Effects Of Anisotropic Stress Research Articles

Mechanical properties and dynamic multifractal characteristics of shale under anisotropic stress using AE technology

The mechanical properties and failure mechanism of shale under anisotropic stress is the key to evaluate the stability of wellbore. Meanwhile, the fracture instability process of shale needs to be characterized by the reasonable monitoring technology. This paper performs a series of uniaxial, biaxial and true triaxial loading tests on shale specimens. A monitoring technology, Acoustic emission (AE), is adopted to record the fracture process of shale. The effects of anisotropic stress on the mechanical behaviors of shale are analyzed. The evolution characteristics of AE event rate are applied to describe the fracture process of shale. The transition from AE quite period to AE active period effectively characterizes the macro-cracking process within shale specimens. The multifractal theory is adopted to analyze the dynamic multifractal characteristics and to quantitatively characterize the fracture instability process of shale. The distribution difference of multifractal parameters can be served as an early warning information for the stable and unstable macro-cracking within shale. The anisotropic stress evidently affects the failure mechanism of shale. The results of this study will be important to ensure the production efficiency of shale gas and the stability of drilling operation.

Cyclic resistance of fly ash influenced by anisotropic stress condition, sand contents, and gravel content

A series of cyclic triaxial tests were performed to study the effect of anisotropic stress, sand content (in a fly ash-sand mixture), and addition of gravel on the cyclic resistance of fly ash. The results indicated that the failure mode of pure fly ash in the presence of isotropic stress was cyclic mobility and that the cyclic resistance of pure fly ash was significantly lower than that of clean Nakdong River sand for given test conditions. The cyclic resistance of pure fly ash decreased when the effective confining stress increased from 100 to 200 kPa, as expected. The failure of pure fly ash conducted in the absence of stress reversal was due to the initial axial strain accumulation and subsequent sudden runaway deformation, and this failure mode differed dramatically from that of fly ash conducted under symmetrical stress reversal. An increase in the anisotropic ratio resulted in a decrease in the cyclic resistance of pure fly ash. Nakdong River sand and gravels were added to pure fly ash and it was examined whether the cyclic resistance of fly ash increased. Addition of sand was observed to decrease the cyclic resistance for 10% and 20% sand content by volume, regardless of the amount of increase in the dry density of the samples. Furthermore, the cyclic resistance of fly ash-gravel mixtures was greater than that of pure fly ash by approximately 17%.

Second-order correction of Klinkenberg equation and its experimental verification on gas shale with respect to anisotropic stress

Shale is an extremely tight and fine-grained sedimentary rock with nanometer-scale pore sizes. The internal nanopore structure gives rise to not merely the ultra-low permeability of shale, but also the significant slip flow (non-Darcy) phenomenon. This study involves a second-order correction of the traditional Klinkenberg equation and its experimental verification, in consideration of the effect of anisotropic stress, using cube-sized shale samples sourced from a typical Chinese sedimentary basin. The effects of two gas slippage factors, Klinkenberg-corrected permeability, and anisotropic stress were also discussed. The results showed an increasing trend of apparent permeability with decreasing pore pressure when the pressures are relatively low, due to the gas slippage effect. The second-order model results in a lower Klinkenberg-corrected permeability compared with that from the traditional Klinkenberg equation and yields a better match with the experimental data. Analysis of the experimental data showed that both the first-order slippage factor A and the second-order slippage factor B rose as stress anisotropy increased, and that A was more sensitive to stress anisotropy compared with B. Interestingly, both A and B first increased slightly and then dramatically as the permeability declined. It is recommended that when the shale permeability is below 10−3 mD, the second-order approach should be taken into consideration. Darcy's law starts to deviate when Kn > 0.01 and is invalid at high Knudsen numbers. The second-order approach alleviates the problem of permeability overestimation compared with the traditional Klinkenberg equation. When subjected to effective stress, shale pores become constricted and the degree of this constriction decreases gradually as the effective stress continues to increase.

Ice lens induced interfacial hydraulic resistance in frost heave

Frost heave and ice lens cause the failure of engineerings and change the soil structure in cold regions. It is known that the water migrates from the unfrozen zone to feed the ice lens, but the mechanism of water migration in the vicinity of the ice lens is not well understood. In this study, the water migration at the interface between the ice lens and soil particles is theoretically investigated. Pore water flows from the soil pores to the unfrozen film between the ice lens and soil particles to expands the integral ice lens. The kinetics of the microscopic flow in the unfrozen film produces the hydraulic resistance at the macroscopic interface between ice lens and soil particles. Based on the global mechanical equilibrium at the ice-water interface, the rate of frost heave is derived for freezing soils with and without a frozen fringe. An expression for the neutral stress in the frozen fringe is developed to predict the initiation of new ice lenses with considering the effect of anisotropic stress of ice. The impacts of overburden pressure, temperature gradient, and interfacial hydraulic resistance on the rate of frost heave are discussed. In a relatively thick frozen fringe, the interfacial hydraulic resistance is insignificant compared to the hydraulic resistance of frozen fringe. However, if the thickness of frozen fringe is less than a critical value the interfacial hydraulic resistance will play a dominant role in the frost heave. This research is expected to facilitate the improvement of frost heave models, studying the influence of the interfacial hydraulic resistance on the characteristic of frost heave with and without a frozen fringe.

The effect of anisotropic stress and non-adiabatic pressure perturbations on the evolution of the comoving curvature perturbation

We derive the equation for the evolution of the curvature perturbation on the comoving time slice, , in the presence of anisotropic and non-adiabatic terms in the energy–momentum tensor of matter fields. The equation is obtained by manipulating the perturbed Einstein’s equations in the comoving time slice. It could be used to study the evolution of the comoving curvature perturbations for systems with an anisotropic energy–momentum tensor, such as in the presence of vector fields, in the presence of entropy, such as in a multi-field system, or in modified gravity theories. As a simple application, after checking that the comoving time slice for a multi-field system does not coincide with the uniform field time slice in general, we use the equation in the case of two minimally coupled scalar fields and derive a closed set of equations for the curvature and entropy perturbations on the comoving time slice.

Theoretical and numerical studies of cryogenic fracturing induced by thermal shock for reservoir stimulation

The phenomenon of cryogenic fracturing in rock via thermal shock is approached theoretically and numerically. This study first proposes a theoretical solution of the temperature and stress distribution in a finite plate based on the thermoelastic theory and then discusses the effect of several parameters, including the heat transfer coefficient, the thermal conductivity and the size of the specimen, on the temperature distribution and the associated thermal stress evolution in the plate-shaped model. In addition to predicting the stress and temperature distribution based on the theoretical solution, numerical modeling of the initiation and propagation of cryogenic fractures in a finite plate is used to further explain the effect of the heat transfer coefficient on the density of the newly formed fractures and the length of the fracture growth. The initiation and propagation of cryogenic fractures around a borehole filled with cold liquid and the effect of anisotropic stress on cryogenic fracturing are also studied via numerical modeling. The fracture mechanics are based on the time-dependent thermal loading (thermal shock) and on the concept of stress release and redistribution, with single- and multiple-fracture propagation starting from quenching-generated flaws at the surface at the beginning of thermal shock. Analysis of the best method for increasing the heat transfer coefficient further indicates that it is feasible to stimulate reservoirs with the cryogenic fracturing technique to create a strong thermal gradient that generates a considerable local tensile stress in the rocks surrounding the pre-existing main fractures or the borehole.

Experimental study of the effect of stress anisotropy on fracture propagation in Eagle Ford shale under water imbibition

Hydration swelling due to the rock-water reaction may create induced fractures to enhance hydrocarbon recovery in shale reservoirs. This paper investigates the effects of anisotropic stress and hydration swelling on fracture generation and propagation in Eagle Ford shale. To explore whether fractures can be induced by hydration under anisotropic stress conditions, sequential imbibition tests and water imbibition tests were conducted. CT scanning was used to obtain cross-sectional images of shale cores with different axial and confining pressures. The sequential imbibition tests indicated that hydration swelling might induce fractures, and more fractures were generated under water imbibition than oil imbibition. This finding shows that hydration swelling is beneficial to fracture generation under anisotropic compressive stress, which challenges the believe that hydration swelling may cause formation damage. Experiments also show that more induced fractures were generated in the samples under anisotropic stress conditions than under isotropic conditions. The greater the stress anisotropy, the larger the induced fracture geometry is. Beddings and natural fractures can facilitate fracture initiation and propagation.

Anisotropic generalization of well-known solutions describing relativistic self-gravitating fluid systems: an algorithm

We present an algorithm to generalize a plethora of well-known solutions to Einstein field equations describing spherically symmetric relativistic fluid spheres by relaxing the pressure isotropy condition on the system. By suitably fixing the model parameters in our formulation, we generate closed-form solutions which may be treated as an anisotropic generalization of a large class of solutions describing isotropic fluid spheres. From the resultant solutions, a particular solution is taken up to show its physical acceptability. Making use of the current estimate of mass and radius of a known pulsar, the effects of anisotropic stress on the gross physical behaviour of a relativistic compact star is also highlighted.

TMR forecasts 5% per annum growth in world iron oxide pigments market

Hydration swelling due to the rock-water reaction may create induced fractures to enhance hydrocarbon recovery in shale reservoirs. This paper investigates the effects of anisotropic stress and hydration swelling on fracture generation and propagation in Eagle Ford shale. To explore whether fractures can be induced by hydration under anisotropic stress conditions, sequential imbibition tests and water imbibition tests were conducted. CT scanning was used to obtain cross-sectional images of shale cores with different axial and confining pressures. The sequential imbibition tests indicated that hydration swelling might induce fractures, and more fractures were generated under water imbibition than oil imbibition. This finding shows that hydration swelling is beneficial to fracture generation under anisotropic compressive stress, which challenges the believe that hydration swelling may cause formation damage. Experiments also show that more induced fractures were generated in the samples under anisotropic stress conditions than under isotropic conditions. The greater the stress anisotropy, the larger the induced fracture geometry is. Beddings and natural fractures can facilitate fracture initiation and propagation.

Mn-Znフェライト単結晶の初透磁率に及ぼす応力効果の異方性

Manganese Zinc ferrite single crystals have been widely used for making video cassette recorder (VCR) heads. The initial permeability is one index relating to the performance of magnetic heads. It was known that the initial permeability of the crystals decreased as a function of residual stresses left after machining of the magnetic heads. At that time, most of the crystals were grown in the Bridgman furnace, so that compositional segregation could not be avoided and no research work was systematically carried out on the relationship between anisotropic stress effects and initial permeability of the crystals. For the tensile and compressive samples we used very uniform single crystals of less than 0.1mol% compositional segregation, produced by solid phase reaction. The variously orientated crystal samples, shaped like picture frames, were cut by ultrasonic machining and etched more than 20μm deep by a heat phosphoric acid to remove residual stresses. The initial permeability was measured during tensile and compressive tests as a parameter of frequency from 100kHz to 100MHz, in a temperature-controlled silicon oil bath. The initial permeability of the crystal samples under uniaxial stress was also obtained theoretically by the magnetocrystalline anisotropy energy due to the rotation magnetization. It was found that the initial permeability at 50MHz might be improved by more than 10dB by applying the tensile stress of 25MPa at <100> direction. This phenomenon may be applied to other applications.

Thickness-dependent charge ordering state in Bi0.4Sr0.28Ca0.32MnO3 thin films under the anisotropic strain

We have achieved clear charge ordering (CO) state in Ca doped Bi0.4Sr0.6−xCaxMnO3 (BSCMO) films on (110)-orientated SrTiO3. The BSCMO film's CO transition temperature (TCO) was tuned to room temperature (RT) by optimum doping Ca (x = 0.32). Asymmetric x-ray reciprocal space mapping was used to determine the structure evolution of the films with thickness, in detail, under the anisotropic stress in films. We found that the CO phase only appears in thicker films, accompanied by an unique distortion in structure, in which twin-domains structure occurs with the [110] axis slightly tilting toward [1–10] and [−110] directions. The results demonstrate the effects of anisotropic stress on the structure and electronic phases in BSCMO films as thickness increases. Our works are valuable for allowing a RT study in the CO film and further operation of the devices for a practical use.

Impact of anisotropic stress of free-streaming particles on gravitational waves induced by cosmological density perturbations

Gravitational waves (GWs) are inevitably induced at second-order in cosmological perturbations through non-linear couplings with first order scalar perturbations, whose existence is well established by recent cosmological observations. So far, the evolution and the spectrum of the secondary induced GWs have been derived by taking into account the sources of GWs only from the product of first order scalar perturbations. Here we newly investigate the effects of purely second-order anisotropic stresses of photons and neutrinos on the evolution of GWs, which have been omitted in the literature. We present a full treatment of the Einstein-Boltzmann system to calculate the spectrum of GWs with anisotropic stress based on the formalism of the cosmological perturbation theory. We find that photon anisotropic stress amplifies the amplitude of GWs by about $150 %$ whereas neutrino anisotropic stress suppress that of GWs by about $30 %$ on small scales $k\gtrsim 1.0 h{\rm Mpc}^{-1}$ compared to the case without anisotropic stress. The second order anisotropic stress does not affect GWs with wavenumbers $k\lesssim 1.0 h{\rm Mpc}^{-1}$. The result is in marked contrast with the case at linear order, where the effect of anisotropic stress is damping in amplitude of GWs.

Liquid crystal anchoring utilizing surface topological effects of self-structured dual-groove patterns

We demonstrate topologically patterned dual-groove surfaces for liquid crystal (LC) surface anchoring, where the dual-groove structure is made by a replica-moulding method from a self-structured poly(dimethylsiloxane) (PDMS) surface. In our method, the micro-groove structure with a periodicity on the order of micrometres is self-structured on the oxidized PDMS mould surface having a photolithographically defined macro-groove pattern owing to a thermally induced anisotropic stress effect. The direction of the micro-groove is determined to be perpendicular to the macro-groove direction and to form a mutually orthogonal dual-groove structure. With the presented method, the relative azimuth anchoring strength ratio (g) between the macro-groove and the micro-groove can be controlled precisely and easily. We investigated the LC anchoring effect created by the self-structured dual-groove pattern based on g, where the monostable LC anchoring surface is provided by the dual-groove surface with a g value close to 0 or infinity, and bistable LC anchoring is promoted as g approaches 1.

Single domain to multidomain transition due to in-plane magnetic anisotropy in phase-separated (La0.4Pr0.6)0.67Ca0.33MnO3thin films

Phase separated perovskite manganites have competing phases with different crystal structures, magnetic and electronic properties. Hence, strain effects play a critical role in determining the magnetic properties of manganite thin films. Here we report the effect of anisotropic stress on the magnetic properties of the phase separated manganite (La$_{0.4}$Pr$_{0.6}$)$_{0.67}$Ca$_{0.33}$MnO$_{3}$. Thin films of (La$_{0.4}$Pr$_{0.6}$)$_{0.67}$Ca$_{0.33}$MnO$_{3}$ grown under anisotropic in-plane stress on (110) NdGaO$_{3}$ substrates display in-plane mangetic anisotropy and single domain to multidomain transition as a function of temperature. Angle dependent magnetization measurements also show that the magnetization reversal occurs mainly through the nucleation $&$ propagation mechanism. By comparing the results with (La$_{0.4}$Pr$_{0.6}$)$_{0.67}$Ca$_{0.33}$MnO$_{3}$ thin films grown on (001) SrLaGaO$_{4}$ substrates, we have confirmed that the magnetic anisotropy is mainly due to substrate induced anisotropic stress. Our results suggest novel avenues for storing magnetic information in nanoscale magnetic media.

Three Dimensional Modeling of Anisotropic Stress Effects in Thermal Oxidation of Silicon

AbstractThree-dimensional analyses of anisotropic stress effects during thermal oxidation of silicon are performed in this study using finite strain formulation. A reaction rate model that conjugates pressure to a crystalline direction dependent activation volume is employed for the modeling of facet formation in an STI MOSFET device. The numerical results agree well with TEM observations. The 3-D nature of stress evolution in fabrication process is demonstrated and the implication of stress relaxation on device performance in a strained SiGe based device is discussed.

薄膜媒体の面内磁気異方性の考察

The macroscopic in-plane magnetic anisotropy induced in Co-alloy thin-film media deposited on textured substrates, arises mainly from the orientation of easy magnetization axis of the Co-alloy in the circumferential direction. This anisotropy correlates with in-plane distortion of the crystal structure of the Cr under layer. Correlation between the substrate surface microstructure and distortion of the Cr crystal structure was investigated as an effect of anisotropic stress caused by the thermal expansion of substrates. We found that the Cr crystal structure becomes distorted, and a large magnetic anisotropy is induced, when the grain size is less than half the width of texture grooves. In this case, the different thermal expansion coefficients of NiP and Al make the thermal strain of the inclined plane of texture grooves larger in the circumferential direction than in the radial slope direction. The stress on Cr grains from the substrate surface therefore becomes anisotropic and induces distortion of the Cr crystal structure. The anisotropic stress also correlates with the in-plane magnetic anisotropy.

Reduction of Superconducting Transition Temperature due to Orthorhombic Distortion in La2-xSrxCuO4

The anisotropic stress effects on superconducting temperature T c have been investigated in single crystals of La 2- x Sr x CuO 4 with x =0.10 and 0.145. Utilizing non-hydrostaticity of the applied pressure, we estimated T c in the tetragonal phase at the ambient pressure. The T c was found to be higher in the tetragonal phase than the T c in the orthorhombic phase. We demonstrate that the orthorhombic distortion suppresses superconductivity and the tetragonal structure with the flat CuO 2 plane is favorable for superconductivity.

Microstructural effects on the friction and wear of zirconia films in unlubricated sliding contact

Friction and wear of sapphire and steel counterfaces sliding on unlubricated thin films of zirconia were studied using conventional multiple pass pin-on-disk testing and single pass testing with a ‘friction microprobe’. The zirconia films were grown by electron-cyclotron-resonance (ECR) oxygen plasma assisted deposition on r-cut (011̄2) sapphire using different growth parameters to produce films with varying degrees of order. The microstructure of the zirconia films is correlated to the widely varying observed friction and wear characteristics. Anisotropic stress effects produced during scratch tests are also presented.

Multicritical points in structural phase transitions

Abstract Recent theoretical (renormalization group) results on phase diagrams and multicritical points in systems undergoing structural phase transitions are reviewed. Particular attention is given to (a) the effects of anisotropic stress on cubic systems, e.g. perovskites undergoing antiferrodistortive phase transitions (bicritical, tricritical, Potts model, Lifshitz points, etc.), (b) phase diagrams of alloys, (c) the effects of electric fields on anisotropic antiferroelectrics.

On the thermodynamics of pressure solution—interaction between chemical and mechanical forces

Parts of a solid under stress show a higher solubility than stress-free parts. In porous, granular formations this effect causes dissolution of grain-to-grain contacts because stresses are concentrated here. This pressure-solution reaction leads to closer packing of the grains, resulting in porosity decrease. The dissolved material will reprecipitate in the remaining pores, thus further reducing porosity. The pressure-solution process comprises three steps: dissolution of the grain at stressed spots, diffusion to stress-free parts, and precipitation. In this article four different mechanisms are compared and evaluated: 1. 1. dissolution caused by compressive stresses inside grain-to-grain boundaries, followed by diffusion through an adsorbed water layer to free pores (Weyl, 1959). 2. 2. Dissolution due to shear stresses at or just outside the rims of grain-to-grain contacts (Bathurst, 1958, 1975). 3. 3. Dissolution of tiny particles abraded from the original grains. 4. 4. Dissolution of plastically deformed parts. A general equation for the effect of anisotropic stress on the solubility of a solid is derived and some sources that led to deviations in earlier derivations are spotted. This equation indicates that high supersaturations may occur inside grain-to-grain contacts. Supersaturations due to stresses at or just outside the rims of grain-to-grain contacts, as well as those of abraded particles, are too low to cause effective pressure solution. This result strongly supports the mechanism proposed by Weyl (1959). It is in disagreement with Bathurst's proposal (1958). The mechanism mentioned in (1) is also supported by geological arguments, in contrast to the hypothesis of plastic deformation causing pressure solution.