Large Volumetric Strains Research Articles

Role of Macroscopic Deformations in Energetics of Vacancies in Aluminum

Electronic structure calculations on million-atom samples are employed to investigate the effect of macroscopic deformations on energetics of vacancies in aluminum. We find that volumetric strain associated with a deformation largely governs the formation energies of monovacancies and divacancies. The calculations suggest that nucleation of these defects is increasingly favorable under volumetric expansion, so much to the point that they become thermodynamically favorable under large positive volumetric strains. On the contrary, on an average, existing vacancies are found to bind preferentially under compressive volumetric strains. Shear deformations did not affect the formation energies of vacancies, but strongly influenced the 110 divacancy binding energies, causing them to orient under energetically preferential directions.

Investigation of an aqueous lithium iodide/triiodide electrolyte for dual-chamber electrochemical actuators

Electrochemical pumping, the electromigration-driven flow of ions and their associated solvent molecules across a permselective membrane, is investigated for the construction of dual-chamber electrochemical actuators. Important features include large volumetric strain, significant pressure generation, and minimal pressure-driven backflow. Aqueous electrolytes have a number of advantages over organic electrolytes such as dimethylformamide; four concentrations of a lithium iodide/triiodide electrolyte are investigated here. Fluid transport decreases as the ionic strength increases, with the waters associated with each cation decreasing from 16 to 6 as [Li +] increases from 0.5 to 3.5 M. As a result, the maximum volumetric strain which might be achieved in a symmetric dual-chamber actuator, about 18%, is for an electrolyte of intermediate concentration, 2 M LiI + 0.5 M I 2. Pressure generation experiments using this electrolyte reached 295 psig (∼20 atm) in 10 min, with about 50% of the available charge consumed. For this pressure, losses measured at open circuit, ca. 13 psi/min, are lower than previously measured losses using a dimethylformamide electrolyte. Simultaneous measurement of pressure generation and fluid transport provides a measure of the pressure-driven backflow, 0.13 μL/min, which compares favorably with those estimated for the porous separators used for electroosmotic-driven flow.

Micromechanical behaviour of Si-based particulate assemblies: A comparative study using DEM and atomistic simulations

In this paper, we investigate the micromechanical behaviour of Si-based particulate systems subjected to tri-axial compression loading. The investigations are based on three-dimensional discrete element modelling (DEM) and simulations. At first, we compare the variation of mean compressive stress for a silicon assembly subjected to tri-axial compression, predicted at two different scales: at the particulate scale, using the DEM simulation (mono-dispersed particulates) and at the atomistic scale using the molecular dynamics (MD) simulation results for silicon mono-crystal reported by Mylvaganam and Zhang (2003) [K. Mylvaganam, L. Zhang, Key Eng. Mater. 233–236 (2003) 615–620]. Both the simulation methods considered the silicon assembly subjected to an identical (tri-axial) loading condition. We observed a good qualitative agreement between the DEM and MD simulation results for the mean compressive stress when the assembly was subjected to small volumetric strain. However, at large volumetric strain, the mean stress of the silicon assembly predicted from MD simulation did not scale-up with the DEM results. This discrepancy could be due to that MD simulation is only valid for particle contacts, which are independent of one another and does not consider the inherent ‘discrete’ nature of particulates and the induced anisotropy prevailing at particulate scale. The micromechanical behaviour of particulate assemblies strongly depends on the inherent discrete nature of the particles, their single-particle properties and the induced anisotropy during mechanical loading. At the second stage, using DEM, we present the evolution of macroscopic compressive stress and several micromechanical features for four cases of the commonly used Si based poly-dispersed particulate assemblies (Si, SiC,Si 3N 4 and SiO 2) under tri-axial compression loading. We also present the evolution of several other phenomena occurring at particulate scale, such as the energy dissipation characteristics due to sliding contacts and the features of fabric structures developed during mechanical loading. The study shows that the single-particle properties of the Si based assemblies considered here significantly affect the micromechanical behaviour of the assemblies and DEM is a powerful tool to get insights on the internal behaviour of discrete particulates under mechanical loading.

Volumetric Shrinkage Strain Measurements in Expansive Soils Using Digital Imaging Technology

Abstract Expansive soils undergo large volumetric shrinkage strains, which eventually lead to high heave movements upon hydration of these soils. Current test methods to determine shrinkage strain potentials of soils are restricted by several limitations including small specimen sizes, molds with rigid walls that restrain shrinkage strains in lateral directions, and manual measurement errors. In this paper, a novel methodology to conduct volumetric shrinkage strain test on cylindrical soil specimens and a digital imaging technique to analyze and determine volumetric shrinkage strains of the test are described. As part of the evaluations of this methodology, volumetric shrinkage strains of four types of medium to high expansive soils were researched. Shrinkage tests were conducted on all four soils at three different moisture contents. Volumetric shrinkage strains were then measured using both digital and conventional manual approaches. Test results showed that the test and the developed measurement methodology provided repeatable and realistic shrinkage strain measurements. Digital measurements provided more accurate results than manual measurements by accounting for even minor shrinkage cracks in the soils. The significance of the digital measurements in relation to the current shrinkage strain characterizations is discussed. Potential geotechnical application areas where these test results could be used are also described.

Role of Microscopic Physicochemical Forces in Large Volumetric Strains for Clay Sediments

In a clay-water-electrolyte system, each particle is under the influence of van der Waals attraction, electrical double-layer repulsion, Born's repulsion, hydrodynamic viscous drag, and gravity. This study investigates the role of microscopic forces in the volumetric behavior of clay sediments. The microscopic forces are implemented in a 2D discrete element method (DEM) framework that uses spheres to represent clay particles. The model is validated with two well-defined problems: (1) an analytical estimation of force-equilibrium for the system of colloidal particles; and (2) the theoretical settling velocity of spherical particles according to Stokes' law. An experimental program is developed to measure the free swell of Georgia kaolinite, Na-montmorillonite, and Ca-montmorillonite. The experimental results are used to validate the DEM framework in its ability to correctly model the volumetric strains inherent to clay sediments. A good agreement between the model prediction and the experimental data is ob...

Prediction of the nonlinear poisson function using large volumetric strains estimated from a finite hyperelastic material law

AbstractThe aim of this study is to evaluate the transverse strain of hyperelastic solids as a function of its longitudinal using a special constitutive equation for compressible hyperelastic materials. In addition, the nonlinear dependence of the Poisson ratio on longitudinal strain was derived. Experimental data from EPDM elastomers subjected to uniaxial tension were used in order to determine the material parameters, which are incorporated in the constitutive law.

Cam-Clay plasticity part III: Extension of the infinitesimal model to include finite strains

The infinitesimal version of the modified Cam-Clay model of critical state soil mechanics is reformulated to include finite deformation effects. Central to the formulation are the choice of a hardening law that is appropriate for cases involving large plastic volumetric strains, and the use of a class of two-invariant stored energy functions appropriate for Cam-Clay-type models that includes, as special cases, the constant elastic shear modulus approximation as well as the pressure-dependent shear modulus elastic model. The analytical model is cast within the framework of finite deformation theory based on a multiplicative decomposition of the deformation gradient. For the fully elasto-plastic case, return mapping is done implicitly in the space defined by the invariants of the elastic logarithmic principal stretches, which requires the solution at each stress point of no more than three simultaneous non-linear equations. A finite element analysis of a strip footing problem is presented to illustrate a prototype example where finite deformation effects significantly impact the predicted response.

The simple tension problem at large volumetric strains computed from finite hyperelastic material laws

In this paper, different formulations of finite isotropic hyperelastic material laws for compressible solids are considered. Material laws with an additive split of the hyperelastic strain energy function into isochoric parts and volumetric parts are often used in the numerical treatment of nearly incompressible solids. It will be shown that this formulation leads to unphysical results in the simple tension problem when we do not restrict ourselves to nearly incompressible materials.

Issues on the constitutive formulation at large elastoplastic deformations, part 2: Kinetics

The coupling between kinematics and kinetics and the invariance requirements under superposed rigid body rotations, determine unambiguously the proper general form of the elastoplastic rate constitutive equations, in terms of the values of the state variables and their rates in reference to the current and any choice of the unstressed configuration. Topics such as the effect of changing the stress rate, small elastic deformations with or without large volumetric elastic strains, rate effects and viscoplasticity, an example on single slip, the effect of the plastic spin constitutive relations and the concept of an effectively unstressed configuration, are addressed in detail. Issues and different approaches debated in the past are discussed, compared and clarified.

A study of vehicle performance in snow

The snow trafficability problem is studied by calculating the energy absorbed by snow while being compacted by vehicle tracks. Using a constitutive law for snow which is valid for large volumetric strains and high strain rates, the rate of energy dissipation due to viscoplastic compaction of the snow is calculated and related parametrically to vehicle speed, track loading and snow density. Energy dissipated by shear effects is neglected. The results indicate that such studies can provide design guidelines for track geometry, track loading, and vehicle speed in terms of snowpack properties.