Characteristics Of Microcracks Research Articles

Effect of the Interaction between Cavities and Flaws on Rock Mechanical Properties under Uniaxial Compression

Cavities and flaws are common types of defects in rock specimens that have an important impact on the mechanical properties of rockmass. In this paper, cement mortar was used to prefabricate a rock-like specimen with two cavities and a single flaw, and the uniaxial compression test was carried out. The process of fracture evolution on the specimen surface was obtained by using photography technology. The evolution regularity of a fracture was monitored by utilizing acoustic emission (AE) technology during the process of the specimen failure. Moreover, three-dimensional (3D) tomograms of specimens after uniaxial compression were obtained by using computerized tomography (CT) scanning technology, investigating the development characteristics of microcracks and the distribution of the final macrofractures. The particle flow code in two dimensions (PFC2D) program was used to simulate the mechanical behavior of brittle rock combining with microcrack propagation. The calculated stress-strain curve, AE features, and fracture distribution of the specimen obtained from the PFC2D simulation were relatively consistent with the experimental results.

Microscopic Characteristics of Fractured Sandstone after Cyclic Freezing‐Thawing and Triaxial Unloading Tests

The understanding of tunneling rock failure characteristics under unloading conditions in the cold region is critical for the proper design of rock tunneling support and mining safety operation. Given this understanding is currently limited, this study aimed to investigate the characteristics of fractured sandstone samples in the microscale after cyclic freezing‐thawing and triaxial unloading tests. The samples were first subjected to different cycles of freezing and thawing, followed by the triaxial unloading test and scanning electron microscopy imaging. The peak strength and damage dilatancy stress were measured from the stress‐strain curves. The microcrack characteristics (number, length, and width) were obtained through the image analysis. The results show that the decrease in peak strength and damage dilatancy stress was more significant by the first 20 freezing‐thawing cycles when the pore pressure gradient is maximum compared to the later freezing‐thawing cycles. The mechanical properties also significantly deteriorated when the severe freezing‐thawing treatments were performed. The fracture section mainly had the morphology of honeycomb‐like microstructure, stripped microstructure, and flocculent microstructure. The cracking extent was mainly influenced by the freezing‐thawing rather than the triaxial unloading test, but the azimuthal angle of microcracks was significantly altered by the triaxial unloading. To properly design rock tunneling support and safe operation of mining in the cold region, both impact of cyclic freezing‐thawing and the excavation operation direction should be considered.

Failure pattern of brittle rock governed by initial microcrack characteristics

Fracturing processes of rock are simulated using a micro- and fracture mechanical-based numerical modeling approach. The numerical model considers material heterogeneity and initial microcrack distributions. The gradual formation of macroscopic fractures by coalesced microcracks is successfully reproduced. Distinct failure modes are observed in the model under different loadings. Agreement is shown between the numerical results and laboratory observations. The influence of microcrack orientations on the fracture patterns is quantified by the numerical models. Possible mechanisms describing the splitting failure of hard brittle rocks under uniaxial compression are proposed and discussed.

Quantitative evaluation of organic porosity and inorganic porosity in shale gas reservoirs using logging data

ABSTRACTReservoir pores space characterization is the core content of shale gas reservoir evaluation. The reservoir pore space of shale gas reservoir is complex, which can be divided into organic pore, clay pore, crack pore, and debris pore. A method for evaluating the four porosity of shale gas reservoir by using logging data is presented in this paper. First, according to the volume model, shale rock is equivalent to three parts: matrix, organic matter, and pore. On this basis, response equations of density logging and neutron logging are constructed. The total porosity of shale reservoirs can be solved by combining the two equations. Second, organic pore is nanopore that remains in organic matter during the conversion of kerogen to oil and gas, which is related to the content and maturity of organic matter. An equation for calculating organic porosity using total organic carbon content and density logging is presented. The scale factor in the equation is obtained from argon ion polishing scanning electron microscope experimental data. Third, based on the GRI porosity experimental data of 8 samples in the study area, the relationship among clay porosity, clay bound water saturation, and clay content was analyzed. It is considered that the clay bound water saturation has a certain correlation with the clay content, but the correlation coefficient is not high, has a positive correlation with the clay content. Based on this, a statistical model of clay porosity, clay content, and total porosity was established. Fourth, based on the dual laterolog response characteristics of microcracks, the fracture porosity of shale reservoirs is calculated by positive and negative simulation proposed by Li et al. In addition, the detrital porosity is equal to the difference between total porosity and organic porosity, clay porosity, crack porosity. The above techniques are applied to the calculation of four porosities of two wells in the Longmaxi Formation of Sichuan basin, China, and good effect has been achieved.

Research on the Macro and Micro Evolution Law of Shear Fracture of Gas-Bearing Coal Under Different Shear Rates

Using a self-researched micro-structure shear testing device for gas-containing coal, this paper studies the characteristics of micro-cracks forming, expanding, and ultimately destroying the sample at different shear rates and then discusses the macro-failure characteristics and the evolution law of micro-cracks in the gas-containing coal at different shear rates. The results show that the shear strength decreases with an increasing shear rate, and the relation between them is a logarithmic relation; the cracking tips are in the upper and lower parts of the coal, and a fine crack appears in the sample when the shear stress is 10% of the maximum shearing stress, while the cracks fully penetrate when the shear stress is 70% of the max; finally, macroscopic damage forms when the shear stress equals the maximum amount. The angle between the direction of the lower cracks and the shear increases with an increasing shear rate; the main fracture of the gas-containing coal is around the coal particles, the fracture surface of the coal is injustice, and the fracture surface roughness becomes worse with an increase in the shear rate. With an increasing shear rate, the gas-containing coal’s cranny ratio and crack fractal dimension increases, the degree of damage increases, and the carrying capacity decreases, which leads to a lower stability and a higher tendency for coal and gas outbursts.

A numerical study of brittle failure in rocks with distinct microcrack characteristics

A numerical model is presented to reproduce the stress induced fracturing process of brittle rocks. Material heterogeneity at the zone level of the model is considered, and distinct initial microcrack lengths and orientations are also included. The rock material is prescribed with a Weibull distribution, and the microcrack lengths and orientations are defined by distinct distribution functions. Stress intensity factor approach is adopted to characterize the stress condition of the microcracks, and is used as a failure criterion for the zones of the model. The mechanical response of rock specimens is investigated on models under different external loading conditions. Obvious nonlinear characteristics before the peak strength can be observed in models under uniaxial compression. Consistency can be found between simulation results and lab test data. The influence of microcrack conditions and heterogeneity levels on the simulation result is also investigated. The macroscopic failure modes of models are studied with different distributions of initial microcracks, and correlations are found between the microcrack conditions and fracturing processes. Especially, scattered zones and obscure fracture patterns are produced in the model defined by exponential and uniform distribution, compared with clear shapes of macroscopic fractures observed in models defined by normal or lognormal distributions. The simulations also show that the differences in the mean value of microcrack orientations obeying certain distribution result in distinct macroscopic fracture patterns, from shear bands at lower mean orientations to tensile fractures at higher mean orientations. Typical fracture patterns observed in laboratory are successfully reproduced in the model, which demonstrates the capability of the model in reproducing faithful rock behaviors through prescribing specific microscopic characteristics.

Generation Mechanism of Nonlinear Rayleigh Surface Waves for Randomly Distributed Surface Micro-Cracks.

This paper investigates the propagation of Rayleigh surface waves in structures with randomly distributed surface micro-cracks using numerical simulations. The results revealed a significant ultrasonic nonlinear effect caused by the surface micro-cracks, which is mainly represented by a second harmonic with even more distinct third/quadruple harmonics. Based on statistical analysis from the numerous results of random micro-crack models, it is clearly found that the acoustic nonlinear parameter increases linearly with micro-crack density, the proportion of surface cracks, the size of micro-crack zone, and the excitation frequency. This study theoretically reveals that nonlinear Rayleigh surface waves are feasible for use in quantitatively identifying the physical characteristics of surface micro-cracks in structures.

Enamel Microcracks Induced by Simulated Occlusal Wear in Mature, Immature, and Deciduous Teeth.

Enamel wear, which is inevitable due to the process of mastication, is a process in which the microcracking of enamel occurs due to the surface contacting very small hard particles. When these particles slide on enamel, a combined process of microcutting and microcracking in the surface and subsurface of the enamel takes place. The aim of this study was to detect microscopic differences in the microcrack behavior by subjecting enamel specimens derived from different age groups (immature open-apex premolars, mature closed-apex premolars, and deciduous molars) to cycles of simulated impact and sliding wear testing under controlled conditions. Our findings indicated that the characteristics of the microcracks, including the length, depth, count, orientation, and relation to microstructures differed among the study groups. The differences between the surface and subsurface microcrack characteristics were most notable in the enamel of deciduous molars followed by immature premolars and mature premolars whereby deciduous enamel suffered numerous, extensive, and branched microcracks. Within the limitations of this study, it was concluded that enamel surface and subsurface microcracks characteristics are dependent on the posteruptive age with deciduous enamel being the least resistant to wear based on the microcrack behavior as compared to permanent enamel.

Microscopic Characterization of Tensile and Shear Fracturing in Progressive Failure in Marble

AbstractCompression‐induced tensile and shear fractures were reported to be the two fundamental fracture types in rock fracturing tests. This study investigates such tensile and shear fracturing process in marble specimens containing two different flaw configurations. Observations first reveal that the development of a tensile fracture is distinct from shear fracture with respect to their nucleation, propagation, and eventual formation in macroscale. Second, transgranular cracks and grain‐scale spallings become increasingly abundant in shear fractures as loading increases, which is almost not observed in tensile fractures. Third, one or some dominant extensional microcracks are commonly observed in the center of tensile fractures, while such development of microcracks is almost absent in shear fractures. Microcracks are generally of a length comparable to grain size and distribute uniformly within the damage zone of the shear fracture. Fourth, the width of densely damaged zone in the shear fracture is nearly 10 times of that in the tensile fracture. Quantitative measurement on microcrack density suggests that (1) microcrack density in tensile and shear fractures display distinct characteristics with increasing loading, (2) transgranular crack density in the shear fracture decreases logarithmically with the distance away from the shear fracture center, and (3) whatever the fracture type, the anisotropy can only be observed for transgranular cracks with a large density, which partially explains why microcrack anisotropy usually tends to be unobvious until approaching peak stress in specimens undergoing brittle failure. Microcracking characteristics observed in this work likely shed light to some phenomena and conclusions generalized in seismological studies.

Quantification of microcrack characteristics and implications for stiffness and strength of granite

Microcracks can affect the mechanical properties of rocks, such as their stiffness and strength. To provide a link between the microstructural parameters and the mechanical behaviour of rock, micromechanical models use parameters that represent a quantitative description of the microcrack population. However, these parameters are difficult to constrain. With the aim of better constraining micromechanical models for rock strength and stiffness, we provide here robust measurements of microcrack characteristics. We developed an algorithm to process optical micrographs of rock, automatically creating binary images of the microcrack network. We applied this procedure to optical micrographs of fine-grained granite samples that have undergone varying degrees of thermal microcrack damage. From these processed images, we calculate the mean microcrack length and the number of microcracks per unit area (and therefore the 2D microcrack density). We also create heat maps showing the spatial distribution of microcracks and their lengths. The results of our automated image analysis are in very good agreement with those of widely-used stereological techniques, and we show that our method can be applied to other rock types (sandstone and andesite) that contain microcracks. Using the measured microcrack characteristics as inputs for Ashby and Sammis’ (1990) 2D micromechanical sliding wing crack model, we predict the uniaxial compressive strength of the granite and compare the predictions with strength measurements made in the laboratory. We find good agreement between the model and the experimental data for granite heated to temperatures below the alpha-beta transition of quartz (~573°C). Rock strength is overestimated above this threshold, possibly due to variations in fracture toughness, which is considered constant in our modelling. Finally, we use the 2D sliding crack model of David et al. (2012) to infer microcrack density and aspect ratio from the mechanical response of the thermally microcracked samples to cyclic stressing. We show a good agreement between inferred and measured crack densities if a scaling factor is introduced.

The Rejuvenating Effect in Hot Asphalt Recycling by Mortar Transfer Ratio and Image Analysis.

Using a rejuvenator to improve the performance of asphalt pavement is an effective and economic way of hot asphalt recycling. This research analyzes the rejuvenating effect on aged asphalt by means of a Mortar Transfer Ratio (MTR) test, which concerns the ratio of asphalt mortar that moves from recycled aggregates (RAP aggregates) to fresh added aggregates when aged asphalt is treated with a regenerating agent and comes into contact with fresh aggregates. The proposed MTR test analyzes the regeneration in terms of the softening degree on aged asphalt when the rejuvenator is applied. The covered area ratio is studied with an image analyzing tool to understand the possibility of mortar transferring from RAP aggregates to fresh aggregates. Additionally, a micro-crack closure test is conducted and observed through a microscope. The repairing ability and diffusion characteristics of micro-cracks can therefore be analyzed. The test results demonstrate that the proposed mortar transfer ratio is a feasible way to evaluate rejuvenator diffusion during hot recycling. The mortar transfer ratio and uncovered area ratio on fresh aggregates are compatible, and can be used to quantify the contribution of the rejuvenator. Within a certain temperature range, the diffusing effect of the rejuvenator is better when the diffusing temperature is higher. The diffusion time of the rejuvenator is optimum when diffusion occurs for 4–8 h. When the rejuvenator is properly applied, the rough and cracking surface can be repaired, resulting in better covered aggregates. The micro-closure analysis visually indicates that rejuvenators can be used to repair the RAP aggregates during hot recycling.

A fracture damage constitutive model for fissured rock mass and its experimental verification

A constitutive model for fissured rock mass is established based on the deformation characteristics of microcracks under compression. The method to determine the model parameters is given. Comparative analyses between the proposed model and experimental results of siltstone indicate that this model can describe the whole process from the initial state to failure state of fissured rock mass well; this model can also simulate the initial phase in the process of fissured rock under compression well. At the same time, this model can also reflect the property of strain softening as well as the damage evolution process. Compared with previous models, this model is better able to capture the general trend which is observed experimentally. The strain energy and damage revolution of fissured rock under compression are also discussed; and the proposed model is capable of predicting the energy and damage behavior, which are in reasonably agreement with the experimental observations.

Modelling of multi-stage nonlinear interaction of micro-crack and ultrasonic based on equivalent elastic modulus

A multi-stage model of nonlinear interaction between micro-crack and ultrasound based on equivalent elastic modulus is presented in this paper. In this model, the interface characteristics of micro-cracks at a micro-level and the relative motion at a macro-level are unified into an elastic modulus of the mesoscopic element. The equivalent elastic modulus is used to characterize the stress-strain of the damage region. Then piecewise function is used to describe the nonlinear interaction between ultrasound and micro-crack. Finally, the wave equation is solved by the finite element simulation. In this manner, the nonlinear interaction law between ultrasound and micro-crack is obtained, and the validity of the model is verified. The simulation results also show that compared with bilinear stiffness model and contact surface model, the multi-stage model can well reflect the distortion of the waveform in one period of ultrasonic wave passing through the micro-crack. In addition, the influences of the crack angle, the crack length and the input amplitude on the second harmonics generation and the third harmonics generation are analyzed. In the end, the comparison and analysis of the experimental test and simulation calculations based on the proposed multi-stage model show that the proposed multi-stage model and the experimental test can well reflect the second harmonic signal produced by the nonlinear interaction of micro-crack and ultrasound, and the second harmonic amplitudes of the experimental test are basically the same as the simulation calculations based on the proposed multi-stage model. Thus, the effectiveness of the proposed multi-stage model is verified. The model provides a new simulation method to quantitatively detect the micro-crack by ultrasonic nonlinear effect.

Fatigue properties of strongly textured bulk NiW

The cyclic plasticity of Ni–5at.%W with a strong cube texture was investigated for two different deformation directions and at two different plastic strain amplitudes of 2.5⋅10−4 and 5⋅10−4 in a symmetric push–pull-mode. To prevent bending of the thin samples during the compression half-cycles, a novel sample suspension was successfully integrated into the mechanical tests.Samples deformed parallel to a 〈001〉or a〈011〉direction of the cube texture displayed significant differences in both plastic behavior (i.e., lifetime, correlation of saturation stress amplitude and plastic strain amplitude) and microcrack characteristics. The majority of these effects can be interpreted as a consequence of the peculiar grain structure of the material, consisting of a “matrix” of cube oriented grains with small embedded grain regions of differing orientations. The hysteresis loops of all samples exhibit a short region of concave curvature shortly before the inversion points, possible reasons of which are discussed.

Quantitative damage evaluation of concrete suffered freezing–thawing by DIP technique

Most of the hydraulic concrete structures in cold regions of China are suffering freezing–thawing (FT) deterioration and the first step to choose repairing measures is to appropriately and quantitatively evaluate the FT damage degree in concrete. The current evaluating parameters for FT damage in concrete, such as resonant frequency and ultrasonic wave velocity are all phenomenological descriptions and cannot reveal the essence of the FT damage, which is the initiation and propagation of microcracks. This paper proposes a method to evaluate FT-damage in concrete based on vacuum–epoxy-impregnation method and digital-image-processing (DIP) technique. Quantitative microcrack analysis and mechanical tests including compressive tests, flexural tests and axial tensile tests were performed for the concrete specimens with/without air-entrainment suffered different FT cycles. Evolutions of microcrack characteristics of the matrix cracks (including Interfacial Transition Zone (ITZ) cracks and mortar cracks) in the concrete specimens were thoroughly investigated. Relationships between the relative mechanical properties and the microcrack density were established. Results show that the length density, area density and crack width of microcracks in the matrix of concrete all increase with the increase of damage degree and the ITZ cracks instead of the mortar cracks are the major part of FT-caused microcracks. The high correlation coefficients between the mechanical properties and microcrack density indicate that quantitative microcrack analysis could be used as a powerful and promising tool of quantifying FT damage in concrete.

Quantitative damage evaluation of AAR-affected concrete by DIP technique

This paper presents a method to evaluate alkali–aggregate reaction (AAR) damage in concrete by digital-image-processing (DIP) technique. A vacuum fluorescent-epoxy impregnation technique is used to highlight microcracks in the AAR-damaged concrete and a Matlab-based software package is developed to quantitatively analyse the microcrack characteristics (area, length, width etc.) in the fluorescent microscopic images of concrete. Six microcrack pattern parameters – the total length, the length density, the total area, the area density, the maximum width and the average width of microcracks – in slices from the concrete specimens that suffered different degrees of AAR damage were obtained. Results show that the length density, the area density and the average/maximum width of microcracks all increase as the degree of AAR damage increases. Relationships between the mechanical properties and the microcrack characteristics are established and results show that both the relative flexural strength and relative axial tensile strength have quadratic correlations with the microcrack density. The degree of AAR damage has a good logarithmic correlation with the microcrack density and the expansion ratio has a linear correlation with the microcrack density. Quantitative microcrack analysis based on DIP techniques could be developed as a new powerful tool of quantifying AAR damage in concrete.

Evaluation of enamel micro-cracks characteristics after removal of metal brackets in adult patients

The purpose of this study was to evaluate and compare enamel micro-crack characteristics of adult patients before and after removal of metal brackets. After the examination with scanning electron microscopy (SEM), 45 extracted human teeth were divided into three groups of equal size: group 1, the teeth having enamel micro-cracks, group 2, the teeth without initial enamel micro-cracks, and group 3, control group to study the effect of dehydration on existing micro-cracks or formation of new ones. For all the teeth in groups 1 and 2, the same bonding and debonding procedures of metal brackets were conducted. The length and width of the longest enamel micro-crack were measured for all the teeth before and after removal of metal brackets. The changes in the location of the micro-cracks were also evaluated. In group 3, teeth were subjected to the same analysis but not bonded. The mean overall width of micro-cracks after removal of metal brackets was 3.82 μm greater than before bonding procedure (P < 0.05). Also, a significant difference was noticed between the width of micro-cracks in first zone (cervical third) and third zone (occlusal third) after debonding procedure (P < 0.05). New enamel micro-cracks were found in 6 of 15 (40 per cent) examined teeth. Greatest changes in the width of enamel micro-cracks after debonding procedure appear in the cervical third of the tooth. On the basis of this result, the dentist must pay extra care and attention to this specific area of enamel during removal of metal brackets in adult patients.

Quantification Studies on Meso-Damage Characteristics of Marble under Uniaxial Compression Condition

The damage and failure of rock under loading are caused by the propagation and coalescent of cracks at mesoscale, so it is helpful to understand the mechanical behaviours of rock by quantitatively investigating on mesostructure of rocks. Based on the theory of damage mechanics, a digital experimental system on rock meso-structure is proposed. In this paper, the digital studies are conducted on the meso-structure of the marble from Jinping, Sichuan with the damage theory. Lots of meso-structure images are obtained through SEM observation. Based on digital image processing theory, some meso-level characteristics of micro-cracks are obtained such as length, azimuth, width, area and circumference. With the statistics theory and the Monte Carlo theory, the meso-level information of micro-cracks and their morphology are simulated in the representative volume element (RVE) of the Jinping specimen.

Nonlinear acoustic characteristics of Fatigue microcracks in al alloy plate

A system consisting of piezoelectric transducers, arbitrary waveform generator, power amplifier, laser vibrometer, digital oscilloscope, and a computer has been constructed to study the nonlinear acoustic characteristics of fatigue microcracks of 2024-T351 aluminum alloy plates. Using two different frequency signals (fH=810 kHz, fL=100 kHz), the intact and microcracked samples are experimented, respectively. The experimental results show that in the intact samples there are only two fundamental frequencies and no harmonics and sidebands occur. In the microcracked samples, there is an abundance of the harmonics and sidebands, and the frequencies fH±fL and fH±2fL rise. These new frequency components can be used to indicate the presence of a microcrack or damage.

In situ visualization of microcracks by a confocal laser scanning microscopy system

Invisible microcracks occur on the surface of wood in the first stage of drying. It is important to investigate the morphological changes with time from generation to propagation in connection with moisture content (MC). In this study, a confocal laser scanning microscopy (CLSM) system with a controlled environment chamber was innovated to visualize the microcracks occurring during drying. Small samples of softwood Cryptomeria japonica, ring-porous Melia azedarach and diffuse-porous acacia hybrid (A. mangium×A. auriculiformis) were used. Microcracks occurred on the transverse surface in all samples with decreasing MC. The points at which microcracks occurred were in ray parenchyma or between tracheids or wood fibres and ray parenchyma. Then, the microcracks propagated in both the bark and pith directions along ray parenchyma and stopped at the maximum shape. After that, they closed with further drying. In the cases of C. japonica and M. azedarach, the microcracks almost closed and some of them could not be detected by CLSM. However, those in acacia hybrid did not close completely in the last stage of drying. These in situ observations clarified the characteristics of microcracks in relation to MC and wood structure.