Orientation Of Microcracks Research Articles

Leveraging photosensitive and thermally stable luminescent Ba2ZnWO6:Eu3+, M+ (M+= Na, K , and Li) nanophosphor for targeted non-invasive and stain-free visualization of cracked tooth syndrome

The cracked tooth syndrome poses a significant challenge in dentistry, thereafter untreated cases often lead to severe complications, such as pulpitis or complete tooth fracture, ultimately contributing to tooth loss. However, the conventional diagnostic methods to visualize microcracks in the tooth suffer from severe drawbacks, such as inaccurate cold stimulation, discomfort with probing, impractical staining techniques, and difficulty in distinguishing harmless craze lines from pathological cracks. To address this challenge, for the first time, we are proposing a novel approach by utilizing luminescent Ba2ZnWO6:Eu3+ (3 mol %), K+ (1 wt %) nanophosphor for improved imaging and diagnosis of cracked tooth syndrome. Herein, the double perovskite structured Ba2ZnWO6:Eu3+ (1–11 mol %), M+ (M+ = Na, K, and Li (1 wt %)) nanophosphors were synthesized via the sonochemical route. The photoluminescence emission spectra of the prepared Ba2ZnWO6:Eu3+ (1–11 mol %) nanophosphors displaying distinct peaks at 583, 595, 613, 662, and 720 nm, which ascribed to transitions from state 5D0 to 7FJ (J = 1–4) state of the Eu3+ ions, respectively. By adopting a strategic charge compensation mechanism, the enhancement in the luminescence emission intensity of about 1.5-fold was achieved after co-doping K+ (1 wt %) with Ba2ZnWO6:Eu3+ (3 mol %) nanophosphor. The photometric studies of the phosphors portray their orange-red emission with excellent quantum efficiency (82.52 %), and color purity (∼ 99 %). The emission intensity was sustained up to 73.71 % at 473 K, indicating excellent thermal stability of the phosphor. The in vitro cytotoxicity assessments of the optimized nanophosphor demonstrated its biocompatibility on normal non-malignant oral fibroblasts. The visualized microcracks in the tooth using optimized Ba2ZnWO6:Eu3+ (3 mol %), K+ (1 wt %) nanophosphor under UV excitation of UV 365 and 395 nm light revealed the orientation of microcracks, crack width, depth of the crack, and microcrack branching without any stain. The aforementioned results demonstrated that the proposed methodology paves the way for a new avenue in dental imaging technology with the potential to revolutionize and improve patient care outcomes.

Dynamic fracture mechanism from a pre-existing flaw in granite: Insights from grain-based modelling

Both heterogeneity and pre-existing flaws are closely associated with the mechanical response of crystalline granite. To uncover the underlying mechanism, grain-based model (GBM) based on particle flow code (PFC), i.e., PFC-GBM, with different contacts between any two grains, was employed to carry out numerical calculations to investigate the effect of a single flaw with various inclination angles on the dynamic behavior of LdB granite. The failure pattern is identified through density and spatial distributions of microcracks, and the primary fracture directions are obtained by microcrack orientations. Microcracks are divided into inter-Grain and intra-Grain types, and these two types are further classified into more subtypes according to mineral composition. IG cracks typically form during the pre-peak stage and tend to emerge earlier than TG cracks. Conversely, TG cracks are predominantly created during the post-peak stage but constitute a significant proportion irrespective of flaw inclination angle. A novel method is also proposed to acquire high-stress particles, which promote the identification of dominant macrocrack paths. Finally, the effect of particle size is also systemically analyzed.

Azimuthal pore pressure response to teleseismic waves: effects of damage and stress anisotropy

SUMMARY Pore pressure oscillations induced by stress variations, including propagating seismic waves from remote earthquakes, have been widely observed in various groundwater systems. The monitored pressure change in wells shows significant water-level oscillations to volumetric strain as well as to S and Love waves. Recent observations demonstrated azimuthal dependence of the pore pressure oscillations with respect to stress indicators and fault zone orientation. Within the fault zone, damage-induced anisotropy is the result of the alignment and orientation of cracks and other internal flaws within the rock. In this work, we provide a complete quantitative description of the pore pressure changes induced by passing seismic waves associated with different orientations and values of principal stress and damage tensor components. The model quantifies the azimuthal dependence of the pore pressure response by a non-dimensional ratio defined as the amplitude of the pressure oscillations induced by a shear strain normalized to the volumetric strain. Three angles and two values are needed to calculate the azimuthal dependence of the pore pressure response: the angle between the directions of the maximum horizontal stress and the seismic event; fault zone orientation; microcrack orientation within the fault zone; and damage and stress values. The model predicts that maximum pore pressure response occurs when microcracks and maximum horizontal stress are in the same orientation, high damage and high stress anisotropy. By adjusting these quantities, we recalculate results of recent seismological studies in the Arbuckle disposal well, Osage County, Oklahoma. The presented model successfully predicts the observed azimuthal dependence in wave-induced fluid pressure response and relates the anisotropic response to tectonic indicators such as the orientations of the maximum horizontal stress, fault zone, and microfractures.

Development of Micro-mechanical Constitutive Model for Alumina at High Strain Rates Using Unified Mechanics Theory

Ceramic materials used in mechanical applications show variations in their properties due to the presence of cracks. Micro-cracks within the material (size, orientation and density) affect the ceramic material’s strength and other mechanical properties. This study developed a micro-mechanics-based model that accounts for the original orientation of micro-cracks and their propagation as wing cracks. Unlike other micromechanics-based models, the current model defines failure based on entropy associated with crack propagation within the material. Entropy is calculated from energy dissipation from crack propagation from the pre-existing flaws in the ceramic. The Unified Mechanics Theory (UMT) is used to define entropy-based damage in the ceramic material, in which a parameter called thermodynamic state index (TSI) is employed to describe the state of the material. A representative volume element (RVE) with a pre-existing flaw is used to calculate the energy dissipated during the wing crack propagation. The effect of various crack lengths and orientations is incorporated with a probability density function. The strain rate effects are implemented using dynamic crack growth law. The stress-strain curve at strain rate from quasi-static to high strain rate (10-3-106) is plotted for Alumina under dynamic compression.

Investigation on the fatigue behavior of 7075 aluminum alloy at different aging states

To study the effect of the aging state on the low-cycle fatigue (LCF) performance of 7075 Al alloy, LCF experiments with different strain amplitudes (Δε/2 = 0.5%- Δε/2 = 0.9%) were conducted for underaged (UA), peak-aging (PA) and overaged (OA) states with similar tensile properties in this work. The effect of the aging state on the cyclic stress response behavior and the LCF lives of 7075 Al alloy was revealed by observing the dislocation configuration after LCF. The microcrack orientation on the surface of LCF samples was observed by the electron backscatter diffraction (EBSD) technique, and the results showed that the soft orientation inside the coarse grains was the main cause of fatigue cracking. Combined with the previous studies, we summarized the dominant factors for the LCF and high-cycle fatigue (HCF) properties of 7075 Al alloys at different aging states, i.e., the LCF properties were determined by tensile properties, while the HCF properties were dominated by dislocation slip mode.

Numerical simulation of effective diffusivity in concrete with random microcracks

Cracks in concrete structures can provide preferential transport pathways for the ingress of aggressive media. Therefore, it is important to understand the relationship between cracks and the durability properties of concrete. Numerical modeling is an efficient and practical means of investigating the effect of cracks, especially microcracks, on the diffusivity of concrete. In this study, a modeling scheme was developed to predict the effective diffusivity of cracked concrete. Numerical samples containing randomly overlapping and nonoverlapping microcracks were generated, and the effects of microcrack geometrical parameters were investigated. The results showed a critical area fraction of microcracks in the diffusion process, which was related to the aspect ratio, orientation, and tortuosity of microcracks. The effective diffusivity was found to be closely related to the crack orientation and tortuosity. Moreover, the tortuosity of nonoverlapping microcracks was observed to be greater than that of overlapping microcracks at the same area fraction.

Ultrasonic inverse identification of surface integrity of aviation functional coatings through material-oriented regularization

Accurately controlling surface integrity of aviation functional coatings including defect, geometry, structure, and property is important to balance the comprehensive performance of aviation parts. In this paper, a new ultrasonic inverse method of surface integrity of aviation functional coatings is presented through a developed material-oriented regularization scheme, which combines model regularization, data regularization, and sensitivity matrix analysis. A sensitivity-matrix-based inversion method of ultrasonic detecting multiple surface integrity parameters is developed using the optimized high-sensitivity characteristic detection signal. Taking ultrasonic inverse detection of elastic constants of plasma-sprayed Al2O3 coating and inverse prediction of porosity of ZrO2 coating as examples, the validity of proposed ultrasonic inversion method is illustrated. Results show that the inverted elastic constants C11, C13, C33, and C44 of Al2O3 coating are 123.81 GPa, 34.38 GPa, 190.20 GPa, and 24.63 GPa, respectively. The relative error between inverted and experimental C33 and C44 are all less than 4.0%. The inverted elastic constants show obvious “inverse” elastic anisotropy, which is due to the preferred orientation of micro-cracks in Al2O3 coating. Compared with actual porosity of ZrO2 coating, the predicted porosity using sensitivity matrix optimized Gaussian process regression algorithm has a relative error of less than 5.3%. The proposed ultrasonic inverse method provides a new path for solving the complex ultrasonic non-destructive detecting problems, which are difficult for traditional “trial-and-error” ultrasonic methods.

Characterization of Fracture Pattern of Indian Coal Measure Rock Under Uniaxial Compression Stress by Statistical Analysis of Fractal Dimension of the Microcrack Orientation

Characterization of Fracture Pattern of Indian Coal Measure Rock Under Uniaxial Compression Stress by Statistical Analysis of Fractal Dimension of the Microcrack Orientation

Genipin does not reduce the initiation or propagation of microcracks in collagen networks of cartilage

ObjectiveWe recently initiated microcracks, i.e. micron-scale cracks in the collagen networks of cartilage, using both single low-energy impacts and unconfined, cyclic compressions. We also tracked the propagation of microcracks after cyclic compressions simulating 12,000 walking strides. In this study, we aimed to determine the effect of one or more genipin treatments on: (1) the initiation of microcracks under mechanical impacts and (2) the subsequent propagation of microcracks under cyclic, unconfined compression. We hypothesized that treatments with genipin would improve the resistance of cartilage to microdamage, specifically reducing both the initiation of microcracks under impact loading and the propagation of microcracks under cyclic compression. DesignWe tested 49 full-thickness, cylindrical osteochondral specimens. We incorporated one or two doses of genipin in between mechanical treatments, i.e. single low-energy mechanical impacts to initiate microcracks and unconfined, cyclic compressions to propagate microcracks. We also imaged specimens using second harmonic generation confocal microscopy, and analyzed the resulting images to quantify changes in morphologies (length, width, and depth) and orientations of microcracks. Finally, we used separate mixed-regression modeling to evaluate the effects of genipin treatments on mechanically induced microcracks. ResultsSpecimens treated with genipin presented significantly longer and marginally deeper microcracks after mechanical impacts. Two doses of genipin caused significantly longer and wider microcracks under propagation verses one dose. ConclusionsOur results do not support our hypothesis: unfortunately treatments with genipin, and the resulting mechanisms of cross-linking, do not provide resistance to microdamage, quantified as the initiation and propagation of microcracks.

Mathematical modeling of pyroxene-magnetite crystalline shales elastic and acoustic properties

The analysis of the results of mathematical modeling of the influence of the format, mineral concentration and fracture of metamorphic crystalline shales of the Pishcha iron ore structure is presented. The aim of this work is to analyze the influence of mineral composition, types, orientation and concentration of mineral inclusions and microcracks on the acoustic and elastic properties of a group of samples of “quartz-magnetite-pyroxene” crystalline shales of Pishchans’ka iron ore structure. Based on the method of conditional moments, mathematical modeling of the influence of the format, orientation and content of mineral grains, as well as the concentration and format of cracking on the acoustic and elastic properties of rocks of the Pishchans`ka iron ore structure was performed. According to the obtained data, a weak effect of changes in the content of rock-forming minerals and a significant effect of different types of fractures on the value of elastic and acoustic anisotropy (10-40%) was proved. Elastic constants of models with layered and chaotic orientation of structural-textural elements are calculated. It is established that most models, as well as basic samples have a rhombic type of acoustic symmetry. When comparing the stereoprojections of the anisotropy parameters of real samples with the stereoprojections obtained during modeling, the authors found that in most samples there is a double system of cracking: chaotic and directed in the area of shale. The results of mathematical modeling showed that for models with ordered crack orientation, the change in the format and concentration of voids is a defining characteristic. This effect is significantly smaller for models with a chaotic arrangement of structural elements. It is proved that models with a combined (layered and chaotically oriented) type of fracture are the closest to real samples. The authors show that this technique allows you to create and operate models close to the real geological environment.

Effects of porosity, orientation and connectivity of microcracks on dispersion and attenuation of fluid-saturated rocks using an upscaling numerical modelling of the squirt flow mechanism

Squirt flow is a wave-induced fluid flow mechanism to account for the velocity dispersion and attenuation of fluid-saturated porous media. Theoretical models of squirt flow cannot deal with complex microcrack-pore networks and thus cannot give accurate dispersion and attenuation curves with respect to frequency. We adopt a numerical oscillatory compressibility test method, based on the quasi-static poroelastic equation of Biot, to model the squirt flow and calculate corresponding P-wave modulus dispersion and attenuation. We also designed nine porous medium models containing different crack-pore configurations and investigated the effects of microcrack porosity, orientation and connectivity on the elastic responses. Modelling results show that not only microcrack porosity but also their orientation has control on the magnitude of the P-wave modulus dispersion and attenuation. Preferentially aligned microcracks may cause frequency-dependent anisotropy in the P-wave modulus and attenuation. Microcrack connectivity has a negligible influence on the dispersion and attenuation magnitude but a large influence on the characteristic frequency of the squirt flow, which is not predicted by theoretical models based on simple representative element volume of crack-pore structure. Therefore, detailed geometry of crack-pore network, of which microcrack porosity, orientation, aspect ratio and connectivity are vital factors, dictates the unique variations in the elastic modulus and associated attenuation with frequency caused by the squirt flow. Pore structure obtained by CT scanning representative of that of the whole rock is needed to obtain more accurate poroelastic responses of the rock with the numerical oscillatory compressibility test method. In this regard, we provide a powerful tool for evaluating dispersion and attenuation of fluid-saturated rock media with complex crack-pore structure, which have potential applications in seismic exploration of hydrocarbon in the subsurface.

Damage Modeling of Solid Oxide Fuel Cells Accounting for Redox Effects

This paper applies a multiscale mechanistic damage model developed for porous brittle ceramics and implemented in finite element analysis (FEA) packages [1] to study progressive damage in solid oxide fuel cells (SOFC) subjected to thermomechanical loading under operating and shutdown states. The damage model captures the micromechanics of stiffness reduction due to material porosity change and microcracking and integrates the as-obtained stiffness reduction law into a continuum damage mechanics (CDM) formulation for the evolution of microcracks up to fracture. An Eshelby-Mori-Tanaka approach (EMTA) [2-3] is first used to model the ceramic containing pores and aligned microcracks regarded as inclusions with negligible stiffness. The stiffness of the actual porous ceramic is obtained from the EMTA solution averaged over all possible microcrack orientations using an orientation averaging method. The evolution of microcracking damage is described by a CDM formulation derived from our previous model [4]. Apart from the operating and shutdown states, additional damage that may occur due to reduction and re-oxidation (redox cycling) of Ni/YSZ anodes is also studied. The reduction of NiO into Ni leads to changes in volume and porosity of the anode layer that consequently change its physical and thermomechanical properties. The volume change or volumetric “swelling” during redox cycling may affect the structural integrity of the anode and may cause fracture of the electrolyte under high tensile stresses leading to operational failure for SOFCs. In the constitutive relations, volumetric swelling is treated in a similar way to thermal expansion using the models developed in Refs [5-6] for fusion energy ceramic materials. However, while thermal expansion vanishes if temperature change tends to zero, swelling is irreversible and significantly contributes to degrade SOFC stacks during redox cycling.This damage model was implemented in the commercial FEA software ABAQUS© and ANSYS© via user subroutines. First, it has been validated through predictions of strength and stress-strain response for the typical SOFC ceramic materials. Figs 1(a) and 1(b) show the predicted stress-strain responses as a function of temperature for dense NiO/YSZ and Ni/YSZ, respectively. These figures also illustrate the predicted responses at 750°C with prescribed initial values of porosity. The damage model predicts significant reductions in strength and elastic modulus with higher initial porosity and higher temperatures, and the predicted trends are in good agreement with the experimental findings [7-8].Next, the damage model was applied to predict the potential for degradation in a generic planar SOFC stack with large active area cells modeled in ANSYS. Stack models with single and multi-cells were simulated in both co-flow and counter-flow configurations. The thermomechanical loads representing the operating and shutdown conditions were simulated. The realistic thermal gradients that occur in an operating stack were imposed in the models by solving the electrochemistry under various operating conditions using the in-house SOFC multiphysics code (SOFC MP-3D) [9-10]. In addition to the operating and shutdown conditions, a constant temperature redox cycle was also simulated to capture overall cell electrode damage due to volumetric swelling of the Ni/YSZ anode in the anode supported cells. The results from single cell stack models showed little damage at operating and shutdown conditions which could be attributed to enough out-of-plane support for the PEN assembly. The results from the multicell stacks (Fig. 2) showed higher damage in counter-flow configuration. Within the stack, the top cells experienced the highest damage and the middle cells experienced the lowest. Overall, no significant damage that may lead to total failure of the cells was observed in the multicell stack models with the operating conditions considered in this study.

Statistical analysis of defects within concrete under elevated temperatures based on SEM image

Concrete is a multiphase composite material consisting of aggregates, mortar, and initial defects. This work aims to quantitatively study the distribution and evolution of initial defects under elevated temperatures. To minimize the effect of hydration process and thermal and endogenous shrinkage, concrete samples cured in water for 12 months is tested. In order to study the effect of external load on the initial distribution of defects within concrete, concrete samples without pre-loading and pre-applied with 30% of the failure load are selected as research objects. The evolution of defects within the concrete samples are characterized using scanning electron microscopy (SEM). By employing an image processing software, the two-dimensional configurations of defects are extracted from the backscattered images obtained by SEM. Each defect is identified and approximated using an optimal ellipse. Thereafter, the defects are automatically recognized and classified into microcracks and pores by utilizing the image processing software and MATLAB programming. Consequently, the distribution and propagation of microcracks and pores in concrete could be quantitatively and separately analyzed. Statistical analysis of area, perimeter, and fitting ellipse parameters (e.g., major axis, minor axis, coordinates of the center, axis orientation, aspect ratio, and solidity) can provide the distribution of initial defects within concrete. It is demonstrated that the area of microcracks consistently increases with temperature. Log-normal and normal distributions may be suitable for the distribution of aspect ratio and solidity of microcracks, respectively, for both concrete samples with and without pre-applied loading under all elevated temperatures (40, 105, 150, 200, and 250 ℃). However, the distributions of area, coordinates of the center, and axis orientation of microcracks may vary with the initial concrete loading condition and temperature. For the pre-loaded concrete sample, the distribution of parameters does not conform to any type of distributions. Moreover, for the sample without pre-loading, the distributions of area, coordinates of the center, and axis orientation of microcracks exhibit good fitting precisions with log-normal, normal, and normal distributions, respectively at lower temperatures (40–150 ℃); however, such relationships do not apply at higher temperatures (200 and 250 ℃). As reference for identifying the variations in microstructures, 150 ℃ is selected as the threshold temperature. The statistical approach employed in this study can lay the theoretical and experimental bases for determining the microstructure evolution in concrete. It can also be used for identifying the meso-defect and macro-defect and establishing their relationship with concrete properties.

Progressive Damage Behaviours of Triaxially Confined Rocks under Multiple Dynamic Loads

Investigation of rock progressive damage under static confinement and strain rates facilitates the generation mechanism of natural fault damage zones. A triaxial Hopkinson bar apparatus is used to perform dynamic triaxial compression tests to examine the damage and degradation process of rocks subjected to multiple impacts. Dynamic mechanical properties are determined under a static triaxial pre-stress of (30, 20, 10) MPa and multiple dynamic loadings, with the repetitive impact velocity of 27 m/s and strain rates from 50 to 150/s. The acoustic characteristics are identified by ultrasonic measurement to qualify the damage values. The micro-crack parameters, including crack area and volumes are detected using synchrotron X-ray micro-computed tomography (μCT) to characterize the progressive damage. In addition, the microcrack orientation, density and fractal dimension are analysed from thin section. Experimental results show that dynamic stress-strain curves can be divided to elastic, nonlinear deformation and unloading phases. Dynamic peak stress, Young’s modulus and ultrasonic wave velocity decrease with increasing impact times. The high frequency of ultrasonic wave is filtered by the induced microcracks. The progressive damage and evolution of fracture networks are associated highly with microcrack initiation, propagation, branching and coalescence. Shear bands are commonly generated in granite, and tensile cracks are dominant in marble, while sandstone is mainly failed by compaction and deformation band. The absorbed energy of rock increases nonlinearly with increasing crack surface and volume. Besides, microcracks propagate primarily along the maximum principal stress; the density and fractal dimension exhibit an anisotropic distribution controlled by true triaxial confinement and dynamic impacts.

Stress level monitoring using SWS measurements of local earthquakes of Garhwal Himalaya

Chamoli district of Garhwal Himalaya is one of the highly seismically active region and probable zone for impending great earthquake in the CSG. In the present study, to understand the upper crustal anisotropy of Chamoli region, shear wave splitting (SWS) analysis is performed for local seismic events. Forevents > 2 MreportedatGhuttu station during the period Nov 2014 to JULY 2015, SWSparameters aremeasured. A considerable temporal difference in delay time was observed during the SWS study with respect to the 4.9Mw magnitude earthquake that occurred in the Chamoli region on 1st April 2015 at 21:23 hrs (UTC). A rise in delay time prior to the earthquake which decreases steadily over time is observed at GTU station. This decrease in delay time can be related to the region’s stress relief. The calculated average delay time (δt) is 3.83 ms/km and a maximum value of 5.37 ms/km for the earthquake of magnitude 4.9Mw. Sfast polarization direction range from N120° to N170° has been observed for the majority of the earthquakes. An average NW-SE orientation of the shallow subsurface micro-cracks along the ray path beneath the Chamoli region has also observed. Present study is a step towards earthquake precursory research in Garhwal-Kumaun region for monitoring of the state of accumulated stress. This study clearly demonstrate that SWS measurements for local earthquakes can be utilized for continuous monitoring of temporal variation in time delay (δt) and probably used for stress forecast for an impending earthquake.

Characterization of Microcrack Orientation Using the Directivity of Secondary Sound Source Induced by an Incident Ultrasonic Transverse Wave

In this paper, characterization of the orientation of a microcrack is quantitatively investigated using the directivity of second harmonic radiated by the secondary sound source (SSS) induced by the nonlinear interaction between an incident ultrasonic transverse wave (UTW) and a microcrack. To this end, a two-dimensional finite element (FE) model is established based on the bilinear stress–strain constitutive relation. Under the modulation of contact acoustic nonlinearity (CAN) to the incident UTW impinging on the microcrack examined, the microcrack itself is treated as a SSS radiating the second harmonic. Thus, the directivity of the second harmonic radiated by the SSS is inherently related to the microcrack itself, including its orientation. Furthermore, the effects of the stiffness difference between the compressive and tensile phases in the bilinear stress–strain model, and the UTW driving frequency, as well as the radius of the sensing circle on the SSS directivity are discussed. The FE results show that the directivity pattern of the second harmonic radiated by the SSS is closely associated with the microcrack orientation, through which the microcrack orientation can be characterized without requiring a baseline signal. It is also found that the SSS directivity varies sensitively with the driving frequency of the incident UTW, while it is insensitive to the stiffness difference between the compressive and tensile phases in the bilinear stress–strain model and the radius of the sensing circle. The results obtained here demonstrate that the orientation of a microcrack can be characterized using the directivity of the SSS induced by the interaction between the incident UTW and the microcrack.

Study of microstructure effect on the nonlinear mechanical behavior and failure process of rock using an image-based-FDEM model

Abstract Rock microstructures, including the mineral heterogeneity and initial microcracks, not only affect its failure process but also influence its nonlinear mechanical behavior. However, in previous studies the rock microstructures especially the width of initial microcracks are usually unreasonably considered, which as a result fail to capture the rock realistic nonlinear mechanical behaviors (such as the gradual crack closure characteristic) and failure process. In this study, an image-based model considering the rock microstructures is developed by an FDEM scheme. The rock microstructures are characterized by a series of image process and then are incorporated into the numerical model. The width of the initial microcracks are considered in the model through inserted cohesive elements with certain thickness. The proposed model is firstly validated, based on which the influences of microcrack density, microcrack width and microcrack orientation on rock mechanical behavior are then discussed. The numerical results indicate that the initial microcrack has a larger influence on the stress distribution than the mineral heterogeneity and its width is a key factor affecting rock crack closure behavior.

Anisotropic influence of fracture toughness on loading rate dependency for granitic rocks

Granite is a significantly anisotropic rock that is affected by the pre-existing microcrack distribution. In rock engineering applications in a variety of fields, it is crucial to understand the influence of the internal microcrack distribution on the mechanical properties of the rock. In this paper, static and dynamic fracture toughness tests were conducted with straight notched disc bend (SNDB) specimens of Youngju granite to investigate the relationship between the microcrack-induced fracture toughness anisotropy and the loading rate dependency. Three groups of specimens were manufactured with three different orthogonal directions, which were determined by the P-wave velocity measurement. In order to conduct the static and dynamic fracture toughness tests, a servo-controlled Material Testing System (MTS) machine and a split Hopkinson pressure bar (SHPB) apparatus were respectively used. In the dynamic test, to achieve the stress equilibrium state in the measurement, a careful pulse shaping technique with a copper and rubber pulse shaper was used. The results of the entire range of tests in this study revealed that the fracture toughness of Youngju granite has a clear loading rate dependency. Further, we found that the preferred orientation of pre-existing microcracks causing strong anisotropy of fracture toughness under static loading rates became significantly weaker in the dynamic loading state. Finally, a possible reason for this relationship between the fracture toughness anisotropy and the loading rate dependency is discussed with different fracturing processes of rock under the static and the dynamic loading states using image analysis for generated cracks.

Crack Evolution in Damage Stress Thresholds in Different Minerals of Granite Rock

Crack evolution in a rock depends on the mineralogy, microstructure and fabric of specific rock type. This study aims to investigate how mineralogy and grain shape affect the microcrack initiation and propagation of granite rock, which contains plagioclase, quartz, k-feldspar, biotite and amphibole, during uniaxial compression loading. Physico-mechanical properties and microcrack features such as linear microcrack density (LMD) and microcrack type were investigated. By acoustic emission (AE) measurements, damage stress thresholds were identified. Then, crack characteristics of a fresh sample were compared with the samples that were loaded until damage threshold stresses. The results demonstrate that at an early stage of loading, pre-existing microcracks growth and LMD of intergranular crack increase. In the elastic phase, all microcracks types increase at the same rate. When the loading reaches to the strength limit of the sample, the total LMD reduced, because cracks start to coalesce and form new and large transgranular microcrack. Investigating crack propagation in mineral shows that at first, microcrack generates in biotite and at last in quartz. Plagioclase has the highest LMD and microcracks usually formed within the cleavage plane, but alteration and inclusions of tiny minerals can drastically change the LMD and orientation of microcracks. Biotite can terminate or let the microcrack to pass through the crystal based on the orientation of the microcrack plane and cleavage microcrack within the mineral. Furthermore, crystals with an aspect ratio higher than two have higher LMD. By getting close to the uniaxial compression strength, more microcracks appear close to the grain boundary which increases the circularity of the grain.

Numerical Analysis of the Nonlinear Interactions Between Lamb Waves and Microcracks in Plate

In this paper, three-dimensional finite-element modeling is conducted to investigate the nonlinear interactions between Lamb waves and microcracks. The simulation research focuses on the influence of microcrack orientation on the propagation direction of generated sum-frequency Lamb waves. The simulation results show that the resonant conditions based on classical nonlinear theory are valid for such interactions, leading to the generation of transmitted and reflected sum-frequency S0 waves (SFSWs). Moreover, the propagation directions of these two SFSWs exhibit different trends with respect to the orientations of microcracks. The transmitted SFSW can be used to detect microcracks, whereas the reflected one can be used to measure their orientations.