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Morteza Hakimi Siboni

Bio: Morteza Hakimi Siboni is an academic researcher from University of Pennsylvania. The author has contributed to research in topics: Dielectric & Finite strain theory. The author has an hindex of 7, co-authored 10 publications receiving 217 citations.

Papers
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Journal ArticleDOI
TL;DR: In this paper, a homogenization framework for electro-elastic composite materials at finite strains is presented, where the effective energy of the composite is written in terms of a purely mechanical component together with a purely electrostatic component, this last one dependent on the macroscopic deformation via appropriate kinematic variables such as the particle displacements and rotations, and the change in size and shape of the appropriate unit cell.
Abstract: This paper presents a homogenization framework for electro-elastic composite materials at finite strains. The framework is used to develop constitutive models for electro-active composites consisting of initially aligned, rigid dielectric particles distributed periodically in a dielectric elastomeric matrix. For this purpose, a novel strategy is proposed to partially decouple the mechanical and electrostatic effects in the composite. Thus, the effective electro-elastic energy of the composite is written in terms of a purely mechanical component together with a purely electrostatic component, this last one dependent on the macroscopic deformation via appropriate kinematic variables, such as the particle displacements and rotations, and the change in size and shape of the appropriate unit cell. The results show that the macroscopic stress includes contributions due to the changes in the effective dielectric permittivity of the composite with the deformation. For the special case of a periodic distribution of electrically isotropic, spherical particles, the extra stresses are due to changes with the deformation in the unit cell shape and size, and are of order volume fraction squared, while the corresponding extra stresses for the case of aligned, ellipsoidal particles can be of order volume fraction, when changes are induced by the deformation in the orientation of the particles.

89 citations

Journal ArticleDOI
TL;DR: In this paper, homogenization estimates for the finite-strain effective response of a certain class of dielectric elastomer composites (DECs) subjected to electromechanical loading conditions are presented.
Abstract: This paper presents homogenization estimates for the finite-strain effective response of a certain class of dielectric elastomer composites (DECs) subjected to electromechanical loading conditions. The DECs consist of a dielectric elastomer matrix phase constrained to undergo plane strain deformations by means of aligned, long, rigid-dielectric fibers of elliptical cross section that are also aligned but randomly distributed in the transverse plane. The estimates for the effective electro-active response are obtained by means of available estimates for the purely mechanical response of such composites, together with a partial decoupling strategy/approximation. Such homogenization estimates can then be used to assess the effect of various microstructural parameters, such as the concentration and cross-sectional shape of the fibers, on the overall electromechanical response of the DECs, when subjected to an electric potential difference across suitably positioned soft electrodes. In addition, three different instability and failure mechanisms are investigated: loss of positive definiteness, loss of strong ellipticity and dielectric breakdown, with the objective of finding an optimal design of the microstructure and constituent properties for maximal electrostriction before failure.

53 citations

Journal ArticleDOI
TL;DR: In this article, the authors investigate the possible development of instabilities in a certain class of dielectric-elastomer composites (DECs) subjected to all-around dead electromechanical loading.
Abstract: In this work we investigate the possible development of instabilities in a certain class of dielectric-elastomer composites (DECs) subjected to all-around dead electromechanical loading. The DECs consist of a dielectric elastomer matrix phase constrained to plane strain deformations by means of aligned, long, rigid dielectric fibers of elliptical cross-section that are also aligned but randomly distributed in the transverse plane. Two types of instabilities are considered: loss of positive definiteness (LPD), and loss of strong ellipticity (LE). LPD simply corresponds to the loss of local convexity of the homogenized electroelastic stored-energy function for the DECs and can be of two types depending on the resulting instability modes. When the modes are aligned with the ‘principal’ solution, the instability corresponds to a maximum in the nominal electric field, possibly followed by snapping behavior. Alternatively, when the modes are orthogonal to the principal solution, the instability corresponds to a...

33 citations

Journal ArticleDOI
TL;DR: In this paper, an analytical expression for the equilibrium rotation of an isolated, rigid inclusion with linear-magnetic behavior embedded in a linear-elastic matrix was obtained under general magneto-mechanical loading conditions.

23 citations

Journal ArticleDOI
TL;DR: In this paper, the authors provide estimates for the effective response of Electro-Active Polymer Composites (EAPCs) consisting of aligned ellipsoidal inclusions of a stiff dielectric material which are distributed randomly in an soft elastomeric matrix with two-point statistics.
Abstract: We provide estimates for the effective response of Electro-Active Polymer Composites (EAPCs) consisting of aligned ellipsoidal inclusions of a stiff dielectric material which are distributed randomly in an soft elastomeric matrix with “ellipsoidal” two-point statistics. The derivation of the results for the electro-mechanical response assumes linearized deformations, but includes non-linear (quadratic) terms in the electric fields. We investigate three different physical mechanisms contributing to the macroscopic electro-mechanical response of the composite: the intrinsic effect of the particles on the Maxwell stress, the inter-particle (dipole) interactions which are accounted for by evaluating the effect of changes in the “shape” of the two-point probability functions with the deformation, and the effect of particle rotations and torques when the geometric and/or anisotropy axis of the particles are not aligned with the applied electric field. Several illustrative examples are provided to empha...

18 citations


Cited by
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TL;DR: In this article, a new homogenization framework for magnetoelastic composites accounting for the effect of magnetic dipole interactions, as well as finite strains, was proposed, which is capable of handling strongly nonlinear effects associated with finite strains and magnetic saturation of the particles at sufficiently high deformations and magnetic fields, respectively.
Abstract: This paper proposes a new homogenization framework for magnetoelastic composites accounting for the effect of magnetic dipole interactions, as well as finite strains. In addition, it provides an application for magnetorheological elastomers via a “partial decoupling” approximation splitting the magnetoelastic energy into a purely mechanical component, together with a magnetostatic component evaluated in the deformed configuration of the composite, as estimated by means of the purely mechanical solution of the problem. It is argued that the resulting constitutive model for the material, which can account for the initial volume fraction, average shape, orientation and distribution of the magnetically anisotropic, non-spherical particles, should be quite accurate at least for perfectly aligned magnetic and mechanical loadings. The theory predicts the existence of certain “extra” stresses—arising in the composite beyond the purely mechanical and magnetic (Maxwell) stresses—which can be directly linked to deformation-induced changes in the microstructure. For the special case of isotropic distributions of magnetically isotropic, spherical particles, the extra stresses are due to changes in the particle two-point distribution function with the deformation, and are of order volume fraction squared, while the corresponding extra stresses for the case of aligned, ellipsoidal particles can be of order volume fraction, when changes are induced by the deformation in the orientation of the particles. The theory is capable of handling the strongly nonlinear effects associated with finite strains and magnetic saturation of the particles at sufficiently high deformations and magnetic fields, respectively.

165 citations

Journal ArticleDOI
TL;DR: In this paper, the authors classify the deformation and instabilities of soft dielectrics into three generic modes: thinning and pull-in, electro-creasing to cratering, and electro-cavitation.
Abstract: Widely used as insulators, capacitors, and transducers in daily life, soft dielectrics based on polymers and polymeric gels play important roles in modern electrified society. Owning to their mechanical compliance, soft dielectrics subject to voltages frequently undergo large deformation and mechanical instabilities. The deformation and instabilities can lead to detrimental failures in some applications of soft dielectrics such as polymer capacitors and insulating gels but can also be rationally harnessed to enable novel functions such as artificial muscle, dynamic surface patterning, and energy harvesting. According to mechanical constraints on soft dielectrics, we classify their deformation and instabilities into three generic modes: (i) thinning and pull-in, (ii) electro-creasing to cratering, and (iii) electro-cavitation. We then provide a systematic understanding of different modes of deformation and instabilities of soft dielectrics by integrating state-of-the-art experimental methods and observations, theoretical models, and applications. Based on the understanding, a systematic set of strategies to prevent or harness the deformation and instabilities of soft dielectrics for diverse applications are discussed. The review is concluded with perspectives on future directions of research in this rapidly evolving field.

148 citations

Journal ArticleDOI
R. R. Birss1
TL;DR: Slepian used an operational "definition" of stress and concluded that the compensating mechanical forces which must be introduced operationally are not derivable from a tensor.
Abstract: for the force on a dielectric in an electrical field E and bounded by a surface S with unit normal n. He used an operational \"definition\" of stress and concluded that the compensating mechanical forces which must be introduced operationally are not derivable from a tensor. It is suggested here that Slepian's analysis is essentially correct and that the difficulty arises because of the choice of an operationally \"defined\" stress. This choice is inconsistent with the existence of an electrical surface stress-which is familiar, in the magnetic analogue, in studies of the form effect-and it is argued here that the Euler-Cauchy definition of stress is the appropriate one. The Definition of Stress.-In authoritative works on continuum mechanics stress is introduced by means of the stress hypothesis of Euler and Cauchy,2 that is, by asserting that, acting upon any imagined closed geometrical surface a within the body, there exists a field of stress vectors t which has an equivalent effect to the (interparticle) forces exerted by the material outside aupon the material within. For a dielectric material the interaprticle (i.e., intermolecular) forces are partly long-range in character and they may therefore contribute not only to t but also to f, the body force per unit volume. For the present purpose, however, the important point to note is that ais an imagined geometrical surface and not a physical surface of separation within the material. An alternative procedure is to use the operational definition of stress in which it is imagined that a physical cut is made in the material along an internal element of surface dd = nda. If means are then introduced for keeping the strains in the material on both sides of the cut the same as they were before the cut was made, then the force introduced by these means is t'do-, where t' is the operationally defined stress vector. In adopting this operational definition, Slepian commented: \"It is not assumed that the cut and the introduced means do not disturb the microstructure and micromechanics of the material. For example, in the case of a fluid the cut and means would cause molecules to be reflected which would otherwise pass through the geometric element of surface dS. It is assumed, however, that in spite of the change in the micromechanics, there is no change in the observable macromechanics.\"' It may also be noted there is a further element of idealization involved in that the cut is imagined to be of finite extent: in practice, as discussed later in this paper, it is only possible to measure the force on an element of volume when the element is completely separated from the rest of the body. For an ordinary elastic material the stress acting at a physical surface of separa-

142 citations

Journal ArticleDOI
TL;DR: In this article, the governing equations for the electromechanically coupled behavior of dielectric elastomers in a thermodynamic framework are presented and the attendant finite-element formulation and implementation are discussed.
Abstract: Dielectric elastomers undergo large deformations in response to an electric field and consequently have attracted significant interest as electromechanical transducers. Applications of these materials include actuators capable of converting an applied electric field into mechanical motion and energy harvesting devices that convert mechanical energy into electrical energy. Numerically based design tools are needed to facilitate the development and optimization of these devices. In this paper, we report on our modeling capability for dielectric elastomers. We present the governing equations for the electromechanically coupled behavior of dielectric elastomers in a thermodynamic framework and discuss the attendant finite-element formulation and implementation, using a commercial finite-element code. We then utilize our simulation capability to design and optimize complex dielectric elastomeric actuators and energy harvesting devices in various settings.

112 citations

Journal ArticleDOI
TL;DR: In this paper, the macroscopic response and stability of a new type of magnetorheological elastomer (MRE) under combined in-plane mechanical and magnetic loading by means of the finite-strain homogenization framework and partial decoupling approximation was analyzed.
Abstract: This paper is concerned with the development of constitutive models for a class of magnetoelastic composites consisting of stiff, aligned cylindrical fibers of a magnetizable material that are embedded firmly in a soft elastomeric matrix. The fibers have elliptical cross section and their (transverse) in-plane axes are also aligned, but their distribution is random and characterized by “elliptical” two-point correlations. Estimates are obtained for the macroscopic response and stability of this new type of magnetorheological elastomer (MRE) under combined in-plane mechanical and magnetic loading by means of the finite-strain homogenization framework and “partial decoupling approximation” of Ponte Castaneda and Galipeau (2011) . The resulting macroscopic magnetoelastic constitutive model accounts for the microstructure of the composite and its evolution under finite strains and rotations, as well as for the nonlinear magnetic behavior of the fibers, including the effect of magnetic saturation. When the loading directions are not aligned with the fiber axes, the model predicts magnetic and mechanical torques on the fibers, leading to their in-plane rotation, which is found to have significant effects on the coupled magnetoelastic response of the composite, including the possible development of macroscopic torques on a given finite-size sample of the composite. To eliminate these macroscopic torques, while maintaining the advantageous effects of the fiber rotations, we also investigate the response of a laminated composite consisting of plus/minus orientations of the fibers relative to the layering direction, and subjected to magnetic and mechanical loadings along the layering direction. The results for the actuation tractions, magnetostrictive strain and magnetoelastic moduli demonstrate that the microstructure of these laminated MRE samples can be designed optimally for significantly enhanced magnetoelastic effects. In particular, the actuation tractions and magnetostrictive strains can be made several times larger than the corresponding tractions and strains for isotropic MREs with spherical (circular) particles.

105 citations