Duc Khoi Vu

Bio: Duc Khoi Vu is an academic researcher from University of Erlangen-Nuremberg. The author has contributed to research in topics: Finite element method & Nonlinear system. The author has an hindex of 12, co-authored 18 publications receiving 599 citations. Previous affiliations of Duc Khoi Vu include RWTH Aachen University & Kaiserslautern University of Technology.

Numerical modelling of non-linear electroelasticity

Abstract: The numerical modelling of non-linear electroelasticity is presented in this work. Based on well-established basic equations of non-linear electroelasticity a variational formulation is built and the finite element method is employed to solve the non-linear electro-mechanical coupling problem. Numerical examples are presented to show the accuracy of the implemented formulation. Copyright © 2006 John Wiley & Sons, Ltd.

Experimental study and numerical modelling of VHB 4910 polymer

Abstract: VHB 4910 is an important polymeric material that has potential use as electro-active polymer in producing actuators. Such polymer, a member of the acrylic polymer group, is a very soft viscoelastic material. In this contribution, a comprehensive mechanical characterization of this important viscoelastic material has been carried out using different standard experiments such as single-step relaxation tests, multi-step relaxation tests and loading–unloading cyclic tests. In modelling the mechanical behaviour, a modified version of the micro-mechanically motivated Bergstrom–Boyce viscoelastic model has been used along with a finite linear evolution law. The model validation shows its excellent capability to predict the experimental results.

Nonlinear electro-and magneto-elastostatics : Material and spatial settings

Abstract: The material and spatial settings of the nonlinear coupling problem of electro- and magneto-elastostatics are discussed in this paper. The governing equations and variational formulations of the problem derived in these two settings using basic equations of electricity, magnetism and elasticity allow the consideration of material defects by the material force method.

A comprehensive characterization of the electro-mechanically coupled properties of VHB 4910 polymer

Abstract: An illustrative documentation of some standard experimental tests of electro-active VHB 4910 polymer under application of purely mechanical and electro-mechanically coupled loadings is presented. VHB 4910 is a very soft polymer that has potential applications as an electro-active polymer in the production of different types of actuators and sensors. The time-dependent viscoelastic phenomenon is ideal in polymers. Therefore, experiments with electro-mechanically coupled loads were conducted considering some standard tests that were usually used for a viscoelastic polymeric material characterization, i.e. loading-unloading tests, single-step relaxation tests, and multi-step relaxation tests. In all experimental cases, the polymer samples were pre-stretched up to several hundred per cent to make them thin enough initially so that the application of the electro-mechanically coupled load can show its effect to a larger extend. The pre-stretched samples were then subjected to various amounts of mechanical as well as coupled deformations at different strain rates. The data produced from several loading-unloading tests, single-step relaxation tests, and multi-step relaxation tests show that the electric loading has profound effect in the time-dependent behaviour of the electro-active VHB 4910 polymer. The data set either from single-step relaxation tests or multi-step relaxation tests can be used to identify electro-viscoelastic parameters for a suitable constitutive model that can capture electro-mechanically coupled behaviours of VHB 4910. For validation, loading-unloading cyclic tests data can be utilized.

Computational electro-elasticity and magneto-elasticity for quasi-incompressible media immersed in free space

Abstract: Summary In this work, a mixed variational formulation to simulate quasi-incompressible electro-active or magneto-active polymers immersed in the surrounding free space is presented. A novel domain decomposition is used to disconnect the primary coupled problem and the arbitrary free-space mesh update problem. Exploiting this decomposition, we describe a block-iterative approach to solving the linearised multiphysics problem, and a physically and geometrically based, three-parameter method to update the free space mesh. Several application-driven example problems are implemented to demonstrate the robustness of the mixed formulation for both electro-elastic and magneto-elastic problems involving both finite deformations and quasi-incompressible media. Copyright © 2016 John Wiley & Sons, Ltd.

Theory of dielectric elastomers

Abstract: In response to a stimulus, a soft material deforms, and the deformation provides a function. We call such a material a soft active material (SAM). This review focuses on one class of soft active materials: dielectric elastomers. When a membrane of a dielectric elastomer is subject to a voltage through its thickness, the membrane reduces thickness and expands area, possibly straining over 100%. The dielectric elastomers are being developed as transducers for broad applications, including soft robots, adaptive optics, Braille displays, and electric generators. This paper reviews the theory of dielectric elastomers, developed within continuum mechanics and thermodynamics, and motivated by molecular pictures and empirical observations. The theory couples large deformation and electric potential, and describes nonlinear and nonequilibrium behavior, such as electromechanical instability and viscoelasticity. The theory enables the finite element method to simulate transducers of realistic configurations, predicts the efficiency of electromechanical energy conversion, and suggests alternative routes to achieve giant voltage-induced deformation. It is hoped that the theory will aid in the creation of materials and devices.

Maximal energy that can be converted by a dielectric elastomer generator

Abstract: Mechanical energy can be converted to electrical energy by using a dielectric elastomer generator. The elastomer is susceptible to various modes of failure, including electrical breakdown, electromechanical instability, loss of tension, and rupture by stretch. The modes of failure define a cycle of maximal energy that can be converted. This cycle is represented on planes of work-conjugate coordinates and may be used to guide the design of practical cycles.

Giant voltage-induced deformation in dielectric elastomers near the verge of snap-through instability

Abstract: Dielectric elastomers are capable of large voltage-induced deformation, but achieving such large deformation in practice has been a major challenge due to electromechanical instability and electric breakdown. The complex nonlinear behavior suggests an important opportunity: electromechanical instability can be harnessed to achieve giant voltage-induced deformation. We introduce the following principle of operation: place a dielectric elastomer near the verge of snap-through instability, trigger the instability with voltage, and bend the snap-through path to avert electric breakdown. We demonstrate this principle of operation with a commonly used experimental setup—a dielectric membrane mounted on a chamber of air. The behavior of the membrane can be changed dramatically by varying parameters such as the initial pressure in the chamber, the volume of the chamber, and the prestretch of the membrane. We use a computational model to analyze inhomogeneous deformation and map out bifurcation diagrams to guide the experiment. With suitable values of the parameters, we obtain giant voltage-induced expansion of area by 1692%, far beyond the largest value reported in the literature.

Modeling Viscoelastic Dielectrics

Abstract: Dielectric elastomers, as an important category of electroactive polymers, are known to have viscoelastic properties that strongly affect their dynamic performance and limit their applications. Very few models accounting for the effects of both electrostatics and viscoelasticity exist in the literature, and even fewer are capable of making reliable predictions under general loads and constraints. Based on the principles of non-equilibrium thermodynamics, this paper develops a field theory that fully couples the large inelastic deformations and electric fields in deformable dielectrics. Our theory recovers existing models of elastic dielectrics in the equilibrium limit. The mechanism of instantaneous instability, which corresponds to the pull-in instability often observed on dielectric elastomers, is studied in a general non-equilibrium state. The current theoretical framework is able to adopt most finite-deformation constitutive relations and evolution laws of viscoelastic solids. As an example, a specific material model is selected and applied to the uniform deformation of a dielectric elastomer. This model predicts the stability criteria of viscoelastic dielectrics and its dependence on loading rate, pre-stress, and relaxation. The dynamic response, as well as the hysteresis behavior of a viscoelastic dielectric elastomer under cyclic electric fields, is also studied.

Homogenization-based constitutive models for magnetorheological elastomers at finite strain

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.