the absolute permeability of rocks under F–T cycling more accurately. Anisotropy in sandstone was enhanced by F–T cycling. After uniaxial compression, the $D_A$ value was the greatest in ice-filled fractured sandstone. In addition, the tree-like fracture structure produced by F–T cycling expanded the range of self-similarity, which enhanced the fractal characteristics of sandstone. However, due to the large frost heave pressure of ice-filled sandstone, fracture expansion accelerated in the later period of F–T cycling, which destroyed the self-similarity. These results assist in understanding the F–T characteristics of porous rock materials. The method described provides a new way to better evaluate and predict F–T-related engineering disasters.

This paper proposes a method for the fractal characterization on fracture volume in coal based on CT scanning experiment, and derives the relation among the fractal dimension $D$, fracture volume and length scale. The principle and methodology were deduced in detail, and the rationality was examined by the classical Mandelbrot fractal equation on fractal structure in porous media. The results demonstrate fracture volume is the comprehensive embodiment of the height, length, and aperture of fracture. Consequently, the estimated fractal dimension $D$ can more comprehensively symbolize the fractal characteristic on fracture volume of coal in three-dimensional space than the fractal dimension for fracture number. Therefore, it is worth to be further studied on establishing the analytical equation for fluids transport characteristic parameters in porous media based on $D$.

The pores and fissures in loaded rock masses are the main channels for underground flow, and may cause serious accidents during the development of groundwater resources. This work presents an efficient method for analyzing the microstructure of the loaded rock mass using fractal theory and computed tomography (CT) scanning. A relation between the microstructure features of the sandstone porosity, fractal dimension, and loading stress is developed using an image identification technique. The results demonstrate that the distribution trends of sandstone samples’ slice porosities in the xz- and yz-directions are nearly identical, and the distribution in the xy-direction differs significantly from those in xz- and yz-directions. The total and connected porosities increase with the increase of stress, and the change can be fitted to straight lines. The fractal dimensions of the pores change significantly with stress or loading stress in the xy-direction.

This paper proposed a method for the fractal characterization of the three-dimensional (3D) fracture tortuosity ($D_{T3})$ in coal based on CT scanning experiment. The methodology was deduced in detail, and the values of $D_{T3}$ of four coal samples were calculated by the rigorous derivation equation established by Feng and Yu. The values of $D_{T3}$ by the proposed method fit the relation of $D_{T3}$ versus the fractal dimension for 3D fracture number $D_{f3}$, and the relation of $D_{T3}$ versus the 3D fracture porosity, indicating the rationality and accuracy of the proposed method on estimation of the $D_{T3}$. The results show that the proposed $D_{T3}$ can comprehensively character the fractal characteristics of fractures tortuosity in 3D space. It is worth to further study for establishing an analytical fractal equation for fluid mass transfer in 3D fractures of porous media based on the $D_{T3}$.

CAD reconstruction of anatomical regions from computerized tomography (CT) scans is a very common approach in orthopaedic biomechanics. The CAD model is discretized into finite volume sub-domains and finite element (FE) analyses are performed in order to predict the distribution of stresses generated by applied loads. However, quality and reliability of numerical results depend on the level of accuracy reached in the meshing process. This paper analyzes some critical parameters that may affect the overall efficiency of the CT–FEM transformation process: scan threshold range, object size, and complexity. An optimization procedure for minimizing geometric errors on size and shape of reconstructed objects is presented. Finally, accuracy of stress predictions is evaluated for FE models that include known amounts of geometric errors. Compression and bending loads are considered. Results show that geometric and stress errors rapidly decrease as the objects to be reconstructed become larger in size. Optimal threshold ranges can be identified clearly for both an epoxy-resin benchmark model and a real bone specimen cut from a human lumbar vertebra. This allows geometric errors to be reduced significantly.