Distribution Of Microcracks Research Articles

A three-dimensional mesoscale model for progressive time-dependent deformation and fracturing of brittle rock with application to slope stability

An understanding of progressive time-dependent deformation is essential for determining appropriate measures to ensure the long-term integrity of rock-masses surrounding engineering structures. We propose a 3D numerical model, that uses the Norton-Bailey creep law and a time-independent damage evolution law, to investigate the progressive time-dependent deformation and fracturing of brittle rock. The model considers material heterogeneity and the concept of mesoscopic renormalization at the mesoscale. The cooperative interaction between microcrack distribution and damage evolution leads to local material degeneration as a function of increasing time in the model. First, the input parameters for the model were calibrated and the model was validated using laboratory experiments. Numerical creep simulations were then performed for a range of constant stresses. Our model can accurately replicate the evolution of strain and strain rate as a function of time, the output of acoustic emission energy, and the emergence of a macroscopic failure plane seen in laboratory experiments. Our simulations also show that the minimum creep strain rate and the time-to-failure increase and decrease, respectively, as stress is increased, also seen in laboratory experiments. For example, increasing the differential stress by 10 MPa increased the minimum creep strain rate increased by an order of magnitude. Finally, we use the proposed 3D model to investigate the time-dependent stability of an engineering-scale rock slope containing faults at the Fushun West Open Pit coal mine. A small increase in fault-adjacent damage was observed after 40 days and, between 60 and 120 days, the damage on the side of the slope increased. After 140 days, the localized growth of the damaged elements split the slope into two segments, resulting in slope failure. Our 3D numerical model highlights potential slope instability in the Fushun West Open Pit coal mine and can be used to investigate slope stability in engineering projects worldwide.

A modeling study of the micro-cracking behavior of brittle crystalline rocks under uniaxial loading considering different scenarios of grain-based models

The grain-based model (GBM) in two-dimensional Particle Flow Code (PFC2D) is widely used to simulate the deformation and brittle failure of crystalline rocks. However, the original GBM consisting of the parallel-bond (PB) and the smooth-joint (SJ) produces unrealistic micro-cracking processes in which excessive microcracks are distributed along the interfaces of mineral grains under uniaxial loading. To solve the problem, this research presents a discussion of the mechanical behavior of six GBMs composed of different contact models including PB, SJ, and flat-joint (FJ). After the uniaxial compressive and direct tensile tests of numerical granites, it is found that the different contact types in GBMs have a limited influence on the tensile strength (TS), elastic modulus (E), and Poisson’s ratio (ν) but have a great impact on the uniaxial compressive strength (UCS) and microcrack pattern. The UCS calculated by the PB/FJ GBM is higher than the actual value. Microcrack distributions simulated by the FJ/SJ and FJ/PB GBMs show that numerous mineral grains are cut across under uniaxial loading instead of their interfaces. While the FJ/SJ and FJ/PB GBMs may well reproduce the brittle failure process of crystalline rocks, it remains to be seen whether they will be applicable to other rock mechanics issues, including triaxial compression, thermal cracking, and hydraulic fracturing.

The alterations of critical pore water pressure and micro-cracking morphology with near-wellbore fractures in hydraulic fracturing of shale reservoirs

The crack initiation and propagation in a laminated shale formation have been widely investigated, but the alterations of critical pore water pressure (including fracture initiation pressure and breakdown pressure) and the micro-cracking morphology with near-wellbore fracture geometry are still unclear. This paper numerically investigates the alterations of critical pore water pressure and the morphology of the hydraulic fractures in shale formation with different near-wellbore fractures. First, a hydro-fracturing model is developed for a laminated shale formation within the framework of PFC2D. Then, this model is validated with the Blanton’s criterion, pressure history and fracture geometry. Finally, the effects of length, shape, and symmetry properties of near-wellbore fractures on the critical pore water pressure and micro-cracking morphology are comparatively investigated. The critical pore water pressure, micro-cracking morphology, aperture and permeability, and the fractal dimension of hydraulic fractures are quantitatively analyzed at three initial near-wellbore fractures. Simulation results indicate that the length and the distribution of near-wellbore fracture branches play a crucial role in pore water pressure variation and micro-crack propagation during hydraulic fracturing. The critical pore water pressure is sensitive to the initial near-wellbore fracture length especially when the initial fracture length exceeds a critical fracture length. The shape of initial near-wellbore fracture could change the distribution of micro cracks and induce large pore water pressure. The symmetrical radial fracture (θ=0°) more likely causes a hydraulic fracture with high permeability while the X-shape fracture (θ=30°) contributes greatly to the complexity of the fracture network and stimulated reservoir volume. These findings on the pore water pressure variation and the micro-cracking morphology are helpful in the optimization design of hydraulic fracturing treatment.

A three‐dimensional multiscale damage‐poroelasticity model for fractured porous media

AbstractThe paper investigates the failure of brittle rocks within a multiscale/multiphysics computational modeling framework so as to explicitly incorporate both microstructural and hydromechanical aspects in their overall (nonlinear) fracture behavior. Herein, the rock is idealized as a microfissured porous medium via a representative elementary volume (REV) containing distributed oblate spheroidal open microcracks and spheroidal nanopores at the lower scales. A two‐scale analytical homogenization procedure is then performed on the REV, assuming a saturated pore space across the scales. The end result is a microstructurally enriched continuum damage‐poroelasticity constitutive model within the generalized Biot's framework that inherits the underlying small‐scale characteristics. A set of damage tensors is derived based on the directional density distribution of microcracks that operate on both the poromechanical and hydraulic properties. Microstructural evolution is modeled following changes in microcrack length, aperture, and number density, including nanoporosity. To this end, a robust localization procedure is used that accurately captures the macroscopic softening due to microcracking events, leading to a nonlinear (but path‐independent) model. To investigate the baseline features of the model, including the salient effects of microcracking on induced anisotropy and deterioration of poroelastic properties, numerical results of material point‐based computations are discussed in detail. Finally, predictions of the current model are validated against experiments on Fontainebleau sandstone.

Study on the Quantitative Measurement Technique of the Micro Fracture Surface

The quantitative study on the distribution of cracks and micro cracks is a major problem in the study of reservoir properties of tight and lithologic regions In view of this problem, the present study is to evaluate the cracks by indirect method, and the direct evaluation method is lack of micro cracks This paper tries to combine the results of electronic scanning electron microscopy and image data processing method, which can directly test the density of micro cracks The test results prove that the method is convenient and feasible, and has a strong reference value for engineering practice and scientific research.

The Effect of Electrolytic Solution Composition on the Structure, Corrosion, and Wear Resistance of PEO Coatings on AZ31 Magnesium Alloy

Plasma electrolytic oxidation coatings were prepared in aluminate, phosphate, and silicate-based electrolytic solutions using a soft-sparking regime in a multi-frequency stepped process to compare the structure, corrosion, and wear characteristics of the obtained coatings on AZ31 magnesium alloy. The XRD results indicated that all coatings consist of MgO and MgF2, while specific products such as Mg2SiO4, MgSiO3, Mg2P2O7, and MgAl2O4 were also present in specimens based on the selected solution. Surface morphology of the obtained coatings was strongly affected by the electrolyte composition. Aluminate-containing coating showed volcano-like, nodular particles and craters distributed over the surface. Phosphate-containing coating presented a sintering-crater structure, with non-uniform distributions of micro-pores and micro-cracks. Silicate-containing coating exhibited a scaffold surface involving a network of numerous micro-pores and oxide granules. The aluminate-treated sample offered the highest corrosion resistance and the minimum wear rate (5 × 10−5 mm3 N−1 m−1), owing to its compact structure containing solely 1.75% relative porosity, which is the lowest value in comparison with other samples. The silicate-treated sample was degraded faster in long-term corrosion and wear tests due to its porous structure, and with more delay in the phosphate-containing coating due to its larger thickness (30 µm).

Mineralogical and geochemical characterization of fossil bones from a Miocene marine Konservat-Lagerstätte

Fossil bones, together with teeth, are the most common remains of vertebrates that could manage to get preserved over geological times, providing information on the diagenetic and fossilization processes that occurred in the depositional paleoenvironment. Fossil bones from the marine vertebrate Konservat-Lagerstätte in the East Pisco Basin and Sacaco area (Peru) show a high variety of different textural and chemical features, suggestive of different processes variably contributing to the fossilization path. At the macroscopic scale, bone samples can be grouped into six different categories on the basis of the color (red to gray to white) and hardness (which relates to the mineralization degree); a variety of case studies can be found between these categories. Microscopically, the original microstructure of the bone tissue, both compact and cancellous, is well preserved in all the studied samples, with differences in cavity fillings, distribution of microcracks, and presence of Fe oxides in the diverse bone types. The bone composition and mineralogy correspond to fluorapatite. Differences in color, mineralization degree and geochemistry can be interpreted in terms of different fossilization paths, from burial at the seafloor to exposure in the present-day desert environment. The fossilization paths are strongly conditioned by the factors controlling the interplay of the mechanisms of apatite dissolution-recrystallization and dolomite precipitation (formation of carbonate concretions) as well as the fixation of iron in finely disseminated sulfides in the very early stages of fossilization.

Laser induced micro-cracking of Zr-based metallic glass using 1011 W/m2 nano-pulses

Low power nanosecond laser irradiation has potential applications in surface processing of metallic glasses (MGs). However, the response of MGs to a relatively low power intensity (1011 W/m2) has not been investigated. In this study, surface microstructures and characteristics of Zr-based MG after low power nanosecond laser irradiation were studied. Micro-cracks with depth in nanoscale and width in microscale were observed on the irradiated MG surface. The effects of laser parameters on the formation and evolution of micro-cracks were further investigated, and surface characteristics were characterized by using X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDS). The results showed that under the peak laser power intensity of 2.3 × 1011 W/m2, when increasing the scanning speed from 1 to 5 mm/s, the maximum width of micro-cracks reduced from 3 μm to be less than 1 μm; further increasing to 10 mm/s, no micro-cracks appeared. When increasing the laser power intensity, the micro-groove structure would be formed with pileup around it and line cracks, open cracks and laser pulse tracks on it. During multi-line laser scanning, the pulse overlap rate also affected the formation and distribution of micro-cracks. At last, the formation mechanism of micro-cracks was discussed. These results would be meaningful for understanding the laser-MG interaction as well as guiding the selection of laser parameters for various applications.

Studies of stress and displacement distribution and the evolution law during rock failure process based on acoustic emission and microseismic monitoring

The evolutions of stress and deformation inside of rock is highly important in studies of the rock failure mechanism but is difficult to obtain via the traditional stress measurement methods due to limited measuring points. Thus, studies of the stress and displacement evolution within rock were conducted based on acoustic emission (AE) monitoring in laboratory experiments. The differences in the distributions and evolution characteristics of the stress field and deformation field before caving in a deep stope were also examined based on in-situ microseismic (MS) monitoring. The results show that the distributions of micro-cracks, apparent stress and deformation inside the rock are highly consistent and can reflect the spatial-temporal evolution characteristics of the stress field and deformation field within the rock. The accumulated apparent volume is more accurately than the strain to reflect the changes in inelastic deformation inside the rock. Based on in-situ MS monitoring, MS activities were found to be closely related to blasting disturbances during the caving process. Before a caving of the deep stope, the distributions of stress and displacement showed obvious differences, reflecting the loosening process of the rock mass. The non-uniformity and the differences in the stress and deformation can deepen the understanding of the rock (mass) failure process.

Analysis of the Crack Evolution Process in Crumb Rubber Concrete Based on Acoustic Emission Technology

In this study, the crack evolution process of crumb rubber concrete (CRC) in a four-point bending tensile test was monitored using the acoustic emission (AE) technique. The peak frequency, distribution characteristics of the AE source, and the damage process based on the AE ring count during the crack evolution were analyzed. The test results revealed that the addition of rubber particles decreased the brittleness of cracks and improved the ductility of the concrete during the macro-cracking process. The AE positioning method could effectively monitor the evolution process of micro-cracks inside the concrete. Additionally, based on the statistical results of the AE source events and energy distribution, the position of macroscopic cracks could be predicted, and the random distribution of micro-cracks could be characterized. By introducing the Weibull distribution to describe the random growth process of micro-cracks, the damage inside the concrete could be qualitatively predicted. As the rubber particle content increased, the inherent micro-cracks inside the concrete also increased. The damage inside the concrete was biased toward the crack nucleation stage.

Influence of Different Types of Small-Size Defects on Propagation of Macro-cracks in Brittle Materials

The presence of defects in the structure requires noticeable attention and understanding of fracture mechanisms in brittle materials has to be established. Defects in the form of holes, macro- and micro-cracks are the main interest of this paper. This work investigates the dual role of holes and micro-crack arrays on toughening and degradation mechanisms in concrete structures. An ordinary state-based peridynamics (PD) model is utilized to analyse the fracture problem at the micro-level. The application of PD shows its advantage in crack-hole, macro- and micro-crack interaction problems since PD can accurately predict the contribution of defects on structural behaviour. The study of the three-point bending problem with five types of holes existing in the structure showed the crack arrest phenomena at the hole boundary and the “attraction” of the crack to propagate towards the hole. For the study of the macro- and micro-cracks interaction problem, various cases of the micro-crack distribution and inclination angles are considered and validated with analytical studies. The PD quasi-static simulations show good agreement with analytical solutions. Moreover, PD dynamic solutions show the capability of PD to capture complex crack propagation paths. It is observed that the presence of micro-cracks and holes ahead of the main crack can suppress its further propagation as well as have an influence on the crack propagation direction. The numerical results demonstrate the efficiency of the PD modelling of multiple crack interaction problems.

Cyclic shock damage characteristics of electrohydraulic discharge shockwaves

To improve the quality of electrohydraulic discharge (EHD) shockwaves cracking effect and reduce its damage, the study examines the cumulative damage characteristics of rock induced by shockwaves generated by EHD. Voltage, current, energy, efficiency, wave velocity, and peak value of incident and transmission shockwave of a liquid–solid interface at different discharge energies are given based on the observation of the shockwaves of established discharge platform. The cyclic impact cracking test is conducted on cement samples. The cement samples are effectively cracked, and the surface formed several cracks. The tests results indicate that the cumulative effects of different cyclic loading modes exist under the action of EHD shockwaves. An upper limit exists on the cumulative effect, and the breakdown voltage affects the maximum effective number of discharges and affects the depth of action. Particle flow code 2D (PFC2D) simulation is used to simulate the cement samples under the same experimental conditions. The development process and shape distribution of the internal microcracks under different loading energies and loading times are analyzed. The results indicate that there are three cumulative effects of microcrack development stages (i.e. microcrack accumulation, cracks development, and slowing down and cessation of crack development). Simultaneously, the results suggest that the cumulative damage effect of EHD shockwave on rock materials can be divided into three zones (i.e. crushing zone, crack zone, and elastic zone), which can be effectively adjusted via controlling the loading voltage, loading times, and working distance. Finally, an experiment-simulation combined control strategy is proposed to avoid the crushing zone and raise the crack zone. The study provides theoretical, experimental and simulation guidance for the application of EHD shockwaves.

Damage estimation of concrete canal due to freeze and thawed effects by acoustic emission and X-ray CT methods

As a detailed inspection of a concrete structure in service, core samples are usually drilled out and then mechanical properties are measured. In this study, damage estimation of freeze-thawed concrete from concrete-core samples is developed, applying acoustic emission (AE) method. By the authors, the quantitative damage evaluation of concrete has been proposed in RILEM TC −212ACD, by applying AE and damage mechanics in the compression test. In this study, characteristics evaluation of cracking damage effects in concrete mechanical properties by detected AE energy in core test. Prior to the compression test, distribution of micro-cracks in a concrete-core sample is inspected by helical X-ray computer tomography (CT). Then, freeze-thawed damaged samples are tested by the compression test with AE monitoring. Concrete-core samples were taken out of a channel structures in Hokkaido, Japan which is extremely developed cracking damage. Thus, it is demonstrated that the concentration of material damage could be evaluated by comparing geometrical characteristics of cracks with the “energy rate” of AE generation, which is analyzed by AE parameter analysis. A relation between AE energy rate, physical properties and geometrical properties of void is correlated, and thus the damage of concrete is qualitatively estimated by AE parameters.

Numerical modeling on micro-to-macro evolution of crack network for concrete materials

Large quantities of microcracks existing in quasi-brittle materials like concrete can be graphically referred as crack network. Specifically, the network evolution includes microcrack growth and coalescence, macrocrack formation and propagation, and the final rupture. In this regard, the present work aims to formulate a numerical approach that is able to describe the whole process of crack network evolution for concrete materials. Particularly postulated is the stochastic distribution of microcracks in concrete, which coincides with the physical phenomenon in concrete. Highlighted are the growth and coalescence of microcracks, as well as the formation and propagation of macrocracks. A numerical case is carried out to study the concrete specimens that are subjected to tensile loading. The thorough process of crack network evolution is clearly simulated through the approach. Also investigated are the influence of aggregate volume fraction on concrete strength and ductility. The proposed approach provides a guide reference for concrete engineering practice, as well as modeling the complete process of concrete failure.

Simulation of chloride diffusion in fly ash and limestone-calcined clay cement (LC3) concretes and the influence of damage on service-life

Numerical simulation of chloride diffusion through high-performance binding systems such as limestone-calcined clay cement (LC3) and its comparison to control and fly ash modified concretes is discussed. The simulation framework considers the pore structure of concrete, the concentration-dependence of diffusion coefficient, and Freundlich binding. The LC3 concrete and the companion fly ash concrete exhibit similar service lives (~8× control mixture), despite the LC3 system having a reduced clinker factor than the fly ash concrete (~0.5, as opposed to 0.7). The diffusion model is augmented with a scalar isotropic damage variable that accounts for random distribution of microcracks under fatigue loading (e.g., in a bridge deck). The impact of damage on service life at different stress levels, for the different concretes is elucidated. The modeling approach can be used to evaluate the influence of binder composition and damage on effective service life of chloride-exposed concrete structures, thereby aiding in binder selection.

Changes in shear-wave splitting at the 2014 Bárðarbunga seismic crisis and dyke intrusion in Iceland compared with earthquakes and other eruptions

We use shear-wave splitting (SWS) above small earthquakes to monitor stress-variations before the 2014 Bárðarbunga seismic crisis and dyke intrusion in central Iceland. SWS is sensitive to stress-induced variations in the internal geometry of the distributions of stress-aligned fluid-saturated microcracks pervading almost all crustal rocks. This allows stress-accumulation before both impending earthquakes and impending volcanic eruptions to be monitored and potentially the events stress-forecast. We compare the behaviour of SWS before Bárðarbunga with the behaviour before large earthquakes and other volcanic episodes where SWS has been monitored. On the basis of the duration of stress-changes, we classify volcanic episodes into three broad classes. During analysis of SWS before Bárðarbunga, temporal variation in polarizations and another retrospective stress-forecast earthquake were recognized.

Strength and ultrasonic properties of cemented waste rock backfill considering confining pressure, dosage and particle size effects

It is of great significance to study the material properties of cemented waste rock backfill (CWRB) for preventing mining damages, solving environmental problems and ensuring engineering security. Consequently, this paper carried out the comprehensively factorial tests to investigate the effects of the confining pressure, dosage of cementitious material and particle size distribution (PSD) of aggregate particles on the strength and ultrasonic properties of CWRB, for which the PSD of aggregate particles obeyed Talbot gradation theory. The results show that the relations between the confining pressure and dosage of cementitious material and the compressive strength and ultrasonic pulse velocity (UPV) of CWRB present the positively linear functions. However, the relations between the PSD of aggregate particles and the UPV, compressive strength, cohesive force and internal friction angle of CWRB perform the quadratic functions. It is considered that there is an optimal PSD of aggregate particles in the CWRB for characterizing its optimum material properties, and using the gradation Talbot index for describing that optimal PSD is between 0.4 and 0.6. The microstructure characteristics show that the CWRB with finer PSD of aggregate particles produces more distributions of microvoids and microcracks, while the CWRB with coarser PSD of aggregate particles contains coarser microvoids and microcracks that cannot be completely filled by hydration products and secondary hydration products. However, the dense hydration products can be formed among the particles in the CWRB with a suitable PSD of aggregate particles to reduce these defects. Its mechanism is considered that the superior PSD of aggregate particles can improve the microstructure of CWRB including the scale and distribution of defects, thereby resulting in the improvements of its strength and ultrasonic properties.

Numerical Investigation and Analysis of Intermediate Principal Stress Effects on Rock Failure Behaviors

A series of numerical experiments have been conducted to investigate the intermediate principal stress effects on rock failure behaviors. The numerical results show that the strength and deformation of the rock samples are significantly affected by the intermediate principal stress. The effects are inconsistent in different intervals. As intermediate principal stress ratio b increases, the rock strength increases initially and finally decreases. When b is approximately equal to 0.5, the strength of the rock sample reaches the maximum. In the microlevel, the intermediate principal stress affects the number and distribution of microcracks. The increase of the intermediate principal stress makes the projection of the microcracks on the loading plane change from uniform to uneven. On the one hand, the intermediate principal stress restricts the propagation of microcracks in the normal direction along the intermediate principal stress (or with a component in this direction), which will lead to an increase in the strength of the rock samples. On the other hand, the propagation of microcracks along the normal direction with small principal stress (or with a component in this direction) is promoted, which leads to a decrease in the strength of the rock sample. End friction can make the intermediate principal stress effect more significant because the friction of the loading end to the rock sample can result in stress deviation between the actual value and experimental value. Inhomogeneity of stress field induced by the change of stress states or end friction forces is the external factor of the intermediate principal stress effect. Also, the inhomogeneity of rock material itself is the internal factor. Intermediate principal stress will promote or restrict the failure of certain directions, thus affecting the overall strength of the rock samples. The numerical results can be very meaningful for stability analysis of rock masses in practical engineering.

果蔬果皮拉伸力学性质与微观结构的研究

Fruit and vegetable peels exert a protective effect on fruits as constituent parts of the outermost tissue and their properties are of great importance to reducing fruit and vegetable mechanical injury. Four kinds of fruit and vegetable peels such as Nagafu apple, Crisp pear, Tainong mango and long eggplant were chosen to perform longitudinal and transverse tests of tensile property by means of electronic universal testing machine. Stress-strain curve, tensile strength, elastic modulus and fracture strain of peels were obtained; and the microstructures of four kinds of peels were scanned using an electron microscope (SEM). The results indicated that cubic polynomials proved superior for quantifying the stress-strain non-linear relationship of peels and the fitting error of tensile strength is less than 10 parts per thousand. Tensile strength, elastic modulus and fracture strain of peels were different in the case of different fruits and vegetables cultivated and different parts of the same peel; fruit and vegetable peels belong to anisotropic heterogeneous materials and have certain strength. The mean values of tensile strength and fracture strain of the long eggplant peel are the biggest in four kinds of peels and that of elastic modulus of Nagafu apple peel is the largest; long eggplant and Nagafu apple peels had better resistance to damage sensibility than Crisp pear peel. The bearing capacity of the peels depends on the number, width and distribution of microcracks on the surface, and the shape of the epidermal cells and fruit dot on peels; the number of microcracks is bigger and the width of microcracks is wider, the tensile strength is smaller and the elastic modulus of peel is bigger with the slippage increase of epidermis cells. This study provides basic technical parameters for mechanical equipment design for fruit and vegetable during harvesting, processing, packaging, storing and transporting and builds the correlations between macro-mechanics properties and microstructures of fruit and vegetable peels.

The plastic flow model in the healing process of internal microcracks in pre-deformed TC4 sheet by pulse current

In this paper, the distribution of microcracks in titanium alloys (TC4) before and after electropulsing treatment (EPT) was observed by X-ray microscopy (XRM). The results show that EPT has obvious healing effect on internal microcracks in the TC4 tensile specimen. Based on Mises yield criterion and the results of finite element analysis in the internal microcrack healing process in titanium alloy, the elastic-plastic deformation characteristics during microcrack healing were analyzed. The results show that the plastic deformation around the microcrack is nonuniform due to the uneven distribution of temperature and thermal compressive stress during EPT. The region with large plastic deformation looks like a thick-walled ellipsoid-shaped ring surrounding the microcrack, where the elongation of the material along the normal direction of the microcrack surface is the main cause of the microcrack closure. The deformation ring plastic flow model was proposed to explain the microcrack closure mechanism by pulse current.