[Show abstract][Hide abstract] ABSTRACT: Stress field is one of the primary factors influencing crack growth in rock. Pores significantly affect the distribution features of stress fields in porous rock. Based on the numerical reconstruction of the porous sandstone, we simulate the crack growth in porous sandstone subjected to splitting loads. The influences of the pores on the stress field during crack growth are analyzed. The distribution features of the stress σy and the evolution of the principal stress σ1 are disclosed in porous rock. The effects of the stress distribution on the crack growth behavior are discussed. It is shown that during crack growth the maximum value of the principal stress σ1 gradually increases for all of the porous models. The maximum values of tensile stress of σy decrease linearly with porosity increasing. This study presents a way to understand and characterize the distribution features of the stress field and the influences on the crack growth in porous rock.
[Show abstract][Hide abstract] ABSTRACT: This paper presents a numerical and experimental investigation on the effects of pore structures on the static mechanical properties of porous sandstone. Three-dimensional (3D) numerical models of porous sandstone with different statistical pore parameters are constructed using a software program and the experimentally acquired data of the parameters. The mechanical properties of sandstone are numerically tested using Brazilian disc split tests to probe the influences of pore structures. A number of physical models having similar statistical characteristics of pores and physical properties with those of the numerical models are produced using modeling materials. Brazilian disc tests and computer tomography tests are performed on the physical specimens with different porosities to clarify the responses of pore structures during failure processes. The numerical analysis implies that pores significantly affect the mechanical properties of porous sandstone such as stress concentration, distribution, and the connection of the failed material elements. It is shown that the statistical distribution of pore radii presents a certain degree of influence on the split failure behavior of porous rock, which is closely related to rock porosity. However, distribution of spatial location of pores has negligible influence on stress distribution, failure mechanisms, and the split tensile strength of the porous media. A porosity of 15% seems to be a threshold porosity, above which the effects of geometrical and statistical characteristics of pore structures on the split mechanical properties of porous rock become significant. The laboratory test verifies that the developed physical models have consistent geometric and statistical characteristics of pores with those of the real sandstone. The measured split mechanical properties of the physical models present good agreement with the predictions of numerical simulations. (C) 2013 American Society of Civil Engineers.
Full-text · Article · Oct 2013 · Journal of Geotechnical and Geoenvironmental Engineering
[Show abstract][Hide abstract] ABSTRACT: The structure of fractures in nature rock appears irregular and induces complicated seepage flow behavior. The mechanism and quantitative description of fluid flow through rock fractures is a difficult subject that has been greatly concerned in the fields of geotechnical, mining, geological, and petroleum engineering. In order to probe the mechanism of fluid flow and the effects of rough structures, we conducted a few laboratory tests of fluid flow through single rough fractures, in which the Weierstrass-Mandelbrot fractal function and PMMA material were employed to produce the fracture models with various fractal roughnesses. A high-speed video camera was employed to record the fluid flow through the entire single rough fracture with a constant hydraulic pressure. The properties of fluid flow varying with the fracture roughness and the influences of the rough structure were analyzed. The components of flow resistance of a single rough fracture were discussed. A fractal model was proposed to relate the fluid resistance to the fracture roughness. A fractal equivalent permeability coefficient of a single rough fracture was formulated. This study aims to provide an experimental basis and reference for better understanding and quantitatively relating the fluid flow properties to the structures of rock fractures.
Full-text · Article · Aug 2013 · Science China Technological Sciences
[Show abstract][Hide abstract] ABSTRACT: The development history and current state of studies on the characteristics and mechanisms of deformation and failure of rock materials were briefly reviewed from the viewpoint of energy. The main scope and the achievable objectives of the energy-based research system were expatiated. It was validated by experiments that the damage process of rocks can be well described by the rock damage evolution equation established based on energy dissipation. It was found from the uniaxial compression and biaxial compression tests that only a small proportion of the total input energy in hard rocks is dissipated before peak load and a large proportion in soft rocks is dissipated before peak load. For both hard and soft rocks, the energy dissipated after peak load accounts for a greater proportion. More energy would be required for rock failure under equal biaxial compression than under unequal biaxial compression. The total absorbed energy is different for rock failure under high-rate loading and low-rate loading. More fragmented failure pattern usually corresponds to higher energy absorption. The mesoscopic analysis on the damage and failure of bedded salt rocks showed that the energy dissipation is prominent and the total absorbed energy for rock failure is low when cracks propagate in the weak mud interlayer while it is contrary when cracks propagate in the salt rock. The energy accumulation, transfer, dissipation and release during the failure process of tunnel with impending failure under disturbance were analyzed theoretically based on the elastoplastic mechanics theory. Furthermore, the spatial distribution of energy dissipation and energy release of fractured rocks under unloading was simulated numerically. It was demonstrated that energy is likely to be released from the weakest surface under compression, which triggers the global failure of rocks.
No preview · Article · Dec 2011 · Science China Technological Sciences
[Show abstract][Hide abstract] ABSTRACT: The characterization of pore structure in rocks is relevant in determining their various mechanical behaviors. Digital image
processing methods integrated with fractal theory were applied to analyze images of rock slices obtained from industry CT,
elucidating the characteristics of rock pore structure and the relationship between porosity and fractal dimensions. The gray
values of pixels in CT images of rocks provide comprehensive results with respect to the attenuation coefficients of various
materials in corresponding rock elements, and these values also reflect the effect of rock porosity at various scales. A segmentation
threshold can be determined by inverse analysis based on the pore ratios that are measured experimentally, and subsequently
binary images of rock pores can be obtained to study their topological structures. The fractal dimension of rock pore structure
increases with an increase in rock pore ratio, and fractal dimensions might differ even if pore ratios are the same. The more
complex the structure of a rock, the larger the fractal dimension becomes. The experimental studies have validated that fractal
dimension calculated directly from gray CT images of rocks can give an effective complementary parameter to use alongside
pore ratios and they can suitably represent the fractal characteristics of rock pores.
Keywordsrock–pore–CT–fractal dimension–image processing
Full-text · Article · Nov 2011 · Chinese Science Bulletin
[Show abstract][Hide abstract] ABSTRACT: The pore characteristics, mineral compositions, physical and mechanical properties of the subarkose sandstones were acquired
by means of CT scan, X-ray diffraction and physical tests. A few physical models possessing the same pore characteristics
and matrix properties but different porosities compared to the natural sandstones were developed. The 3D finite element models
of the rock media with varied porosities were established based on the CT image processing of the physical models and the
MIMICS software platform. The failure processes of the porous rock media loaded by the split Hopkinson pressure bar (SHPB)
were simulated by satisfying the elastic wave propagation theory. The dynamic responses, stress transition, deformation and
failure mechanisms of the porous rock media subjected to the wave stresses were analyzed. It is shown that an explicit and
quantitative analysis of the stress, strain and deformation and failure mechanisms of porous rocks under the wave stresses
can be achieved by using the developed 3D finite element models. With applied wave stresses of certain amplitude and velocity,
no evident pore deformation was observed for the rock media with a porosity less than 15%. The deformation is dominantly the
combination of microplasticity (shear strain), cracking (tensile strain) of matrix and coalescence of the cracked regions
around pores. Shear stresses lead to microplasticity, while tensile stresses result in cracking of the matrix. Cracking and
coalescence of the matrix elements in the neighborhood of pores resulted from the high transverse tensile stress or tensile
strain which exceeded the threshold values. The simulation results of stress wave propagation, deformation and failure mechanisms
and energy dissipation in porous rock media were in good agreement with the physical tests. The present study provides a reference
for analyzing the intrinsic mechanisms of the complex dynamic response, stress transit mode, deformation and failure mechanisms
and the disaster mechanisms of rock media.
Keywordsporous media-three-dimensional finite element model-rock media-stress wave-failure mechanism-energy dissipation
[Show abstract][Hide abstract] ABSTRACT: A number of porous models having the similar statistical characteristics of pores and physical properties with natural sandstones have been produced using reactive powder concrete (RPC) and polystyrenes. Spit-Hopkinson-Pressure -Bar tests and CT scans have been carried out on the models with the various porosities to probe the performance of wave propagations and the responses of pores and the matrix during wave propagations. It is shown that porosities significantly influence wave propagations. For an identical impact strain rate, the greater the porosity is, the larger the amplitude of the reflected wave appears, the more the peak in the reflected wave presents, and the smaller the amplitude of the transmitted wave turns out. A single peak emerges in the reflected wave when the porosity falls down to 5%. The larger the impact strain rate, the much remarkable the phenomena. The energy-dissipated ratio of porous models, i.e., W
, linearly increases with the increment of porosities. The ratio is sensitive to the impact strain rate. Differences in the performance of wave propagations and energy dissipation result from the varied mechanisms that pores response to impacts. For the porosity less than 10%, the mechanism appears to be a process fracturing the matrix to generate new surfaces or pores. Energy has primarily been dissipated in creating new surfaces or pores. No apparent pore deformation takes place. The impact strain rate takes little effect on pore geometry. For the porosity of 15% or more, the mechanism works depending on the impact strain rate. When a low impact strain rate applies, the mechanism still appears to crack the matrix to generate surfaces or pores, but the amount is lower as compared to the case with a low porosity. If a large impact stain rate applies, the mechanism combines both fracturing the matrix and deforming the pores, with the deforming pores predominating. The vast majority of energy has been dissipated to deform pores. Only high porosity and impact strain rate can bring significant deformation to the pores. The proposed eccentricity of pores is capable of characterizing the geometry of pores and its change during wave propagations.
No preview · Article · May 2009 · Science in China Series E Technological Sciences
[Show abstract][Hide abstract] ABSTRACT: The geometric features and the distribution properties of pores in rocks were investigated by means of CT scanning tests of
sandstones. The centroidal coordinates of pores, the statistic characterristics of pore distance, quantity, size and their
probability density functions were formulated in this paper. The Monte Carlo method and the random number generating algorithm
were employed to generate two series of random numbers with the desired statistic characteristics and probability density
functions upon which the random distribution of pore position, distance and quantity were determined. A three-dimensional
porous structural model of sandstone was constructed based on the FLAC3D program and the information of the pore position and distribution that the series of random numbers defined. On the basis
of modelling, the Brazil split tests of rock discs were carried out to examine the stress distribution, the pattern of element
failure and the inosculation offailed elements. The simulation indicated that the proposed model was consistent with the realistic
porous structure of rock in terms of their statistic properties of pores and geometric similarity. The built-up model disclosed
the influence of pores on the stress distribution, failure mode of material elements and the inosculation of failed elements.
No preview · Article · Nov 2008 · Science in China Series E Technological Sciences