Bai-Xiang Xu

On electromechanical stability analysis of dielectric elastomer actuators

Based on the total stress concept, explicit results of the equilibrium state and the critical electric field are obtained for dielectric elastomer actuators. Criticality is discussed in the frame of structure stability and electric breakdown. Specific results are given for four commonly used material models.

Dynamic analysis of dielectric elastomer actuators

An analytical model is proposed for the dynamic analysis of a homogeneously deformed dielectric elastomer actuator (DEA) with a standard sandwich structure. The equation of motion for the DEA is obtained by the Euler-Lagrange equation. Numerical results of the model are presented to show the vibration and oscillation behaviour of the system. Resonance phenomenon and damping effects are investigated. Results are discussed in comparison with those of the related topics in the literature.

Recent General Solutions in Linear Elasticity and Their Applications

A review is given on the progress in the study of general solutions of elasticity and their applications since 1972. Apart from summarizing and remarking the development of the general solution method in literature, this review aims to present the readers with a systematic and constructive scheme to develop general solutions from given governing differential equations and then to prove their completeness and investigate their nonuniqueness features. The effectiveness of the constructive scheme manifests itself in the fact that almost all the classic solutions, including not just classic displacement potentials but also classic stress functions, can be rederived by using this scheme. Furthermore, thanks to the systematic features of the scheme, it produces a constructive approach to study the completeness and nonuniqueness of general solutions and possesses more flexibility, which facilitates the extension of elastic general solution methods to more general systems governed by elliptic differential equations. Under the framework of this scheme, a comprehensive review is presented on wide application of general solutions in a variety of research areas, ranging from problems with different materials, isotropic or anisotropic, to various coupling problems, such as thermoelasticity, magnetoelasticity, piezoelectric elasticity, porous elasticity, and quasicrystal elasticity, and to problems of different engineering structures, for instance, the refined theories for beams and plates. There are 213 references cited in this review article. DOI: 10.1115/1.2909607

On the origin of inhomogeneous stress and strain distributions in single-crystalline metallic nanoparticles

Abstract The internal stress state in a facetted nanoparticle of fcc copper is investigated by means of finite element calculations based on a linear elastic continuum description. By comparing with atomistic simulations using the embedded-atom method we can show that the elastic anisotropy, particle geometry and surface stresses determine the internal stress state. The calculated internal stresses are much lower than predictions by the Laplace–Young equation. Even under positive surface stresses a negative hydrostatic pressure may develop within the particle, which can be attributed to the strong elastic anisotropy of copper.

The quasi Eshelby property for rotational symmetrical inclusions of uniform eigencurvatures within an infinite plate

Although the Eshelby property does not hold for the non-ellipsoidal inclusions, some special properties have been found for the rotational symmetrical inclusions recently. In the present paper, an infinite plate containing a rotational symmetrical inclusion with uniform eigencurvatures is investigated. We mathematically prove that the rotational symmetrical inclusion in an infinite plate possesses the quasi Eshelby property. Namely, for an N-fold rotational symmetrical inclusion (N is an integer greater than 2 and unequal to 4), the arithmetic mean of curvatures at N rotational symmetrical points in the inclusion is identical with that of a circular inclusion and independent of the orientation of the inclusion. Meanwhile, in two corollaries, we point out that the curvature at the centre, the averaged curvature over the inclusion domain, and the line integral average of curvatures along any concentric circle of the inclusion are all the same as the arithmetic mean.

Enhanced electrocaloric effect near polymorphic phase boundary in lead-free potassium sodium niobate ceramics

The electrocaloric (EC) effect in lead-free (1-x)(K0.48Na0.52)(Nb0.95Sb0.05)O3-xBi0.5(Na0.82K0.18)0.5ZrO3 ceramics was investigated using an indirect thermodynamic method. Large EC temperature changes were obtained in the vicinity of a polymorphic phase boundary at 40 kV/cm, e.g., 0.32 K at 359 K for x = 0.03, 0.51 K at 350 K for x = 0.04, and 0.48 K at 300 K for x = 0.05, respectively. These values are larger than the previous results at inter-ferroelectric phase transition and, more interestingly, are found to be comparable to those usually explored at the Curie temperature. The operational temperature window is broad near the polymorphic phase boundary due to the diffuseness of the phase transition. The enhanced electrocaloric effect is attributed to the formation of nanodomains near the polymorphic phase boundary, which reduces domain wall energy and facilitates the polarization rotation. The construction of a polymorphic phase boundary and the arrangement of coexisting phases at the nanoscale may ope...

Domain wall stability in ferroelectrics with space charges

Significant effect of semiconductor properties on domain configurations in ferroelectrics is demonstrated, especially in the case of doped materials. Phase field simulations are performed for ferroelectrics with space charges due to donors and electronic charge carriers. The free charges introduced thereby can act as a source for charge compensation at domain walls with uncompensated polarization bound charges. Results indicate that the equilibrium position of a domain wall with respect to its rotation in a head-to-head or a tail-to-tail domain configuration depends on the charge defect concentration and the Fermi level position.

Positive and negative electrocaloric effect in BaTiO3 in the presence of defect dipoles

The influence of defect dipoles on the electrocaloric effect (ECE) in acceptor doped BaTiO3 is studied by means of lattice-based Monte-Carlo simulations using a Ginzburg-Landau type effective Hamiltonian. Oxygen vacancy-acceptor associates are described by fixed local dipoles with orientation parallel or antiparallel to the external field. By a combination of canonical and microcanonical simulations the ECE is directly evaluated. Our results reveal that in the case of antiparallel defect dipoles the ECE can be positive or negative depending on the dipole density. Moreover, a transition from a negative to positive ECE can be observed when the external field increases. These transitions are due to the delicate interplay of internal and external fields and are explained by the domain structure evolution and related field-induced entropy changes. The results are in good qualitative agreement to those obtained by molecular dynamics simulations employing an ab initio based effective Hamiltonian. Finally, a modified electrocaloric cycle, which makes use of the negative ECE in the presence of defect dipoles, is proposed to enhance the cooling effect.

A constraint-free phase field model for ferromagnetic domain evolution.

A continuum constraint-free phase field model is proposed to simulate the magnetic domain evolution in ferromagnetic materials. The model takes the polar and azimuthal angles (ϑ1,ϑ2), instead of the magnetization unit vector m(m1,m2,m3), as the order parameters. In this way, the constraint on the magnetization magnitude can be exactly satisfied automatically, and no special numerical treatment on the phase field evolution is needed. The phase field model is developed from a thermodynamic framework which involves a configurational force system for ϑ1 and ϑ2. A combination of the configurational force balance and the second law of thermodynamics leads to thermodynamically consistent constitutive relations and a generalized evolution equation for the order parameters (ϑ1,ϑ2). Beneficial from the constraint-free model, the three-dimensional finite-element implementation is straightforward, and the degrees of freedom are reduced by one. The model is shown to be capable of reproducing the damping-dependent switching dynamics, and the formation and evolution of domains and vortices in ferromagnetic materials under the external magnetic or mechanical loading. Particularly, the calculated out-of-plane component of magnetization in a vortex is verified by the corresponding experimental results, as well as the motion of the vortex under a magnetic field.

Phase-field study of electrochemical reactions at exterior and interior interfaces in Li-ion battery electrode particles

This work is supported by the “Excellence Initiative” of the German Federal and State Governments and the Graduate School of Computational Engineering at the Technische Universitat Darmstadt. The author Xu would particularly like to thank the Adolf Messer Foundation for awarding her the Adolf Messer Prize and for the financial support.