Electro-mechanically Coupled Pure-shear Cyclic Deformation And Low-cycle Fatigue Of Dielectric Elastomers

Dielectric elastomers(DEs)are a class of electro-active soft materials,possessing desirable capabilities of giant voltage-induced deformation,fast responsiveness and high energy density.Dielectric elastomer based devices,including soft actuators,energy harvestors and sensors,have been extensively developed in various engineering fields,such as soft robotics,aerospace and biomedical devices.The DE has strong hyperelasticity,non-linear inelasticity and electro-mechanical coupling characteristics.When being used in practice,the DE-based devices are often subjected to an electro-mechanically coupled cyclic loading and present a complicated cyclic deformation behavior,which leads to a big challenge in the accurate actuation and reliability assessment of the devices.In recent two decades,researchers have performed many experimental and theoretical works on the basic mechanical and electro-mechanical properties of DEs in pure mechanical and electro-mechanically coupled fields,respectively.The hyperelasticity,viscoelasticity,electro-mechanically coupled effect and instability of DEs have been intensively investigated.However,the cyclic deformation and fatigue behaviors of DEs in both pure mechanical and electro-mechanically coupled fields are rarely studied;especially,the experimental observations and theoretical modeling of DEs'ratchetting and fatigue behaviors have not been reported.Therefore,in this work,the cyclic deformation and fatigue behaviors of DEs are investigated by performing systematic experimental observations and theoretical modeling.This work provides helpful theoretical guidelines for structural optimization,safety assessment and fatigue life prediction of DE devices.Main contents of this work are briefly summarized as follows:(1)Experimental observations on pure-shear cyclic deformation behaviors of VHB^TM4910(3M^TM)DE under pure mechanical and electro-mechanically coupled loading conditions.Firstly,in the pure mechanical field,systematic strain/stress-controlled cyclic tests were performed to reveal the cyclic deformation characteristics of the DE,e.g.,the cyclic softening and ratchetting,as well as their dependences on the loading level and loading rate.Then,the electro-mechanically coupled cyclic deformation behaviors of the DE,especially the effect of electro-mechanical coupling on the cyclic softening and ratchetting,were comprehensively investigated by conducting a series of cyclic tests with various combined electro-mechanical loading modes.(2)Constitutive models for the cyclic large deformation behaviors of DEs in both pure mechanical and electro-mechanical fields.Firstly,based on the experimental observations of pure mechanical cyclic deformation behaviors of DEs,a visco-hyperelastic-plastic constitutive model was proposed within a framework of large deformation theory by incorporating the hyperelasticity,finite viscoelasticity and plasticity of DEs,simultaneously.Then,the proposed visco-hyperelastic-plastic constitutive model was extended to an electro-mechanically coupled one by considering the quasi-linear dielectric polarization behavior of DEs.Finally,by comparing the experimental data with simulated results,the newly developed constitutive models were validated to be able to reasonably predict the pure mechanical and electro-mechanical coupling cyclic deformation behaviors of DEs,including the cyclic softening and ratchetting,as well as their dependences on the loading level,loading rate and electro-mechanical loading conditions.(3)Experimental observation and theoretical modeling of electro-mechanically coupled low-cycle fatigue failure of DEs.Firstly,the low-cycle fatigue failure behavior of DEs was revealed by performing a series of electro-mechanically coupled fatigue tests with various mean stresses and stress amplitudes.Based on the experimental observations,an electro-mechanical fatigue life-prediction model was then proposed by employing a configurational stress-based fatigue damage predictor.The proposed model predicts the experimental results reasonably well.