Stephan Rudykh's blog

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Journal Club Theme of January 2014: Electro- and magneto-active deformable composites

Dielectric Elastomers (DE) are promising materials for developing soft machines (e.g., a human-made octopus). The principle of actuation has gained to DE the name "artificial muscles" since they can undergo large deformation when excited by an electric field. Another class of active materials that can be actuated by external field is Magnetoactive Elastomers (MAE). Although MAE and DE share mathematical similarities, the physics is different. Electric field can induce polarization in elastomers, and hence generate electrostatic stresses within the material. As a response to the electrically induced stresses, the material deforms. For MAE the situation is different: elastomers are magnetically inactive, and a similar to DE effect is achieved by mixing the elastomer with magnetically active particles (e.g., carbonyl iron, nickel or Terfenol-D). Thus, due to the magnetic interaction of the particles embedded in a soft matrix, the composite can deform and modify overall stiffness as a response to a magnetic field. The performance of these composites can be further enhanced by optimizing microstructures. Indeed, a similar idea applies for designing DE composites with enhanced properties. These composites, once manufactured can solve the bottle-neck problem of DE technology - the need in extremely high electric fields for meaningful actuations, and potentially lead to a breakthrough in the technology.


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Multiscale instabilities in soft heterogeneous dielectric elastomers

by S. Rudykh, K. Bhattacharya and G. deBotton, Proc. R. Soc. A 470: 20130618.


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PACAM XIV – Mini-symposium on “Mechanics of Soft Active Materials” – Call for abstracts – Deadline 15 November 2013

PACAM XIV - Mini-symposium on "Mechanics of Soft Active Materials" - Call for abstracts - Deadline 15 November 2013

http://pacamxiv.org/submit-abstract/
Dear Colleagues:
We cordially invite you to participate in The Pan American Congress of Applied Mechanics (PACAM) and present research in our mini-symposium on "Mechanics of Soft Active Materials." The mini-symposium will bring together researchers to present state-of-the-art advances, as well as inspire and discuss ideas in the field of mechanics of highly deformable active materials. The areas of interest include but are not limited to
- Electroactive polymers (EAP) and Dielectric elastomers (DE)
- Magnetorheological Elastomers (MRE)


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Wrinkling of Interfacial Layers in Stratified Composites

by Y. Li, N. Kaynia, S. Rudykh and M. C. Boyce  Massachusetts Institute of Technology Wrinkling of Interfacial Layers in Stratified Composites:           Wrinkling vs. long-wavelength instability envelope for multi-layered soft composite: Predictive analytical and finite element models are presented to reveal the instability mechanics governing reversible wrinkling of interfacial layers. The biomimetic guidelines for the design of reversible, tunable, and multi-functional layered soft composites with transforming interfacial layers are developed and verified via 3D printed prototypes.                      Wrinkling of Interfacial Layers in Stratified Composites:          Wrinkling vs. long-wavelength instability envelope for multi-layered soft composite: Predictive analytical and finite element models are presented to reveal the instability mechanics governing reversible wrinkling of interfacial layers. The biomimetic guidelines for the design of reversible, tunable, and multi-functional layered soft composites with transforming interfacial layers are developed and verified via 3D printed prototypes.       


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Analysis of microstructural induced enhancement of electromechanical coupling in soft dielectrics

Electroactive soft elastomers require huge electric field for a meaningful actuation. We demonstrate, by means of numerical simulation, that this can be dramatically reduced and large deformations can be achieved with suitably designed heterogeneous actuators. The mechanism by which the enhancement is attained is illustrated with the aid of both idealized and periodic models.


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Instabilities in Magnetoactive Materials

Stability of anisotropic magnetorheological elastomers in finite deformations: A micromechanical approach

Stephan Rudykh and Katia Bertoldi


Journal of the Mechanics and Physics of Solids 61 (2013) 949–967

Abstract:


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Stability of anisotropic electroactive polymers with application to layered media

The stability of anisotropic electroactive polymers is investigated. A general criterion for the onset of instabilities under plane-strain conditions is introduced in terms of a sextic polynomial whose coefficients depend on the instantaneous electroelastic moduli. In a way of an example, the stable domains of layered neo-Hookean dielectrics are determined. It is found that depending on the direction of the electrostatic excitation field relative to the lamination direction, the critical stretch ratios at which instabilities may occur can be either larger or smaller than the ones for the purely mechanical case.

DOI: 10.1007/s00033-011-0136-1


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Snap-through actuation of thick-wall electroactive balloons


Snap-through actuation of thick-wall electroactive balloons

Stephan Rudykh (a), (c), Kaushik Bhattacharya  (c) and Gal deBotton (a), (b)

(a) Department of Mechanical Engineering, Ben-Gurion University, 84105 Beer-Sheva, Israel

(b) Department of Biomedical Engineering, Ben-Gurion University, 84105 Beer-Sheva, Israel

(c) Division of Engineering and Applied Science, California Institute of Technology, Pasadena, CA 91125, United States

  
  
Abstract

Solution to the problem of a spherical balloon made out of an electroactive polymer which is subjected to coupled mechanical and electrical excitations is determined. It is found that for certain material behaviors instabilities that correspond to abrupt changes in the balloon size can be triggered. This can be exploited to electrically control different actuation cycles as well as to use the balloon as a micro-pump.

 

DOI:10.1016/j.ijnonlinmec.2011.05.006


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A paper on macroscopic stability of fiber composites

http://www.springerlink.com/content/wx185g5301300521/fulltext.pdf

 

Macroscopic instabilities of fiber reinforced composites undergoing large deformations are studied. Analytical predictions for the onset of instability are determined by application of a new variational estimate for the behavior of hyperelastic composites. The resulting, closed-form expressions, are compared with corresponding predictions of finite element simulations. The simulations are performed with 3-D models of periodic composites with hexagonal unit cell subjected to compression along the fibers as well as to non-aligned compression. Throughout, the analytical predictions for the failures of neo-Hookean and Gent composites are in agreement with the numerical simulations.

It is found that the critical stretch ratio for Gent composites is close to the one determined for neo-Hookean composites with similar volume fractions and contrasts between the phases properties.

During non-aligned compression the fibers rotate and hence, for some loading directions, the compression along the fibers never reaches the level at which loss of stability may occur.

 


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