Evolution Of Anisotropy Research Articles

Perfect fluid with shear viscosity and spacetime evolution

We investigate the anisotropic evolution of spacetime driven by perfect fluid with off-diagonal shear-viscosity components. We consider the simplest form of the equation of state for fluid, for which the pressure and the shear stress are proportional to the energy density individually. At late times, compared with the usual Friedmann universe, we find that the spacetime expands less rapidly as the energy density drops faster due to the transfer to the shear stress. Very interestingly, for some ranges of the equation-of-state parameters, we find that the initial big-bang singularity can be removed.

A shear history model for capturing the liquefaction resistance of sands at various cyclic stress ratios

Various constitutive formulations have been developed over the years to reproduce the cyclic resistance of sands. A common challenge for existing models is the accurate simulation of the cyclic strength of sands for a wide range of initial conditions and different cyclic stress levels when adopting a single calibration. Many liquefaction models tend to overpredict the resistance of the soil under large-amplitude loading, while underestimating the strength at low-amplitude cyclic shearing. This manifests itself in slopes of simulated cyclic resistance ratio curves (CRR-curves) which are steeper than experimental studies indicate. This paper provides a discussion on the effects of large-amplitude and low-amplitude cyclic shearing on a granular material based on micromechanical and experimental investigations presented in the literature. A constitutive model with a shear-history threshold is proposed, which accounts for a shift of the apparent angle of phase transformation under cyclic loading. In addition, a novel expression for a deviatoric fabric tensor is introduced to describe the evolution of shear-induced fabric anisotropy while a soil is dilating and contracting. Combining these two features in one formulation within the bounding surface plasticity framework enables an accurate prediction of cyclic strength of sands under a wide range of cyclic stress ratios.

Thermodynamic Constitutive Model of the Gas Hydrate-Bearing Sediments Considering Anisotropy

Gas hydrate-bearing sediments (GHBS) are considered a significant potential energy source. However, the decomposition of hydrates can lead to various geological hazards. Therefore, a comprehensive investigation into the mechanical properties of GHBS is essential to ensure the safe extraction of gas hydrate. This paper presents a constitutive model for GHBS that incorporates anisotropy, based on the theory of thermodynamics. To account for the effects of hydrate filling and cementing, two parameters are introduced into the dissipation function of the model. The filling effect is expressed through the densification mechanism, which increases the density of the host sediment. The cementing effect is represented by the enhanced expansion of the yield surface. The yield function incorporates the spacing ratio [Formula: see text] and the teardrop shape parameter [Formula: see text], which govern the shape of the yield surface, as well as the anisotropy angle [Formula: see text], which signifies the anisotropic evolution law. The physical significance of these parameters is also elucidated. The anisotropic evolution is described by an exponential function. The proposed model is compared to both the test results and the existing constitutive model, and it is found that it provides more accurate predictions of the mechanical properties of GHBS.

Scanning the time and space evolution of anisotropy in single molecular crystals based on 3-(furan-2-yl)-2-(4-{[(2-hydroxy-5-methylphenyl)methylidene]amino}phenyl)prop-2-enenitrile

The 3-(furan-2-yl)-2-(4-{[(2-hydroxy-5-methylphenyl)methylidene]amino}phenyl)prop-2-enenitrile (LAXYUI)), acting as a model, is a type of outstanding organic fluorophore. Using a transient absorption approach, the excitation relaxation behavior of LAXYUI in the monomolecular system and ordered crystals was identified. The polarization optical properties of LAXYUI crystals, including anisotropy of absorption and photoluminescence, polarization PL decay at different temperatures, and the evolution of optical anisotropy as a function of propagation length, are thoroughly investigated using a series of home-built polarization optical techniques. Our results can help people better understand the time- and space-dependent optical anisotropy that occurs in organic crystals, which will be important in the future to improve or broaden the performance of polarization optical systems based on organic crystals.

Lessons learned from matching 3D DEM and experiments at macro, meso and fabric scales for triaxial compression tests on lentils

A series of discrete element method (DEM) triaxial compression simulations on specimens of an anisometric granular material starting from distinct initial fabric anisotropy states are conducted and compared with physical experiments on lentils at different scales, assisted by operando X-ray tomography measurements. A quantitative reproduction of the group of experimental results is achieved by appropriate idealization and determination of particle shape, boundary conditions, contact parameters, and initial state. The reliability of DEM in quantitative representation of macro scale stress–strain response, meso scale strain localization, and micro scale fabric anisotropy evolution is thus comprehensively validated against measurements from physical experiments, which is a step forward from comparisons only at the macro or particle kinematics level. Several key factors that govern realistic DEM simulations are also identified. The friction coefficient between particles during specimen generation, particle shape, and specimen preparation method can significantly affect the initial state of DEM specimens. Differences in initial state at macro and micro scales and boundary conditions can strongly influence the stress–strain response. However, the evolution of fabric anisotropy appears to be insensitive to changes in initial state and boundary conditions.

Coupling model for the gradient loading creep behavior of nickel-based single-crystal superalloys

The emergency situation caused by the abnormal service of an aeroengine, such as gradient loading, directly leads to blade rupture. Considering the effect of gradient loading on creep, a coupling creep model of nickel-based single crystals under gradient loading conditions is proposed based on crystal plastic theory and thermodynamic damage theory. Internal state variables—elastic strain, plastic strain and damage—are introduced to describe the thermodynamic irreversible process of a system. An anisotropic creep damage evolution equation represented by the damage strain energy release rate Y is deduced by the energy conversion method. The gradient loading creep behavior predicted by the finite element calculation is in good agreement with the experimental results.

Spectro-spatial evolution of the CMB. Part II. Generalised Boltzmann hierarchy

In this paper, we formulate a generalised photon Boltzmann hierarchy that allows us to model the evolution and creation of spectral distortion anisotropies caused by energy release in the early Universe. We directly build on our first paper in this series, extending the thermalisation Green's function treatment to the anisotropic case. We show that the problem can be described with the common Boltzmann hierarchy for the photon field extended by new spectral parameters — a step that reduces the complexity of the calculation by at least two orders of magnitude. Our formalism describes the effects of i) Doppler and potential driving, ii) spectral evolution by Compton scattering, iii) perturbed thermalisation and iv) anisotropic heating on the distortion anisotropies. We highlight some of the main physical properties of the equations and also outline the steps for computing CMB power spectra including distortion anisotropies. Limitations and extensions of the formulation are also briefly discussed. The novel Boltzmann hierarchy given here is the basis for a series of companion papers studying how distortion anisotropies evolve in the perturbed Universe and which physical processes could be constrained using future CMB imaging techniques.

Characterizing rock transverse anisotropic spatial variations using digital drilling

Transverse anisotropy plays an important role in the engineering behavior of rock masses. This study examines the transverse anisotropic spatial variations and evaluates them using the coefficient of variation (CV) and anisotropy index (AI) through digital drilling tests. Four types of rocks, shale (SH), mudstone (MU), sandstone (SA), and schist (SC), are studied in four directions, revealing significant transverse anisotropy at the cutting point. The results demonstrate that the effect of the depth per rotation on the difference is weaker compared to the scaled weight and torque. The ranking of transverse anisotropy in the tested rocks is: MU > SA > SH > SC. Transverse anisotropy changes along the drilling depth vary across rock types. With the increase of drilling depth, SH experiences a decrease, rebound, and another decrease. MU shows an initial increase followed by a decrease. For SA, the degree of transverse anisotropy decreases and is sustained at a lower extent. SC exhibits slight fluctuations with the exception of the abrupt change. The time when the drilling bit is in perfect contact with the rock and the cutting efficiency in different drilling directions are the main factors causing the transverse anisotropy. The intricate evolution of transverse anisotropy should be worth investigating on a wider scale.

Optimization, formation, and evolution of the photoinduced curing gradients and in-situ lamellar gaps in additive manufacturing of ZrO2 ceramics: From curing to sintering behaviors

The photoinduced curing gradients and in-situ lamellar gaps were related to the three-dimensional curing mechanism of stereolithography and researched in this work. Based on the photocuring mathematic theories presented by our team, photoinduced curing gradients were influenced by printing parameters such as laser power and frequency, scanning angle, and printing thickness. Among these curing gradients, the inadequate curing area was the main factor affecting the surface quality and physical properties of ceramics. The in-situ lamellar gaps were derived from the inadequate curing area. The lamellar gap size of green bodies was enlarged in brown bodies (sintering at 1170 ℃) and shrunk in sintered bodies (sintering at 1500 ℃). The correlations between the surface quality and printing parameters were clarified by establishing mathematical formulations of the lamellar gap size. Furthermore, anisotropic volume shrinkage, anisotropic surface topography, and density and hardness evolutions were associated with the wrapped particle density differences induced by the photoinduced curing gradients. Finally, the optimal printing parameters (75% of laser power, laser frequency of 150 kHz, scanning angle of 90º, printing thickness of 25 μm, and scanning speed of 2.5 m/s) were obtained by the orthogonal experimental analysis. This study provides a distinct understanding of the high-quality forming and extreme-performance manufacturing of stereolithography.

Effect of applied stress level on anisotropy in creep-aging behavior of Al–Cu–Li alloy

In the present work, the anisotropic creep aging (CA) behavior and microstructural evolution of Al–Cu–Li alloy are investigated under different stress levels. The results indicate that as the stress level increases, the anisotropic creep behavior of Al–Cu–Li alloy changes significantly. As the applied stress level increases, the in-plane anisotropy (IPA) value of creep strain decreases and then increases. In addition, the strength also exhibits significant anisotropy, and as the applied stress level increases, there are differences in the strength changes of samples with different orientations at different creep aging times. Electron backscatter diffraction (EBSD), Energy-dispersive x-ray spectroscopy (EDS) and transmission electron microscopy (TEM) methods are used to study the evolution of texture and microstructure, as well as their effects on the anisotropy evolution of materials. Microscopic observation reveals that the anisotropy changes in creep and strength can be attributed to the difference in dislocation density and grain boundary slip phenomenon during the initial creep stage in samples with different orientations, as well as the stress orientation effect of precipitated phases, especially T1 phase. The study reveals a new mechanism of anisotropic CA mechanism in Al–Cu–Li alloy under different applied stress levels.

Anisotropic evolutions of the magnetohydrodynamic Richtmyer-Meshkov instability induced by a converging shock.

The investigation of the converging shock-induced Richtmyer-Meshkov instability, which arises from the interaction of converging shocks with the interface between materials of differing densities in cylindrical capsules, is of significant importance in the field of inertial confinement fusion (ICF). The use of converging shocks, which exhibit higher efficiency than planar shocks in the development of the RMI due to the Bell-Plesset effects, is particularly relevant to energy production in the ICF. Moreover, external magnetic fields are often utilized to mitigate the development of the RMI. This paper presents a systematic investigation of the anisotropic nature of the Richtmyer-Meshkov instability in magnetohydrodynamic induced by the interaction between converging shocks and perturbed semicylindrical density interfaces (DI) based on numerical simulations using Athena++. The results reveal that magnetic fields with β=1000, 100, and 10 (β is defined as the ratio of the plasma pressure to the magnetic pressure) lead to an anisotropic intensification of magnetic fields, anisotropic accelerations of various shock waves [including the converging incident shock (CIS), transmitted shock (TS), and reflected shock (RS)], and anisotropic growth of the DI with subsequent anisotropic vorticity distribution. Upon closer inspection, it becomes evident that these phenomena are strongly interconnected. In particular, the region where the wave front of the CIS impacts the middle point of semicylindrical DI, where the magnetic field is more perpendicular to the fluid motion, experiences a more significant amplification of the magnetic fields. This generates higher-pressure jumps, which in turn accelerates the shock wave near this region. Furthermore, the anisotropic amplification of the magnetic fields reduces the movement of the RMI near the middle point of semicylindrical DI and leads to the anisotropic formation of RMI-induced bubbles and spikes, as well as vortices. By examining vorticity distributions, the results underscore the crucial role of magnetic tension forces in inhibiting fluid rotation.

Anisotropic inflation in dipolar Bose–Einstein condensates

Early during the era of cosmic inflation, rotational invariance may have been broken, only later emerging as a feature of low-energy physics. This motivates ongoing searches for residual signatures of anisotropic space-time, for example in the power spectrum of the cosmic microwave background. We propose that dipolar Bose–Einstein condensates (BECs) furnish a laboratory quantum simulation platform for the anisotropy evolution of fluctuation spectra during inflation, exploiting the fact that the speed of dipolar condensate sound waves depends on direction. We construct the anisotropic analogue space-time metric governing sound, by linking the time-varying strength of dipolar and contact interactions in the BEC to the scale factors in different coordinate directions. Based on these, we calculate the dynamics of phonon power spectra during an inflation that renders the initially anisotropic Universe isotropic. We find that the expansion speed provides an experimental handle to control and study the degree of final residual anisotropy. Gravity analogues using dipolar condensates can thus provide tuneable experiments for a field of cosmology that was until now confined to a single experiment, our Universe.

Effect of stress level on anisotropy in creep ageing behaviour of 2195-T84 Al[sbnd]Li alloy

In the present work, the anisotropic creep ageing behaviour and microstructural evolution of 2195-T84 AlLi alloy are investigated under different tensile stresses. It is found that the anisotropic creep behaviour of 2195-T84 AlLi alloy changes significantly with the increase of the applied stress. Microstructural observations confirmed that the movable non-octahedral slip systems increases with the applied stress. The obtained results show that with the increase of the applied stress, the dislocation density in the alloy increases obviously, and the θ' phase coarsens significantly in the late stage of creep ageing process. In addition, there is obvious dislocation slip band in the precipitation free zone (PFZ) with high stress level. The non-octahedral slip and grain boundary slip under high stress level change the dislocation mobility of different orientations, which is the main reasons for the change of the creep anisotropy of the alloy. The results revealed a new mechanism accountable for the distinctly anisotropic creep ageing behaviour.

Test and DEM study on cyclic hysteresis characteristics of stereoscopic geogrid–soil interface

The bearing resistance provided by the geogrid's transverse ribs is a non-negligible aspect of the strength mechanism in mobilizing the geogrid–soil interface. Therefore, studying its influence on the response mechanism of geosynthetic-reinforced soil structures under cyclic loading is crucial. The stereoscopic geogrids were manufactured using 3D printing technology by quantitatively thickening the transverse ribs of planar geogrids. To investigate the cyclic hysteresis relationship and stress–dilatancy phase-transformation characteristics of the stereoscopic geogrid–coarse particle interface, cyclic direct shear tests were conducted. Additionally, a discrete element method (DEM) was employed to study the evolution of shear bands and fabric anisotropy at the interface under cyclic loading. The results of the study indicate that the stress–displacement phase angle of the stereoscopic geogrid in the horizontal direction of cyclic shear is smaller compared to the planar geogrid. Furthermore, thickening the transverse ribs decreases the stress–dilatancy phase-transformation angle of the interface. The thickness of the interface shear band in the stereoscopic geogrid is greater than that of the planar geogrid. Moreover, as the transverse-rib thickness increases, the principal direction of the average normal contact force and average tangential contact force under cyclic loading also increases.

Microscopic analysis of the nonlinear stiffness of granular materials at small-to-medium strain

The nonlinear stiffness of dense and loose granular materials at small-to-medium strain is investigated using the three-dimensional discrete element method (DEM). The threshold strain of the nonlinear elasticity of granular materials, i.e., the Y2 surface in the kinematic yielding surface framework, is obtained from the simulated drained triaxial tests. The analytical stress-force-fabric relationship is used to quantitatively examine the evolution of fabric anisotropy. Results show that the after reaching Y2 surface, the contact normal anisotropy begins to increase from a steady low level, and evident slippage begins to occur at strong contacts. The threshold strains of the contact normal anisotropy and strong contact slippage increase linearly with the confining pressure, which is consistent with the threshold strain of the nonlinear elasticity of granular materials. The creation of new contacts and disruption of existing contacts during loading are investigated. A three-stage micromechanism of the observed nonlinear stiffness behavior is proposed.

Plastic Evolution Characterization for 304 Stainless Steel by CQN_Chen Model under the Proportional Loading.

In this paper, the CQN_Chen function is used to characterize the plastic anisotropic evolution of 304 stainless steel (SS304). The uniaxial tensile tests along different loading directions are conducted to experimentally investigate the anisotropic hardening behavior for SS304. The experimental data indicates that the anisotropy of SS304 is weak. The convexity analysis is carried out by the geometry-inspired numerical convex analysis method for the CQN_Chen yield locus during plastic deformation. The Hill48, SY2009 and CQN functions are used as the comparison to evaluate the accuracy of the CQN_Chen function in characterizing plastic evolution. The predicted values are compared with the experimental data. The comparison demonstrates that the CQN_Chen function can accurately characterize anisotropic hardening behavior under uniaxial tension along distinct loading directions and equibiaxial tension. Simultaneously, the CQN_Chen model has the capacity to adjust the yield surface shape between uniaxial tension and equibiaxial tension. The CQN_Chen model is recommended to characterize plastic evolving behavior under uniaxial tension along different directions and equibiaxial tension.

Investigation of anisotropy evolution of an aluminium‑lithium alloy with increasing pre-deformation in creep age forming

The 3rd generation aluminium‑lithium (Al-Li) alloy demonstrates promising potential in various applications due to its advanced synergy of strength, toughness, corrosion resistance and weight reduction. However, like its predecessors, this alloy also suffers from the anisotropy issue which can exert significant impact on the manufacturing processes. This study seeks to investigate the anisotropy evolution of a T8 temper 3rd generation Al-Li alloy when subjecting to increased tensile pre-deformation at 0°, 45°, and 90° to the rolling direction. Uniaxial creep-ageing and tensile tests were conducted to reveal the anisotropic behaviours of the material during creep and tensile deformation. Electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) methods were used to investigate the microstructural evolutions and their influence on the material anisotropy evolvement with treatments of tensile pre-deformation and creep-ageing. It was found that with the increase of tensile pre-deformation level, the creep deformation increased at 0° while decreased at 45° and 90°, due to different configurations of the dislocation substructures at 0°, 45° and 90°. Furthermore, the in-plane anisotropy (IPA) was reduced in both the creep deformation and material strength with increasing tensile pre-deformation and subsequent creep-ageing. The enhanced creep deformation at 0° and decreased deformation at 45° and 90°, as well as the reduction of anisotropy, can be attributed to the reconfiguration of dislocation structures and different extents of precipitation promotion at different directions when subjected to tensile pre-deformation and creep-ageing.

The effect of thickness on the defects and anisotropy of thin-wall Hastelloy X fabricated via laser powder-bed fusion

Laser powder-bed fusion (LPBF) possesses unique advantages in manufacturing complex structures. The mechanical properties of thin-wall structures are crucial for the application. This study investigates the influence of wall thickness on defects and mechanical properties of LPBF Hastelloy X (HX) superalloy. As the wall thickness decreases from 1.5 mm to 0.25 mm, the diameter of sub-grains inside the molten pool increases, exhibiting a progressively enhanced <100>//transverse direction (TD) texture in thin wall structure. Meanwhile, decreasing wall thickness leads to increased porosity and cracks due to unstable molten pool and higher residual stress. The crack initiates at columnar to equiaxed transition (CET) position and propagates along the building direction (BD). The tensile strength of the LPBF HX specimens in the horizontal and vertical directions is 773.5 MPa and 642.1 MPa with thickness of 1.5 mm, respectively, while the elongation of both specimens is close to 35%. When the wall thickness decreases to 0.25 mm, the elongation significantly decreases due to the evolution of microstructure anisotropy and the cracks along the BD, reaching 10.1% and 24.3% for horizontal and vertical specimens. This work illustrates the defects, microstructural evolution and the mechanical properties anisotropy in thin-wall structures of LPBF HX, providing a foundation for adjusting and optimizing the performance of subsequent thin-wall printing processes.

Effect of interlayer mixed zone and effective stress on permeability anisotropy of NGH turbidite reservoir

Resulting from the bedding structure of turbidite reservoir, permeability anisotropy is an essential parameter for natural gas hydrate (NGH) exploitation. Fundamentally, the evolution of permeability anisotropy under increasing effective stress are investigated in this study. Taking the turbidite sediments in northern Cascadia as an example, a series of experimental tests and co-simulations of discrete element method (DEM) and computational fluid dynamics method (CFD) are conducted. For layered turbidite sediments, an interlayer mixed zone will be formed in the interface between fine-grained layer and coarse-grained layer after loading. Fine particles will migrate to the coarse layer, blocking the horizontal preponderance flow paths and further reduce the permeability anisotropy. Moreover, the development of interlayer mixed zone mainly occurs in initial compaction stage and slows down in higher stress level, for which the permeability anisotropy shows a staged reduction with increasing effective stress. Considering the effect of interlayer mixed zone, a predictive model of permeability anisotropy is proposed and verified by the experimental results, which has higher accuracy than traditional “layered cake” model. The finding of this research also confirms the mixing effect in deep sediments, which may affect the geological history records in target sedimentary sequence.

Growth Anisotropy and Morphology Evolution of Line Defects in Monolayer MoS2 : Atomic-Level Observation, Large-Scale Statistics, and Mechanism Understanding.

Understanding the growth behavior and morphology evolution of defects in 2D transition metal dichalcogenides is significant for the performance tuning of nanoelectronic devices. Here, the low-voltage aberration-corrected transmission electron microscopy with an in situ heating holder and a fast frame rate camera to investigate the sulfur vacancy lines in monolayer MoS2 is applied. Vacancy concentration-dependent growth anisotropy is discovered, displaying first lengthening and then broadening of line defects as the vacancy densifies. With the temperature increase from 20 °C to 800 °C, the defect morphology evolves from a dense triangular network to an ultralong linear structure due to the temperature-sensitive vacancy migration process. Atomistic dynamics of line defect reconstruction on the millisecond time scale are also captured. Density functional theory calculations, Monte Carlo simulation, and configurational force analysis are implemented to understand the growth and reconstruction mechanisms at relevant time and length scales. Throughout the work, high-resolution imaging is closely combined with quantitative analysis of images involving thousands of atoms so that the atomic-level structure and the large-area statistical rules are obtained simultaneously. The work provides new ideas for balancing the accuracy and universality of discoveries in the TEM study and will be helpful to the controlled sculpture of nanomaterials.