Elastic Energy Field Research Articles

Mechanism of grain boundary serration during hot deformation of AZ31 alloy: Role of grain boundary dislocations and grain boundary sliding

Despite the industrial significance of grain boundary serration for improving creep resistance and retarding crack propagation, the mechanism of grain boundary serration during hot deformation is still unclear. The results of several studies indicated that increased inhomogeneity of the stored energy under high-stress deformation suppressed grain boundary serration. Thus, traditional strain-induced boundary migration, in which the stored energy difference between adjacent grains acts as the driving force for grain boundary serration, must be modified for hot deformation. The experimental results of this study suggested that dissociated grain boundary dislocations from lattice dislocations, rather than the elastic energy of the lattice dislocations, cause the nucleation of serrated grain boundaries. Grain boundary sliding on the serrated grain boundary asymmetrically generated an elastic energy field and an accommodated serrated grain boundary. The presence of an elastic energy field was predicted by dislocation climbing and statically distributed dislocations near the elastic energy field. Since the asymmetrically deformed serrated grain boundary exhibited sub-migration behavior in the elastic energy field, it was concluded that the applied shear stress (applied by grain boundary sliding) on the serrated grain boundary dominated the growth of serrated grain boundaries until dynamic recrystallization occurred.

Deck the Walls with Anisotropic Colloids in Nematic Liquid Crystals.

Nematic liquid crystals (NLCs) offer remarkable opportunities to direct colloids to form complex structures. The elastic energy field that dictates colloid interactions is determined by the NLC director field, which is sensitive to and can be controlled by boundaries including vessel walls and colloid surfaces. By molding the director field via liquid-crystal alignment on these surfaces, elastic energy landscapes can be defined to drive structure formation. We focus on colloids in otherwise defect-free director fields formed near undulating walls. Colloids can be driven along prescribed paths and directed to well-defined docking sites on such wavy boundaries. Colloids that impose strong alignment generate topologically required companion defects. Configurations for homeotropic colloids include a dipolar structure formed by the colloid and its companion hedgehog defect or a quadrupolar structure formed by the colloid and its companion Saturn ring. Adjacent to wavy walls with wavelengths larger than the colloid diameter, spherical particles are attracted to locations along the wall with distortions in the nematic director field that complement those from the colloid. This is the basis of lock-and-key interactions. Here, we study ellipsoidal colloids with homeotropic anchoring near complex undulating walls. The walls impose distortions that decay with distance from the wall to a uniform director in the far field. Ellipsoids form dipolar defect configurations with the colloid's major axis aligned with the far field director. Two distinct quadrupolar defect structures also form, stabilized by confinement; these include the Saturn I configuration with the ellipsoid's major axis aligned with the far field director and the Saturn II configuration with the major axis perpendicular to the far field director. The ellipsoid orientation varies only weakly in bulk and near undulating walls. All configurations are attracted to walls with long, shallow waves. However, for walls with wavelengths that are small compared to the colloid length, Saturn II is repelled, allowing selective docking of aligned objects. Deep, narrow wells prompt the insertion of a vertical ellipsoid. By introducing an opening at the bottom of such a deep well, we study colloids within pores that connect two domains. Ellipsoids with different aspect ratios find different equilibrium positions. An ellipsoid of the right dimension and aspect ratio can plug the pore, creating a class of 2D selective membranes.

Tunable colloid trajectories in nematic liquid crystals near wavy walls

The ability to dictate the motion of microscopic objects is an important challenge in fields ranging from materials science to biology. Field-directed assembly drives microparticles along paths defined by energy gradients. Nematic liquid crystals, consisting of rod-like molecules, provide new opportunities in this domain. Deviations of nematic liquid crystal molecules from uniform orientation cost elastic energy, and such deviations can be molded by bounding vessel shape. Here, by placing a wavy wall in a nematic liquid crystal, we impose alternating splay and bend distortions, and define a smoothly varying elastic energy field. A microparticle in this field displays a rich set of behaviors, as this system has multiple stable states, repulsive and attractive loci, and interaction strengths that can be tuned to allow reconfigurable states. Microparticles can transition between defect configurations, move along distinct paths, and select sites for preferred docking. Such tailored landscapes have promise in reconfigurable systems and in microrobotics applications.

A phase field modeling based study of microstructure evolution and its influence on thermal conductivity in polycrystalline tungsten under irradiation

In this work, we study effects of neutron irradiation and elastic tensile loading on microstructure evolution in polycrystalline tungsten by using a phase field modeling based approach. The proposed model involves the interaction of point defects with grain boundaries (GBs), thermal resistance of GBs, and voids. The void evolution analysis shows that the elastic energy field affects the shape and growth rate of voids. The overall trend of porosity evolution during irradiation follows the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation. Voids nucleation and growth can reduce both elastic strain energy and chemical free energy in the system. In addition, increase of porosity could significantly degrade the effective thermal conductivity of analyzed tungsten polycrystalline microstructures. The observations comply with the predictions from the theoretical models.

Curvature-driven assembly in soft matter.

Control over the spatial arrangement of colloids in soft matter hosts implies control over a wide variety of properties, ranging from the system's rheology, optics, and catalytic activity. In directed assembly, colloids are typically manipulated using external fields to form well-defined structures at given locations. We have been developing alternative strategies based on fields that arise when a colloid is placed within soft matter to form an inclusion that generates a potential field. Such potential fields allow particles to interact with each other. If the soft matter host is deformed in some way, the potential allows the particles to interact with the global system distortion. One important example is capillary assembly of colloids on curved fluid interfaces. Upon attaching, the particle distorts that interface, with an associated energy field, given by the product of its interfacial area and the surface tension. The particle's capillary energy depends on the local interface curvature. We explore this coupling in experiment and theory. There are important analogies in liquid crystals. Colloids in liquid crystals elicit an elastic energy response. When director fields are moulded by confinement, the imposed elastic energy field can couple to that of the colloid to define particle paths and sites for assembly. By improving our understanding of these and related systems, we seek to develop new, parallelizable routes for particle assembly to form reconfigurable systems in soft matter that go far beyond the usual close-packed colloidal structures.This article is part of the themed issue 'Soft interfacial materials: from fundamentals to formulation'.

Elastic strain energy release from fragmenting grains: Effects on fault rupture

As a confined static grain mass is increasingly stressed, elastic strain energy accumulates in the grains; when local stress somewhere exceeds grain strength, grain breakage radiates this stored energy to the surrounding grains as a brief high-intensity pulse of pressure energy. Local grain stresses in a stressed static fault-zone follow a known probability density distribution in which the maximum local grain stress resulting from a given applied stress increases with fault-plane area. Tectonic stress buildup on a fault increases the probability of local stresses sufficient to break grains. Brittle failure of a grain releases strain energy that can trigger cascading failure of a large fault at relatively low applied stress, culminating in fault rupture, if the fault strength distribution is sufficiently homogeneous. Such ruptures can occur at lower driving stresses in larger faults. This process correctly explains the magnitude of the failure stress of the peninsula segment of the San Andreas fault, so may significantly affect fault rupture strength. Grain fragmentation also plays a significant role in dynamic fault friction; in dense comminuting granular flows, grain fragmentation causes continuous recycling of elastic-strain energy. During cataclastic fault slip, widespread grain comminution thus generates and maintains an intense high-frequency elastic energy field that causes dynamic weakness. This process is sufficiently powerful to explain the magnitude of the dynamic slip resistance of the San Andreas fault.

A transversely isotropic composite with a statistical distribution of spheroidal inclusions: a geometrical approach to overall properties

In the literature, the determination of global elastic properties of composites with ellipsoidal inclusions is based on the averaged stress, strain and elastic-energy fields (e.g. Compos. Sci. Technol. 27 (1986) 111). These are related to the local fields of the inclusion, the matrix, and the inclusion-matrix interface. In this study, we propose a method to obtain the global elastic properties of any transversely isotropic composite directly from the elastic properties of the matrix and the inclusions. Thus, it is not necessary to refer to the stress and strain applied globally or generated locally. The inclusions can have any transversely isotropic probability distribution of orientation. The problem is entirely geometrized and is treated in terms of averages of Walpole's (Adv. Appl. Mech. 21 (1981) 169) components of the fourth-order tensors describing the problem. We give a general numerical solution for any transversely isotropic statistical distribution of orientation, and also provide a validation of our method by applying it to some known cases and by retrieving the known exact solutions from the literature.

LATTICE DYNAMICS OF THE BINARY APERIODIC CHAINS OF ATOMS III: PROPERTIES OF VIBRATIONAL EIGENVECTORS

We numerically study the localization and multifractal properties of normalized vibrational eigenvectors (NVEV), denoted by U, of the microscopic harmonic model of lattice dynamics in Thue-Morse (TM), generalized Fibonacci, octagonal, dodecagonal, Severin, circle and Rudin-Shapiro (RS) binary chain of atoms. Eigenvalues and NVEV are determined with the help of the Dean and Wu-Zheng algorithm, respectively, with free end boundary conditions for chains containing 103<N<104 atoms. The first FM(L) and second SM(L) moment, and the reduced participation ratios Λ red (L) are derived at 1≤L≤N for varying model parameters. All the chains studied show sinusoidal-like and packet-like extended NVEV with Λred(L)≃1, FM(L)≃N/2 and [Formula: see text] The new extended eigenstates called dimmerized NVEV have been found out in the case of the TM chain. The surface localized NVEV with Λred(L)≪1, FM(L)≃1 or FM(L)≃N and the strong tendency to localization of NVEV in RS chain have been observed. The critical NVEV, which dominate in the optical region of phonon spectra, are objects with a broad multifractal (αmin, αmax) spectra. If magnitudes of model parameters are increased then, first, [Formula: see text] and [Formula: see text] at L≪N and, second, [Formula: see text] and [Formula: see text] at L≃N. It is numerically shown that the multifractal spectra α′—f′ characterizing the curdling of the elastic energy field ε(L) are in excellent qualitative and quantitative agreement with the multifractal spectra of the critical NVEV.

Strain-energy-density factor applied to mixed mode crack problems

This paper deals with the general problem of crack extension in a combined stress field where a crack can grow in any arbitrary direction with reference to its original position. In a situation, when both of the stress-intensity factors,k 1,k 2 are present along the crack front, the crack may spread in any direction in a plane normal to the crack edge depending on the loading conditions. Preliminary results indicate that the direction of crack growth and fracture toughness for the mixed problem of Mode I and Mode II are governed by the critical value of the strain-energy-density factor,S cr. The basic assumption is that crack initiation occurs when the interior minimum ofS reaches a critical value designatedS cr. The strain-energy-density factorS represents the strength of the elastic energy field in the vicinity of the crack tip which is singular of the order of 1/r where the radial distancer is measured from the crack front. In the special case of Mode I crack extensionS cr is related tok 1c alone asS cr = (κ − 1)k 1 2 /8μ. In general,S takes the quadratic forma 1 1 k 1 + 2a 1 2 k 1 k 2 +a 2 2 k 2 whose critical value is assumed to be a material constant. The analytical predictions are in good agreement with experimental data on the problem of an inclined crack in plexiglass and aluminum alloy specimens. The result of this investigation provides a convenient procedure for determining the critical crack size that a structure will tolerate under mixed mode conditions for a given applied stress.