Instability Of Dielectric Elastomer Structures And Devices Subjected To Electromechanical Loading

Dielectric elastomer(DE)is a new and promising smart material which processes the advantages including light,high energy density,high efficiency,low cost,and quiet operation.It is widely used in smart actuators,electric generators,sensors,displays,and so on.Stability is a key problem for a long-term operation with high performances.This thesis mainly focuses on the effect of the snap-through stability,wrinkling,and viscoelasticity of the material on the instability of several typical dielectric elastomer structures and devices.DE short-length balloons are widely used in engineering.We study the effect of prestretch on the snap-through stability of an inflated short-length balloon.We study the shapes of the short-length DE balloon subjected to different prestreches and pressures.In the experiment,the snap-through instability is observed also with the localized bulge of the short-length DE balloon.By the theoretical model based on continuum mechanics,we find the shorter the balloon,the better the stability,and large prestretch is useful to restrain the snap-through stability.A series of such short-length balloons consist a DE peristaltic pump(DEPP)system which can drive the water inside.Our study reveals that large prestretch,small aspect ration,and large voltage will restrain the instability of the DEPP module.Further,we study the performance of the DEPP made of two modules.We actuate the DEPP by a series of complementary signals.In the experiment,the square wave voltage is found to be the best driving signal for the DEPP.We observe the mechanical resonance of the DEPP with the driving voltage and the optimal frequency of the DEPP is about 1.5 Hz which is in the same range of the human heart rate.In the experiments,the maximum average flow rate,the instant flow rate,the pump volume per cycle are about 2.5 L/min,3.2 L/min,and 0.09 L.It takes only 1 second for the DEPP to change its working frequency from 1 to 1.5 Hz,which is quite quick.The inflated diaphragm is another common structure of DE actuators.When we apply a square wave voltage to the inflated diaphragm,wrinkle nucleates.We study the critical voltage,nucleation site and propagation of wrinkle.Results show that with the increase of the pressure,the critical voltage of wrinkle will increase.Meanwhile,the nucleation site of wrinkle will move from the center to the boundary of the inflated diaphragm.What's more,with the increase of voltage,the stripe-like wrinkle will transfer to labyrinth wrinkle.We build up a simple theoretical model to predict the propagation of the wrinkle with the increase of the voltage.We also study the effect of the prestretch on the wrinkling of the inflated diagram.Results show the prestretch plays an important role in the nucleation sites and morphology of the wrinkles.Wrinkle always nucleates from the boundary of the inflated diaphragm with small pressure.For larger prestretch,the wrinkle always nucleates from the center of the inflated diaphragm.At last,we obtain the relationship of the snap-through instability,wrinkle,and electric breakdown.A phase diagram of such relationship is given in this study which will provide a guide in the design of such DE actuators.We also use a prestretched rectangular DE membrane bonded to a rigid frame to generate stable stripe-like wrinkles with static voltage.We study the wavelength and the amplitude of the wrinkles by experiment and theory.Results show that theoretical prediction agrees well with the experimental results.The wavelength can be tuned by the prestretch of the membrane and the geometry of the rigid frame and the amplitude of the wrinkle can be tuned by the applying voltage.Based on the principle of wrinkles,we build up a smart window of which the transparency can be tuned by applying voltage.We also develop a trumpet-like DE device which is able to generate radial wrinkle.It consists of a prestretched circular DE membrane where a solid plastic plate is placed in the center of the membrane with its boundary fixed.When we apply a vertical displacement to the circular membrane with voltage,radial wrinkle will nucleate.With the increase of the applying voltage,the nucleation sites of the wrinkles move from the center to the boundary of the stretched DE membrane.We build a theory to analyze the nucleation and obtain an algebraic expression to predict the wavelength of the wrinkles.Results show this expression works very well.At last,we investigate the effect of the viscoelasticity on the wrinkling behavior of an inflated DE diaphragm.Results show the critical voltage of the wrinkle decreases as the relaxation of the DE,and at last they achieve a stable value.What's more,the nucleation sites will move from the top of the inflated diaphragm to the boundary as the viscoelastic relaxation of the DE.It is noted that the critical voltage for the circumferential wrinkle is always smaller than that for radial wrinkle.High electromechanical load enables the inflated DE diaphragm to reach its stable state quickly.And the nucleation site always occurs on the boundary of the DE diaphragm with the high electromechanical load.This study will help the future design of the DE transducers in avoiding or harnessing the instability.