Influences of initial anisotropy and principal stress rotation on the undrained monotonic behavior of a loose silica sand

Triaxial compression and extension tests have been conducted under different initial anisotropy conditions to investigate the undrained response of a crushed silica sand. The loose to medium specimens were prepared using the moist tamping method. Five stress paths with different stress ratios (q/p^') were employed to prepare anisotropically consolidated specimens. Several specimens were consolidated under a specific condition in which a stress rotation occurred under undrained monotonic shearing similar to a reversed cyclic shear stress loading during an earthquake. The effects of initial induced anisotropy at consolidation on the onset of liquefaction, phase transformation, and critical state are investigated within the framework of Anisotropic Critical State Soil Mechanics (ACSSM). In addition, fabric evolution during shearing towards the critical state is evaluated using bidirectional bender element tests. The results illustrate the fact that there is a unique anisotropic critical state representing anisotropic fabric, irrespective of initial anisotropy, and the states of stress. Similar to the critical state line, the phase transformation line has the same loci for different initial anisotropies.

Three-dimensional magnetic reconnection in particle-in-cell simulations of anisotropic plasma turbulence

We use three-dimensional (3-D) fully kinetic particle-in-cell simulations to study the occurrence of magnetic reconnection in a simulation of decaying turbulence created by anisotropic counter-propagating low-frequency Alfvén waves consistent with critical-balance theory. We observe the formation of small-scale current-density structures such as current filaments and current sheets as well as the formation of magnetic flux ropes as part of the turbulent cascade. The large magnetic structures present in the simulation domain retain the initial anisotropy while the small-scale structures produced by the turbulent cascade are less anisotropic. To quantify the occurrence of reconnection in our simulation domain, we develop a new set of indicators based on intensity thresholds to identify reconnection events in which both ions and electrons are heated and accelerated in 3-D particle-in-cell simulations. According to the application of these indicators, we identify the occurrence of reconnection events in the simulation domain and analyse one of these events in detail. The event is related to the reconnection of two flux ropes, and the associated ion and electron exhausts exhibit a complex 3-D structure. We study the profiles of plasma and magnetic-field fluctuations recorded along artificial-spacecraft trajectories passing near and through the reconnection region. Our results suggest the presence of particle heating and acceleration related to small-scale reconnection events within magnetic flux ropes produced by the anisotropic Alfvénic turbulent cascade in the solar wind. These events are related to current structures of the order of a few ion inertial lengths in size.

Laboratory researches of the magnetic-anisotropic method for monitoring the stress-strain state of pipelines

Оценка напряженно-деформированного состояния - один из важнейших этапов при определении работоспособности металлоконструкций, условий их безопасной эксплуатации и остаточного ресурса. Однако до настоящего времени отсутствуют хорошо апробированные технологии, которые позволяли бы определять данную характеристику стальных трубопроводов и корпусов оборудования при диагностировании в условиях эксплуатации. Цель статьи - анализ возможности применения для этого магнитно-анизотропного метода. Метод основан на фиксации магнитных полей рассеяния, возникающих над поверхностью ферромагнитного металла, который под влиянием внешней нагрузки становится анизотропным либо меняет параметры своей анизотропии. Проведена оценка погрешности измерений при использовании магнитно-анизотропного метода. Описаны результаты лабораторных наблюдений за изменением магнитных свойств образцов из трубной стали в упругой и упругопластической областях. Показано, что измерения свойств материала с применением магнитно-анизотропного метода можно трактовать в рамках общепринятых моделей поведения металла под нагрузкой. Указано на необходимость калибровки оборудования на трубных сталях с учетом марки и, возможно, химического состава стали, способа производства трубы, исходной анизотропии металла перед нагружением. Для фиксации перехода металла в упругопластическое состояние следует измерять параметры магнитной анизотропии металла до и после нагрузки либо при возрастании нагрузки фиксировать момент перехода каким-либо альтернативным методом. Assessment of the stress-strain state is the most important stages in determining the serviceability of steel structures, conditions for their safe operation and calculation of residual life. However, until now there are no well-tested technologies that would allow determining this characteristic of steel pipelines and equipment bodies during diagnostics under operating. The purpose of this article is to analyze the possibility of applying the magnetic-anisotropic method for this purpose. The method is based on the record of stray magnetic fields arising over the surface of the ferromagnetic metal, which becomes anisotropic or changes its anisotropy parameters under the external load influence. The error of measurements using this method was evaluated. The results of laboratory observations of changes in the magnetic properties of pipe steel samples in the elastic and elastoplastic domains are described. It is shown that measurements of material properties using the magneticanisotropic method can be interpreted within the framework of generally accepted models of metal behavior under the load influence. The need for the calibration of the equipment on pipe steels taking into account the grade and possibly the chemical composition of the steel is indicated, as well as the method of pipe production, and the initial anisotropy of the metal before loading. To record the transition of metal to the elastoplastic state, the magnetic anisotropy parameters of the metal should be measured before and after the load or the moment of transition should be recorded by alternative method when the load increases.

Optimization of the geometry of specimens with transversal groove for materials with initial anisotropy

The goal of the study is determination of the optimal geometric dimensions of rectangular anisotropic specimens with a transverse groove when tested for uniaxial tension under conditions close to plane deformation. A test scheme in which the shear fracture mechanism is implemented in the center of the groove under conditions of plane deformation is presented. The optimal geometric dimensions of the specimen (namely, the depth, width and length of the transversal groove) were determined proceeding from numerical simulation of the experiment in the ABAQUS software package in the Explicit mode. It is shown that the initial anisotropy of the material significantly affects the deformed state in the groove. The larger the ratio of the Lankford coefficients along the rolling direction and across this direction, the closer the deformed state in the groove to the plane deformation. It is also shown that change in the geometry of the specimen with a transverse groove can provide better results in implementation of the plane deformation in the region of specimen fracture. The value of the parameter of the deformed state approaches zero when the thickness and width of the groove decrease, as well as when the width of the specimen increases. It is shown that the possibility of forming plane deformation in the groove due to the choice of specimen geometry is limited from above for any initial anisotropy. A moment of «saturation» is observed, at which the ratio of principal strains in the center of the groove does not go any further to zero despite further change in the geometry. A table of geometric dimensions of rectangular specimens with a transverse groove recommended for tensile testing under conditions of plane deformation for all types of anisotropic and isotropic sheet materials is presented.

Impacts of pre-initial conditions on anisotropic separate universe simulations: a boosted tidal response in the epoch of reionization

ABSTRACT To generate initial conditions for cosmological N-body simulations, one needs to prepare a uniform distribution of simulation particles, the so-called pre-initial condition (pre-IC). The standard method to construct the pre-IC is to place the particles on the lattice grids evenly spaced in the three-dimensional spatial coordinates. However, even after the initial displacement of each particle according to cosmological perturbations, the particle distribution remains to display an artificial anisotropy. Such an artefact causes systematic effects in simulations at later time until the evolved particle distribution sufficiently erases the initial anisotropy. In this paper, we study the impacts of the pre-IC on the anisotropic separate universe simulation, where the effect of large-scale tidal field on structure formation is taken into account using the anisotropic expansion in a local background (simulation volume). To quantify the impacts, we compare the simulations employing the standard grid pre-IC and the glass one, where the latter is supposed to suppress the initial anisotropy. We show that the artificial features in the grid pre-IC simulations are seen until z ∼ 9, while the glass pre-IC simulations appear to be stable and accurate over the range of scales we study. From these results we find that a coupling of the large-scale tidal field with matter clustering is enhanced compared to the leading-order prediction of perturbation theory in the quasi-non-linear regime in the redshift range 5 ≲ z ≲ 15, indicating the importance of tidal field on structure formation at such high redshifts, e.g. during the epoch of reionization.

Influence of Initial Anisotropy, Stress Path and Principal Stress Rotation on Monotonic Behavior of Clean and Mixed Sands

It is widely accepted that soil behavior is complicated taking into account soil anisotropy owing to the fact that this phenomenon arises from oriented soil fabric or structure forged in the deposition stage. In this study, a review of major findings of authors’ previous studies are presented with the main focus on soil anisotropy using extensive experimental results incuding Triaxial (TXT), Simple Shear (SSA), and Hollow Cylinder (HCA) apparatus. Effects of initial anisotropy, fabric evolution, stress path, principal stress rotation and intermediate stress state are evaluated for a crushed silica sand. In addition, the effects of Portland cement content and granulated rubber contents on anisotropic behavior of the sand are investigated. Bender elments are mounted on triaxial specimens both in vertical and horizontal directions to measure the shear wave velocity and hence maximum shear modulus at the end of consolidation as well as during shearing up to large strains at critical state condition, as an index of evaluating the fabric evolution. The effects of principal stress rotation and stress paths reveals the crucial role of soil anisotropy on the behavior of clean sand. However, adding either cement or granulated rubber to the sand has considerably decreased anisotropy.