Influence of stretch and temperature on the energy density of dielectric elastomer generators

Application of dielectric elastomers(DE) has remarkably increased in mechatronics because they are suitable candidates for energy harvesting due to their low cost,light weight, and high energy density. The dielectric elastomer generators(DEGs) exhibit high performance regardless of the applications scale. However, functioning as a generator, a DE may lose its efficiency due to several failure modes including material rupture, loss of tension(LT), electrical breakdown(EB), and electromechanical instability(EMI). The failure modes confine the area of allowable states for generation process.Dielectric constant and dielectric strength of such elastomers depend on the amount of applied deformation and also working temperature, which are often ignored in theoretical simulations. In this paper, variations of the above-mentioned parameters are considered in mechanical and electrical modellings to investigate their effects on energy density and efficiency of generators. Obtained results show that, ignoring the variations of material dielectric constant and dielectric strength leads to overestimation of the specific energy.Furthermore, it is shown that, for an acrylic-based generator, the specific energy sharply decreases with temperature rise.     

Rippling effect on the structural response of electrostatically actuated single-walled carbon nanotube based NEMS actuators

Several nonlinear phenomena have shown to have significant effect on the electromechanical performance of single-walled carbon nanotube (SWCNT) based nanoelectromechanical (NEMS) devices. To name few: the van der Waals forces, the Casimir forces, the tip charge concentration and the rippling phenomenon. Some of these effects have been take care of in previous investigation; however, some have been disregarded in the mechanical models suggested for simulation of the SWCNT based structures. In this paper, the influence of rippling deformation on the vibration characteristics of SWCNT based actuators is investigated using a nonlinear Euler-Bernoulli beam theory that incorporates the effect of rippling deformation using an improved function including some correcting terms for the SWCNT curvature (rippling deformation). The influence of the Casimir and the van der Waals attraction forces are considered in the proposed model as well as the size-dependent behavior assuming the so-called Eringen nonlocal elasticity theory. The dynamic response of CNT is investigated based on time history and phase portrait plots of the CNT based nano-actuator. It is shown that the rippling deformation can significantly decrease the static as well as the dynamic pull-in voltage of the SWCNT based actuator. The rippling deformation of SWCNT decreases the dynamic pull-in time as well. Effect of various factors such as the DC actuation load and the Casimir attractive forces on the dynamic stability and the pull-in characteristics of the nano-actuator are examined. Results of the present study are beneficial to accurate design and fabrication of electromechanical CNT based actuators. Comparison between the obtained results and those reported in the literature by experiments and molecular dynamics, verifies the integrity of the present numerical analysis.     

On the dynamic electromechanical loading of dielectric elastomer membranes

Dielectric elastomer actuators (DEAs) have received considerable attention recently due to large voltage-induced strains, which can be over 100%. Previously, a large deformation quasi-static model that describes the out-of-plane deformations of clamped diaphragms was derived. The numerical model results compare well with quasi-static experimental results for the same configuration. With relevance to dynamic applications, the time-varying response of initially planar dielectric elastomer membranes configured for out-of-plane deformations has not been reported until now. In this paper, an experimental investigation and analysis of the dynamic response of a dielectric elastomer membrane is reported. The experiments were conducted with prestretched DEAs fabricated from 0.5 mm thick polyacrylate films and carbon grease electrodes. The experiments covered the electromechanical spectrum by investigating membrane response due to (i) a time-varying voltage input and (ii) a time-varying pressure input, resulting in a combined electromechanical loading state in both cases. For the time-varying voltage experiments, the membrane had a prestretch of three and was passively inflated to various predetermined states, and then actuated. The pole strains incurred during the inflation were as high as 25.6%, corresponding to slightly less than a hemispherical state. On actuation, the membrane would inflate further, causing a maximum additional strain of 9.5%. For the time-varying pressure experiments, the prestretched membrane was inflated and deflated mechanically while a constant voltage was applied. The membrane was cycled between various predetermined inflation states, the largest of which was nearly hemispherical, which with an applied constant voltage of 3 kV corresponded to a maximum polar strain of 28%. The results from these experiments reveal that the response of the membrane is a departure from the classical dynamic response of continuum membrane structures. The dynamic response of the membrane is that of a damped system with specific deformation shapes reminiscent of the classical membrane mode shapes but without same-phase oscillation, that is to say all parts of the system do not pass through the equilibrium configuration at the same time. Of particular interest is the ability to excite these deformations through a varying electrical load at constant mechanical pressure.     

Pull-in instability of a typical electrostatic MEMS resonator and its control by delayed feedback

Pull-in instability of the electrostatic microstructures is a common undesirable phenomenon which implies the loss of reliability of micro-electromechanical systems. Therefore, it is necessary to understand its mechanism and then reduce the phenomenon. In this work, pull-in instability of a typical electrostatic MEMS resonator is discussed in detail. Delayed position feedback and delayed velocity feedback are introduced to suppress pull-in instability, respectively. The thresholds of AC voltage for pull-in instability in the initial system and the controlled systems are obtained analytically by the Melnikov method. The theoretical predictions are in good agreement with the numerical results. It follows that pull-in instability of the MEMS resonator can be ascribed to the homoclinic bifurcation inducing by the AC and DC load. Furthermore, it is found that the controllers are both good strategies to reduce pull-in instability when their gains are positive. The delayed position feedback controller can work well only when the delay is very short and AC voltage is low, while the delayed velocity feedback will be effective under a much higher AC voltage and a wider delay range.     

基于板理论的层级褶皱结构失效模式分析

本文基于弹性板理论和夹层板理论对二级层级褶皱结构失效模式进行了分析。通过对基本构件进行受力分析得到了载荷与结构变形之间的关系。根据6种失效模式的定义,从极限载荷或极限应力角度出发,分析了在压缩载荷和剪切载荷工况下的各种失效模式,给出了结构单胞对应的等效正应力和等效切应力表达式。由最小失效强度得到了各失效模式之间的占优关系,并构建了失效机理图来阐释这一机制。最后通过与有限元分析结果比较,分析了本文公式的精度,与数值解吻合较好。     

The response of clamped-clamped microbeams under mechanical shock

We present modeling, simulation, and characterization for the dynamic response of clamped-clamped microbeams under mechanical shock. A Galerkin-based reduced-order model is utilized and its results are verified by comparing to finite-element results. The results indicate that the response of a microbeam to mechanical shock is inherently non-linear because of the dominating effect of mid-plane stretching. The effect of the shock pulse shape is investigated. It is concluded that the shape of the shock pulse can result in significant dynamic amplification in the response of the microbeam even in cases where the shock load is considered quasi-static.The combined effect of the electrostatic force and mechanical shock is investigated. The results show that this combined effect can lead to early instability in microelectromechanical systems (MEMS) devices through dynamic pull-in. This could explain some of the reported experimental evidences for the existence of strange modes of failure of MEMS devices under mechanical shock and impact. These failures are characterized by overlaps between moving microstructures and stationary electrodes, which cause electrical shorts. The shock-electrostatic interaction is shown to be promising to design smart MEMS switches triggered at predetermined level of shock and acceleration. Finally, the mechanical shock combined with the packaging effect of MEMS devices is analyzed. A single-degree-of-freedom model representing the motion of the package, which is mounted over a printed circuit board, coupled with the continuous beam model is utilized. Our results reveal that neglecting the effect of the package motion on the response of microbeams can overestimate or underestimate their response. It is concluded that a poor design of the package may result in severe amplification of the shock effect leading to a device failure.     

评估TiN薄膜与基材结合的划痕试验及有限元模拟

通过有限元模型模拟划痕试验得到的结果表明∶切应力的起伏变化?膜/基界面处切应力差值?接触区附近膜层表面张应力?高载下的几种应力集中等,对膜/基体系的失效都有重要的作用.通过模型计算临界载荷下的膜/基界面处切应力差值,可用来评价膜层与基材的结合强度;提出了划痕试验中膜/基体系失效的2种机制.不同性能基材的TiN膜/基体系划痕试验结果,可验证本文有限元模拟的有效性,并表明临界载荷是膜/基结合强度?体系承载能力?内聚结合性能等的综合反映;低载往复摩擦磨损试验的结果进一步证实,用划痕试验的临界载荷评估膜/基结合强度具有局限性.     

机载平台式惯导系统导电环故障机理分析及解决措施

导电环是平台式惯导系统惯性平台的重要组成部分,主要承担惯性平台台体内各种信号与系统的传输和交联,导电环发生故障直接会导致惯导系统失效。针对导电环三种故障模式:接触电阻变大、绝缘强度降低以及烧坏,本文采用微动磨损和电接触理论,结合作者的实际工作经验,对故障机理进行了分析,提出了相应的改进措施。这些措施均在生产中得到落实。落实措施后的故障统计表明,采取的改进措施有效,导电环的故障率与原来相比降低了38%。     

An alternative reduced order model for electrically actuated micro-beams under mechanical shock

This study focuses on the effect of mechanical shock on dynamic pull-in instability of eclectically actuated micro-beams through an alternative reduced order model (ROM). The model's predictions for dynamic pull-in voltages are compared with available finite element (FE) results and six modes Galerkin approximations in the literature. It is shown that present results for high shock accelerations agree with FE predictions better than those obtained using six modes approximations. Furthermore, the present model can remove the limitation of previous methods in capturing dynamic pull-in instability for cases under enormous shock accelerations.     

POZD涂层方形钢筋混凝土板抗接触爆炸试验研究

为研究聚异氰氨酸酯噁唑烷聚合物高分子材料（polyisocyanate oxazodone，POZD）涂层方形钢筋混凝土板在接触爆炸作用下的破坏模式和抗爆性能，对POZD涂层方形钢筋混凝土板进行接触爆炸条件下试验研究。试验中采用建筑结构中楼面设计常用的钢筋混凝土板为研究对象，通过11次独立的爆炸试验，分析了不同POZD涂层厚度对抗爆性能的影响，观测了钢筋混凝土板在不同装药量和不同POZD涂层厚度条件下的破坏模式和破坏特征，研究结果表明:涂层POZD钢筋混凝土板的主要破坏模式为钢筋混凝土板正面爆炸成坑，背面POZD涂层的圆锥状鼓起。POZD涂层鼓起主要是在爆炸冲击波作用下POZD涂层从基体板脱离并出现较大塑性变形所致。当冲击波荷载强度超过POZD材料的极限抗拉强度时，在涂层锥尖处形成较小的圆孔装剪切破坏，涂层的其他区域保持完好，从而让钢筋混凝土板不会产生较大范围的震塌破坏。在强冲击波荷载作用下利用POZD涂层仍然能够保持大变形、高塑性特性，可以通过自身的大变形很好地延长爆炸荷载的作用时间和耗散时间，吸收较大冲击波能量，从而约束混凝土震塌碎片，提高钢混混凝土板的抗爆性能。随着POZD涂层厚度增加，板的抗接触爆炸作用下的抗爆能力越强，临界震塌破坏装药量越多。研究结果可为工程应用及毁伤评估提供参考。     

Instabilities in multilayered soft dielectrics

Experimental observations clearly show that the performance of dielectric elastomeric-based devices can be considerably improved using composite materials. A critical issue in the development of composite dielectric materials toward applications is the prediction of their failure mechanisms due to the applied electromechanical loads. In this paper we investigate analytically the influence of electromechanical finite deformations on the stability of multilayered soft dielectrics under plane-strain conditions. Four different criteria are considered: (i) loss of positive definiteness of the tangent electroelastic constitutive operator, (ii) existence of diffuse modes of bifurcation (microscopic modes), (iii) loss of strong ellipticity of the homogenized continuum (localized or macroscopic modes), and (iv) electric breakdown. While the formulation is developed for generic isotropic hyperelastic dielectrics, results are presented for the special class of ideal dielectrics incorporating a neo-Hookean elastic response. The effect of material properties and loading conditions is investigated, providing a detailed picture of the different possible failure modes.     

Pattern formation on a stretchable substrate

We propose a theoretical exploration on phase separation of a binary monolayer of molecules on a uniformly stretched substrate. By extending the previous phase-field model to involve the effect of such a finite pre-stretch, the stability of the monolayer is analyzed, and the temporal evolution of the compositional domains is simulated. Our results indicate that the pre-stretch provides an efficient way to order the phase separation patterns. Therefore, by properly adjusting the principal stretch of the substrate and the composition of the monolayer, the resulting surface pattern may be well tailored.     

Analytical modeling of static behavior of electrostatically actuated nano/micromirrors considering van der Waals forces

In this paper,the effect of van der Waals(vdW)force on the pull-in behavior of electrostatically actuatednano/micromirrors is investigated.First,the minimum potential energy principle is utilized to find the equation governing the static behavior of nano/micromirror under electrostatic and vdW forces.Then,the stability of static equilibrium points is analyzed using the energy method.It is foundthat when there exist two equilibrium points,the smaller oneis stable and the larger one is unstable.The effects of different design parameters on the mirror’s pull-in angle andpull-in voltage are studied and it is found that vdW forcecan considerably reduce the stability limit of the mirror.Atthe end,the nonlinear equilibrium equation is solved numerically and analytically using homotopy perturbation method(HPM).It is observed that a sixth order perturbation approximation can precisely model the mirror’s behavior.The results of this paper can be used for stable operation design andsafe fabrication of torsional nano/micro actuators.     

A compliant mechanism with variable stiffness achieved by rotary actuators and shape-memory alloy

The aim of this article is to study the consequences of the active stiffening of a compliant mechanism on the workspace created by the deformation of its structure. In connection with recent soft robotics research integrating shape-memory alloys (SMAs), the variation in stiffness over time is here obtained by the thermal activation of a nickel–titanium SMA spring. The workspace is created by the deformation (in the strength of materials sense) controlled by two rotary actuators acting on a structure comprising two angled flexible beams. In addition to a natural variation in the elasticity modulus of the SMA component during its thermal activation, its shape reconfiguration adds a structural deformation modifying the workspace. The existence of a common area between the workspaces of the mechanism corresponding to the non-activated and activated modes of the SMA is preserved. Several compliance maps are determined from measurements using a laser tracker targeting a given position of the loaded structure. The impact of SMA pre-stretch on stiffness variability is compared to that of a change in Young’s modulus. Variations in the stiffness distributions between the two modes reveal interesting properties (stiffness sign inversion, anisotropy) for the future optimal design of compliant mechanisms with high versatility, associating the spatial positions of the effector with variable stiffness values.     

Experimental slowing of flexural waves in dielectric elastomer films by voltage

Shape and physical properties of dielectric elastomers are changeable by voltage. Theoretical works show that these changes can be harnessed to tune the propagation of superposed elastic waves. We experimentally demonstrate this concept by manipulating waves in a dielectric elastomer film, focusing on the flexural mode at low frequencies. To this end, we design an experimental apparatus to pre-stretch, actuate, excite waves at low frequencies in a VHB™ 4910 film, and measure the velocity of the fundamental flexural mode. Our results show that the excited wave velocity is slowed down by the applied voltage, and provide experimental proof of concept for the application of deformable dielectrics as tunable waveguides.     

丙烯酸弹性体的率相关分数阶黏弹性模型研究

丙烯酸弹性体VHB 4910作为一种重要的介电弹性体,在软体机器人、致动器、俘能器和智能隔振器等领域有很好的应用前景.但材料的非线性黏弹性对其力学行为有显著影响.近来分数阶模型在复杂材料的建模中取得了成功.本文基于分数阶有限变形Kelvin-Voigt流变学模型建立弹性体的三维张量本构,并进一步推导单向拉伸情况下的本构...     

A dynamic finite element method for inhomogeneous deformation and electromechanical instability of dielectric elastomer transducers

We present a three-dimensional nonlinear finite element formulation for dielectric elastomers. The mechanical and electrical governing equations are solved monolithically using an implicit time integrator, where the governing finite element equations are given for both static and dynamic cases. By accounting for inertial terms in conjunction with the Arruda–Boyce rubber hyperelastic constitutive model, we demonstrate the ability to capture the various modes of inhomogeneous deformation, including pull-in instability and wrinkling, that may result in dielectric elastomers that are subject to various forms of electrostatic loading. The formulation presented here forms the basis for needed computational tools that can elucidate the electromechanical behavior and properties of dielectric elastomers that are used for engineering applications.     

元件与模式相对重要度对静强度可靠度的影响分析

本文通过研究指出,在运用增量载荷法寻找每级可能破坏元件时,必须考虑每个元件变异系数的影响,以避免漏掉极限承载能力很大但可靠度很低的失效模式;同时指出,结构失效的各个模式对系统可靠性的影响不仅与其本身发生失效的概率有关,而且与这个模式的重要程度有关,并提出了采用加权系数来计及每个失效模式的重要程度对结构系统可靠性影响的方法;最后用算例说明了本文方法的合理性。     

Effects of the van der Waals force,squeeze-film damping,and contact bounce on the dynamics of electrostatic microcantilevers before and after pull-in

The operational range of microcantilever beams under electrostatic force can be extended beyond pull-in in the presence of an intermediate dielectric layer. In this paper, a systematic method for deriving dynamic equation of microcantilevers under electrostatic force is presented. This model covers the behavior of the microcantilevers before and after the pull-in including the effects of van der Waals force, squeeze-film damping, and contact bounce. First, a polynomial approximate shape function with a time-dependent variable for each configuration is defined. Using Hamilton’s principle, dynamic equations of microcantilever in all configurations have been derived. Comparison between modeling results and previous experimental data that have been used for validation of the model shows a good agreement.     

杆系结构静强度和疲劳失效机理及可靠性分析

根据疲劳载荷造成的疲劳累积损伤对结构极限承载力的影响情况，讨论了结构系统承载能力的可靠度计算方法，分析了结构系统中每一个单元在静载和疲劳载荷作用下的两种失效模式，并考虑了二者之间的相关性对该单元可靠性的影响，具体分析了结构系统在这两种载荷作用下的失效机理，给出了在这两种载荷作用下结构系统的可靠性分析方法. 算例表明，在不同的使用年限内，静载和疲劳载荷对结构系统可靠性的影响是不同的；在结构系统主要失效路径中既有单元静强度失效又有单元疲劳失效，这是符合结构系统使用真实情况的.