Date of Award

5-31-2018

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Mechanical Engineering - (Ph.D.)

Department

Mechanical and Industrial Engineering

First Advisor

Shawn Alexander Chester

Second Advisor

Matthew P. Adams

Third Advisor

Siva P.V. Nadimpalli

Fourth Advisor

I. Joga Rao

Fifth Advisor

Anthony D. Rosato

Sixth Advisor

David Shirokoff

Seventh Advisor

Pushpendra Singh

Abstract

Soft dielectrics are electrically-insulating elastomeric materials, which are capable of large deformation and electrical polarization, and are used as smart transducers for converting between mechanical and electrical energy. While much theoretical and computational modeling effort has gone into describing the ideal, time-independent behavior of these materials, viscoelasticity is a crucial component of the observed mechanical response and hence has a significant effect on electromechanical actuation. This thesis reports on a constitutive theory and numerical modeling capability for dielectric viscoelastomers, able to describe electromechanical coupling, large- deformations, large-stretch chain-locking, and a time-dependent mechanical response. This approach is calibrated to the widely-used soft dielectric VHB 4910, and the finite-element implementation of the model is used to study the role of viscoelasticity in instabilities in soft dielectrics, namely (1) the pull-in instability, (2) electrocreasing, (3) electrocavitation, and (4) wrinkling of a pretensioned three dimensional diaphragm actuator. Results show that viscoelastic effects delay the onset of instability under monotonic electrical loading and can even suppress instabilities under cyclic loading. Furthermore, quantitative agreement is obtained between experimentally measured and numerically simulated instability thresholds.

Filled rubber-like materials are important engineering materials, and they are widely used in aerospace, automotive, and other industries. However, their nonlinear, inelastic, and rate-dependent constitutive behavior is not fully understood and modeled with varying degrees of success. Much of the previous literature has focused on either capturing quasi-static stress-softening behavior or rate-dependent

viscous effects, but generally not both concurrently. This thesis develops a thermody- namically consistent constitutive model which accounts for both of those phenomena concurrently. A set of comprehensive mechanical tensile tests are conducted on the filled rubber Viton. The constitutive model is then calibrated to the experimental data, and numerically implemented into the finite element package Abaqus by writing a user material subroutine UMAT. The constitutive model is validated by comparing a numerical simulation prediction with an inhomogeneous deformation experiment. As an extension to the study of Viton, this thesis also develops a constitutive model to quantitatively capture thermal recovery of the Mullins effect. The model is then calibrated to experiments in the literature, and numerically implemented by writing a user material subroutine for the finite element program Abaqus/Standard. Lastly, simulation results suggest that the unanticipated behaviors due to recovery of Mullins effect are possible.

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