Mesostructure Characteristics Research Articles

Study on the characterization of rock heterogeneity and simulation of fracture process based on the mesoscopic structure

This study aims to explore the effect of rock heterogeneity caused by meso-structure differences on the macro-mechanical properties and fracture evolution process. With the help of computed tomography, the meso-structure characteristics of coarse-grained red sandstone are obtained, starting from the meso-structure level. The digital characterization of heterogeneous rocks is completed with the use of image-processing methods, and a rock equivalent crystalline model is constructed. A numerical simulation study is performed with the particle flow PFC3D 5.0 program. The results show that (1) the main form of rock heterogeneity is a meso-structure difference, and this determines the rock’s macro-mechanical properties and failure characteristics. The meso-structure difference mainly plays a role in the post-peak damage weakening stage of rock, and the slight difference leads to damage in different development modes. (2) The crack initiation starts at the cemented and crystalline grid surfaces, which is randomly distributed. The crack evolution process is selective and affected by the types and distribution of the surrounding mineral particles. The development of cracks is dominated by secondary cracks, distributed in strips, in the area of mineral particles with strong mechanical properties. In contrast, cracks are densely developed and spread around in a cluster in the area of mineral particles with weak mechanical properties.

Accuracy Improvement for Two-Dimensional Finite-Element Modeling while Considering Asphalt Mixture Meso-Structure Characteristics in Indirect Tensile Test Simulation

AbstractA two-dimensional finite-element simulation consumes less computational resources and has higher efficiency than a three-dimensional simulation. However, the existence of great differences ...

Experimental study on deformation characteristics of rock and coal under stress-fluid interactions in coal mine underground water reservoir

ABSTRACT Deformation response of rock and coal under long-term stress-fluid interactions not only directly affects the storage capacity of coal mine underground water reservoir but also seriously threatens its stability. In this paper, sandstone, mudstone, and coal fragments collected from the goaf of panel 22616 proposed underground reservoir in Daliuta coal mine were prepared to conduct experiments and to measure rock and coal bulking and compaction properties under stress-fluid interactions. Results show that the bulking factor of sandstone, mudstone, and coal fragments at loose state in order is 1.587–1.828, 1.561–1.787, and 1.435–1.753, respectively. The bulking factor of rock and coal fragments increases with the size of lumps, and that of mixed lumps is between the values of different single lumps. In addition, the bulking factor of same lumpiness rock and coal fragments with different strength is evidently different: the bulking factor of coal is the smallest, followed by that of mudstone and sandstone, respectively. Compared with natural rock and coal fragments, the bulking factor range of saturated sandstone, mudstone, and coal is 0.382–0.53, 0.41–0.627, and 0.506–0.807 with an average increase of 9.45%, 16.67%, and 13.34%, respectively. Certainly, bulking factor and axial stress of rock and coal basically satisfy the logarithmic relationship, regardless of strength, lumpiness, and water-bearing state. Mineral compositions and mesostructure characteristics of rock and coal were observed, meanwhile, quality and chemical composition of mine water were examined to clarify the relationship between internal composition, structure, and external deformation under stress-fluid interactions. These provide a theoretical basis for storage capacity estimation and stability control of coal mine underground water reservoir.

Improving the calculation accuracy of FEM for asphalt mixtures in simulation of SCB test considering the mesostructure characteristics

ABSTRACT Fracture simulation of asphalt mixtures based on three-dimensional finite element (3D-FE) simulation considering the mesostructure consumes too much computing resources and has low efficiency, which limits the popularisation and application of asphalt mixture fracture characteristics simulations. In contrast, two-dimensional finite element (2D-FE) simulation needs less computational resources and has high efficiency. However, 2D-FE simulation results significantly impacted by aggregate content and particle distribution in the scanned images and an arbitrary 2D image could hardly reflect the mesostructure of the whole asphalt mixture specimen. Therefore, two internal structure indicators, such as aggregate content indicator (ACI) and aggregate distribution indicator (ADI), were defined for image assessment, and an image selection process was proposed based on these two indicators for 2D-FE modelling method. By comparing the results obtained from 3D-FE simulations, 2D-FE simulations with or without image selection process, and laboratory tests, it can be found that the results of 2D-FE simulations with the image selection process are more consistent with laboratory test than other two simulation processes. Thus, it can be verified that the image selection process can highly enhance the accuracy of 2D-FE simulation while reducing the number of models needed and improving computational efficiency.

Experimental Study on the Mechanical Performance of Grouted Specimen with Composite Ultrafine Cement Grouts

An experimental investigation was conducted in order to understand properties of grouts and grouted specimens. Three different cement types, i.e., composite ultrafine cement (CUC) that was independently developed, ultrafine cement (UC), and Portland cement (PC) were injected into broken red sandstone specimens on the basis of a self-designed grouting test equipment. After the grouting test, the effects on the mechanical behavior of grouted specimens were studied using an uniaxial compression test, macroscopic failure analysis and mesostructure analysis, as well as a comparison of the mechanical and structural properties of three types of grouted specimens was presented. The test results show that the compressive strength of grouted specimens improved in comparison to the rock residual strength, and the compressive strength of PC, UC and CUC grouted specimens increased by 23.0%, 59.6% and 101.5%, respectively. The failure modes of all grouted specimens were brittle failure, but only the CUC grouted specimen was failed along the new failure surfaces, indicating that CUC grouts can better bond the original cracks. The mesostructure characteristics obtained through the Digital 3D Video Microscope reveal the superior filling effect of the CUC grouts as well as verifying the macroscopic mechanical behavior.

Effect of Ni loading on SBA-15 synthesized from palm oil fuel ash waste for hydrogen production via CH4 dry reforming

The successful synthesis of SBA-15 using silica source extracted from palm oil fuel ash (POFA) was proven with the presence of mesostructure characteristics as evidenced by low angle XRD, N2 adsorption-desorption isotherms and TEM. Different amounts of Ni were loaded on the synthesized SBA-15(POFA) using the impregnation method at 80 °C. The influence of Ni loading over the Ni/SBA-15(POFA) physiochemical properties and CO2 reforming of CH4 (CRM) were investigated in a stainless steel fixed-bed reactor at 800 °C and atmospheric pressure with 1:1 CO2:CH4 volumetric feed composition. An increment in Ni loading on SBA-15(POFA) from 1 to 5 wt% decreased the BET surface area and crystallinity of catalyst as proven by N2 adsorption–desorption and XRD analysis. The catalytic performance of CRM followed the sequence of 3 wt% > 5 wt% > 2 wt% > 1 wt% -Ni/SBA-15(POFA). This result was owing to the even distribution of Ni and good Ni–O–Si interaction of 3 wt% Ni/SBA-15(POFA) as proved by TEM, FTIR and XPS. Lowest H2/CO ratio and catalyst activity and stability of 1 wt% Ni/SBA-15(POFA) were due to the weaker Ni–O–Si interaction and small amount of basic sites that favor the reverse water gas shift (RWGS) reaction and carbon formation. The recent finding indicates that a quantity as small as 3 wt% Ni loaded onto SBA-15(POFA) could elicit outstanding catalytic performance in CRM, which was comparable with 10 wt% Ni loading catalysts reported in literature.

Study on the sound absorption coefficient model for porous asphalt pavements based on a CT scanning technique

In the study of the noise reduction performance of porous asphalt pavements, it is usually necessary to establish an acoustic model to develop the experiment and the theoretical analysis. However, although the traditional acoustic model is rational, there are some limitations in its accuracy and further research should be carried out. Through CT scanning, image processing and three-dimensional model building, the mesostructure characteristics of porous asphalt concrete (PAC) specimens were identified, the air-void parameters of PAC specimens were obtained, and a prediction model of the sound absorption coefficient and air-void parameters was established. In view of the difficulty involved in obtaining air-void parameters in engineering, a macroscopic prediction model of the sound absorption coefficient and air voids was established. The study showed that PAC specimens contained a large number of voids, most of which were smaller than 1 mm in equivalent diameter and were mostly distributed within 0.4–0.5 mm. Through correlation analysis, several parameters that highly correlated with the sound absorption coefficient were found out: measured air voids, reconstructed air voids, total air-void volume, average air-void volume, total air-void surface area and average air-void surface area. A prediction model for the sound absorption coefficient was established based on measured air voids, total air-void volume, average air-void volume, total air-void surface area, and average air-void surface area. In addition, a prediction model of the sound absorption coefficient and measured air voids was proposed for practical engineering purposes. The above two models can predict the sound absorption coefficient of PAC accurately, which may provide a reference in the study and application of PAC in noise reduction and impact energy absorption.

Deformation behaviors and meso–structure characteristics variation of the weathered soil of Pisha sandstone caused by freezing–thawing effect

The deformation behaviors of soil caused by freezing–thawing (F–T) effect play a key role in determining the damage for engineering structures in seasonal frozen soil regions. Based on the laboratory tests, investigations were made into the deformation behaviors of the weathered soil of Pisha sandstone, as well as the meso–structure characteristics variation before and after F–T effect. The F–T tests demonstrate that there exists an initial frost heave moisture content (IFHMC) and an initial thawing settlement moisture content (ITSMC) in the soil. When the moisture content of the soil below the IFHMC and the ITSMC, the frost heave and the thawing settlement phenomenon will not occur. On the contrary, the soil will expand during the thawing process. Once the moisture content exceeds the value of IFHMC and ITSMC, the frost heave ratio and thawing settlement coefficient increase with moisture content. The frost heave ratio displays a growing trend with dry density, while the thawing settlement coefficient decreases first and then increases with dry density. There is a critical dry density that corresponds to the minimum value of thawing settlement coefficient. With the decreases of freezing temperature, the unfrozen water amount in soil drops, and the frost heave ratio and the thawing settlement coefficient increase gradually, presenting a non–linear increasing development tendency. The meso–structure tests show that some changes occurred in the pore structure and soil particles of the specimen. The larger moisture content of the specimen is, the more sensitive to the pore structures change and soil particles position variation.

XRCT characterization of mesoscopic structure in poured and tapped cohesive powders and prediction by DEM

Predicting structural and flow characteristics of cohesive powder systems is a technical challenge in many powder technology applications. The discrete element method (DEM) is a promising approach to model the mesoscale structure and flow behavior of cohesive granular systems. Advances in experimental techniques such as X-ray computed tomography (XRCT) have enabled characterization of powder systems with sub-micron resolution. Combined with standard bulk powder characterization methods, this technique allows for mesoscopic structural characteristics of cohesive powder systems to be compared directly with DEM simulation. In this study, DEM and XRCT are used to characterize the structural characteristics of two moderately cohesive roughly spherical “model” powders – glass and stearic acid – under loosely poured and tapped consolidated conditions. The DEM simulations are based on a constitutive elasto-plastic model that makes use of the JKR theory to account for cohesive interparticle surface forces. Powder density and mesoscopic void structure obtained from DEM simulation are compared to characterization results obtained from XRCT image analysis. The results show that DEM can accurately predict the extent of tapping induced densification and mesostructure characteristics in cohesive powders, with superior agreement for stearic acid.

Concrete meso-structure characteristics and mechanical property research with numerical methods

Concrete meso-structure significantly affects its mechanical property, as crack generally starts and develops basically along the interface between aggregate and mortar on the meso-scale. The cracking path is highly dependent on the interface structure, the main component of concrete meso-structure. Since the interface is formed due to aggregate distributing in mortar mix, the relation between aggregate surface area and interface perimeter could be considered as a representative index of concrete meso-structure. As it is too complicated to adopt traditional methods to represent this relation, in order to better understanding the influence of meso-structure change on mechanical property, fractal theory is introduced to explore the relationship, with the main focus on the characteristics of the concrete meso-structure and its influence on mechanical property. Results show that the specimen size has great impact on meso-structure and mechanical property, while the influence of aggregate volume fraction can be neglected. It is more obvious in larger specimen size that aggregate volume fraction increase could contribute a lot to bearing capacity improving and whole stress level descending. Besides, aggregate volume fraction increase could improve the local cracking stress, but reduce the cracking strain, resulting in cracking brittleness.

Energy radiated by seismic events of different scales and geneses

Extensive data comprising about 1500 seismic events with the moment magnitudes M W from–3.5 to 9.2 have been analyzed for identifying the implications of the event size, the type of faulting in the source, and tectonic situations for the efficiency of the radiation. It is shown that there are several hierarchy levels with different patterns of scaling relationships describing the changes in the parameters of seismic events with the event size. This is due to the specificity of the hierarchy in the macroscopic characteristics of the rock mass. The size and mechanism of the earthquake determine the general trends in the variations of its radiation efficiency. The role of the macroscopic parameter controlling the efficiency of a seismic source is played by the stiffness of a fault or a fracture. The scaling relationship of this parameter determines several hierarchical levels within which the changes in the characteristics of the earthquakes follow the different laws. The huge scatter in the values of the scaled energy (the ratio of the radiated seismic energy to the seismic moment, energy-to-moment ratio) about the average requires additional study. Quite probably, the value of the scaled seismic energy is determined by the mesostructure and physicomechanical characteristics of the fault’s core. Small variations in these factors may lead to drastic changes in the stress drop amplitude and in the rupture propagation velocity up to the emergence of different regimes of deformation.

Uniformity of temperature variation in coal during methane adsorption

Uniformity of methane adsorption by coal is important for assessing both the maximum storage capacity and damage caused by nonuniform thermal expansion. Therefore, this study examines the structural characteristics and temperature variation on the surface of coal during methane adsorption at 1 and 1.5 MPa. The results showed that the temperature increase in different surface regions associated with methane adsorption obeys the normal distribution law, and it is closely related to its meso-structure characteristics. That is, the vitrinite of the coal matrix has a fast adsorption rate and a better developed microscopic structure, resulting in a higher adsorption capacity and more pronounced temperature rise. In contrast, clay minerals present in coal have a lower methane adsorption capacity and slow adsorption rate, resulting in a smaller temperature rise.

Controlled fabrication of ordered mesoporous titania/carbon fiber composites with high photoactivity: Synergistic relationship between surface adsorption and photocatalysis

Ordered mesoporous TiO2 (OMPT) supported on carbon fibers (OMPT/CFs) were fabricated by supercritical deposition using liquid crystal as soft template. The OMPT/CFs were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS) and nitrogen sorptometry, and their photocatalytic activity was studied through the degradation of acid orange 7 (AO7) aqueous solution. The results indicate that OMPT is dispersed on the CFs with crystalline size of 10–20nm and pore size of 2–5nm. Although with increasing amount of TiO2 coatings the surface area of OMPT/CFs decreased relative to that of CFs, OMPT/CFs-n (n=3) (n is the number of precipitation/drying cycles used to prepare the composite), which had the maximum TiO2 content, still possessed a surface areas of 538.6m2/g because of the mesostructure characteristics of titania. The OMPT/CFs showed high activity in the photodegradation of AO7 in aqueous solution in comparison with P25, pure OMPT and OMPT–CF mixture. Such activity was facilitated by the synergistic relationship between surface adsorption characteristics and photocatalytic potential. Because of the mutual constraint between loading and surface area on the photocatalytic efficiency, the optimal loading of OMPT in OMPT/CFs-2 for AO7 degradation is 15.2wt%. The photocatalytic behavior could be described in terms of a modified Langmuir–Hinshelwood model.

Comprehensive Structure Analysis of Ordered Carbon Nanopipe Materials CMK-5 by X-ray Diffraction and Electron Microscopy

The structure of ordered mesoporous carbon materials CMK-5 was analyzed in detail by transmission electron microscopy (TEM) and X-ray diffraction (XRD) analysis by applying the recently developed continuous density function technique. The materials present two-dimensional (2-D) hexagonally ordered arrays of carbon nanopipes formed within the mesopores of SBA-15 silica templates used for their fabrication. The series of CMK-5 carbons were synthesized by applying the templates of different pore widths and two carbon precursors: furfuryl alcohol and acenaphthene. All the materials were well-ordered, exhibiting five distinct XRD reflections, high nitrogen BET specific surface areas (about 2500 m2/g), and large pore volumes (about 2 cm3/g). TEM analysis and XRD modeling allowed precise determination of the mesostructure characteristics such as the nanopipe diameter, wall thickness, and shape. Around 15% of the space between the nanopipes was found to be occupied by carbon interconnections formed inside the complementary pores of the SBA-15 templates. The structure parameters determined correlated well with the synthesis procedures and were fully consistent with the nitrogen adsorption and thermogravimetric data. The differences in the properties and the formation mechanisms of the materials synthesized by applying furfuryl alcohol and acenaphthene carbon precursors were analyzed based on the whole scope of data obtained.