Optimal electromechanical bandgaps in piezo-embedded mechanical metamaterials

Elastic mechanical metamaterials are the exemplar of periodic structures. These are artificially designed structures having idiosyncratic physical properties like negative mass and negative Young’s modulus in specific frequency ranges. These extreme physical properties are due to the spatial periodicity of mechanical unit cells, which exhibit local resonance. That is why scientists are researching the dynamics of these structures for decades. This unusual dynamic behavior is frequency contingent, which modulates wave propagation through these structures. Locally resonant units in the designed metamaterial facilitate bandgap formation virtually at any frequency for wavelengths much higher than the lattice length of a unit. Here, we analyze the band structure of piezo-embedded negative mass metamaterial using the generalized Bloch theorem. For a finite number of the metamaterial units coupled equation of motion of the system is deduced, considering purely resistive and shunted inductor energy harvesting circuits. Successively, the voltage and power produced by piezoelectric material along with transmissibility of the system are computed using the backward substitution method. The addition of the piezoelectric material at the resonating unit increases the complexity of the solution. The results elucidate, the insertion of the piezoelectric material in the resonating unit provides better tunability in the band structure for simultaneous energy harvesting and vibration attenuation. Non-dimensional analysis of the system gives physical parameters that govern the formation of mechanical and electromechanical bandgaps. Optimized numerical values of these system parameters are also found for maximum first attenuation bandwidth. Thus, broader bandgap generation enhances vibration attenuation, and energy harvesting can be simultaneously available, making these structures multifunctional. This exploration can be considered as a step towards the active elastic mechanical metamaterials design.

     

一系螺旋弹簧动刚度对车辆-轨道耦合振动影响分析

建立准确表征一系悬挂轴箱螺旋弹簧波动特性的力学模型,运用动刚度矩阵法求解,研究其对悬挂系统隔振性能影响。结合基于格林函数法的车辆-轨道耦合动力学模型,引入弹簧刚度频变特性,对比分析考虑一系螺旋弹簧频变刚度前后车辆动力学性能之间的差异。结果表明,动刚度矩阵法可以精确求解螺旋弹簧随频率变化的动刚度特性,在一阶模态振动频率后弹簧刚度值呈现103等级的剧烈变化,该结果与有限元模型结果一致;一系螺旋弹簧的动态频率特性导致轮轨激励由车轮至构架的振动位移传递率提高到接近于1,而对车体的振动传递率提高到了10-3左右;在整车车辆-轨道动力学计算中,其对轮轨振动影响较小,但车体与构架出现了较高的高频振动能量峰值。包含一系悬挂动刚度的车辆模型更接近实际,为了降低车辆振动,应尽量提高一系螺旋弹簧自振频率并降低动刚度变化幅值。       

边界摩擦条件下含有预紧的对合碟簧隔振单元的振动特性

为研究对合碟簧的摩擦对隔振单元振动特性的影响,设计了一种存在预紧的对合碟簧隔振单元,建立了考虑边界摩擦条件下对合碟簧组的刚度模型。通过准静态加载试验验证了该刚度模型的正确性;并提出了该碟簧隔振单元的非线性动力学模型,应用平均法对对合碟簧隔振单元的自由振动进行分析,得出影响系统自由振动频率的各项因素;对基础激励下对合碟簧隔振单元的受迫振动进行分析计算。结果表明:在小位移振动条件下,该碟簧隔振单元可近似等效成非线性黏性阻尼系统,且附加支持力系数仅影响系统脱离频率,刚度三次项系数值的选取越大越不利于系统隔振;在大位移振动条件下,计算得到了系统存在异常跳跃状态与全频域内传递率系数T′d≤0状态,并说明这两种状态与黏性阻尼系数以及附加支持力系数间的联系。     

聚氨酯硬泡连续屏障近场隔振效果分析

传统的连续屏障在缓解环境振动方面已取得比较理想的效果,但其整体稳定性差,对低频的隔振效果不佳。基于此背景,提出了锚杆约束的聚氨酯硬泡连续屏障。通过有限元数值分析方法,对聚氨酯硬泡连续屏障和混凝土连续屏障分别在地面简谐激励和桩振源简谐激励作用下的近场隔振性能进行研究。研究表明:地面振源激励下,混凝土连续墙和聚氨酯硬泡连续屏障对中高频振动有较好的隔振效果,混凝土连续墙对低频(10 Hz)振动隔振效果不佳,聚氨酯硬泡连续屏障对5 Hz~10 Hz的低频振动有很好的隔振效果;桩振源激励下,两种屏障的隔振效果比地面振源激励时效果更好,两种屏障对中高频振动及5 Hz~10 Hz的低频振动有较好的隔振效果。     

超低频非线性隔振系统的研究

通过分析正负刚度并联隔振机理,提出了一种新型超低频被动隔振系统。该系统采用负刚度机构与正刚度弹簧并联,具有高的支撑刚度和极低的运动刚度,且系统刚度呈现非线性,解决了传统被动隔振系统存在的超低频隔振难题。通过对该系统力学特性进行分析,给出了系统刚度表达式及在平衡位置处的零刚度条件,为新型超低频非线性隔振器的设计提供了理论指导。     

Vibration isolation of LNG containment systems due to sloshing with glass fiber composite

Since the stainless steel primary barriers of Cryogenic Containment System (CCS) for Liquefied Natural Gas (LNG) ship failed by impact loads due to LNG cavitation during operation, the stiffness and strength of the primary barrier was increased much to endure the impact pressure of LNG, which increased the transfer of impact load to foam insulation panels under the primary barrier. The increase of impact load to the foam insulation panels will induce crack propagation into the insulation foam panels due to the low toughness of insulation foams at cryogenic temperatures.In this work, an impact isolation system was developed using glass fiber composite mats to lower the impact load transmission into the foam insulation panels. An optimal design process for the vibration isolation system was conducted by a numerical method. For reliability of the impact isolation system, cryogenic fatigue tests of the glass fiber composite mats were performed. Also, an impact test equipment with low impact duration was developed to simulate the efficiency of the impact isolation system.     

From Geometric Transformations to Auxetic Metamaterials

The paper introduces a new alternative towards fabrication of auxetic metamaterials (materials with negative Poisson’s ratio) controlled by geometric transformations. These transformations are derived from the theory of small (infinitesimal) elastic deformation superimposed on finite elastic deformations. By using this theory, a cylindrical region filled with initial deformed foam is transformed through deformation into a cylindrical shell region filled with auxetic metamaterial. As an example, the realization of the seismic cloak device becomes a practical possibility.     

泡沫材料厚度对其刚度及阻尼系数的影响

根据隔振分析模型,建立了相应的实验测量系统,测量并分析了泡沫材料的刚度及阻尼系数。采用激振台对泡沫材料施加激励,由2个加速度计拾振,获得了由泡沫材料组成的缓冲包装系统的振动传递比曲线,应用非线性拟合方法计算得到了阻尼比,并求得了刚度和阻尼系数。实验结果表明:泡沫材料刚度随其厚度增加而递减,阻尼系数则在一定范围内随厚度增加而降低;泡沫材料厚度的增加会降低缓冲包装系统隔振的下限频率,在一定范围内越厚,隔振效果会越好,但当厚度超过了特定值,隔振效果反而有所下降。     

Miura折叠超材料在简谐激励下的振动响应

折叠超材料基于折叠结构发展而来,具有超常的物理性能。应用ADAMS分析典型的三种结构模型,在力和位移两种简谐激励下,不同折痕刚度对Miura折叠超材料振动响应的影响。结果表明,单自由度结构的Miura模型在两种激励下频率响应类似,通过改变折痕刚度可以改变其共振频率,达到很好的低频隔振效果。多自由度模型可以通过增加堆叠数量降低共振频率。为Miura折叠结构的隔振设计提供一定理论参考。     

蜂窝结构力学超材料弹性及抗冲击性能的研究进展

具有手性蜂窝结构的力学超材料是近年来发展起来的高性能工程材料,它具有轻质、高比刚度、负泊松比、结构参数可调以及力学性能稳定等优点。其不仅可以实现面内变形,面外承载的双重力学作用,还具有出色的隔振、吸声降噪以及控制弹性波的传播等工程应用潜质,在智能结构、车辆船舶、航空航天等领域具有巨大的发展潜力。本文从其弹性和抗冲击两个力学性能方面进行综述。首先介绍并评述了近年来蜂窝结构力学超材料的面内杨氏模量、负泊松比特性以及面外剪切模量等弹性性能的理论分析研究进展。在抗冲击性能方面,从力学模型建立和有限元分析的角度出发,对手性蜂窝结构力学超材料在冲击载荷作用下的整体变形及其抗冲击性能的研究现状分别进行了评述。最后指出针对蜂窝结构力学超材料弹性及冲击性能的研究,可进一步建立内部韧带变形及力的传递力学模型以及深入探索冲击过程吸能机理等,以期为该类力学超材料内部韧带和节点环结构的优化设计提供参考。     

聚氨酯泡沫垫层隔振系统的数值模型

从聚氨酯隔振垫层的粘弹性本构关系出发,通过数值分析的方法,对垫层隔振系统的动力学试验进行模拟计算,结果显示:垫层隔振系统在各种频率的周期性外载激励下,其隔振力与位移都表现出典型的迟滞非线性特征,与试验所得结果有相同的变化趋势,表明计算模型和方法可行.以此为基础,可对垫层隔振系统的动力学特性进行更多的数值研究,为此类隔振器的动力学设计和动力优化提供理论依据和实践方法.     

基于几何非线性的果蔬运输隔振系统设计

目的 针对运输过程中振动与冲击造成果蔬损伤和品质下降的问题,基于非线性振动理论,提出一种几何非线性结构的果蔬运输隔振系统。方法 首先,基于水平斜弹簧负刚度机构与垂向正刚度弹簧并联的方式设计果蔬运输隔振结构;其次,分析其静力学特性,推导该非线性隔振装置的刚度特性;最后,建立果蔬运输车模型的运动微分方程,采用谐波平衡法研究该新型果蔬运输车的隔振特性。结果 研究结果表明,通过合理的结构参数设计,该隔振装置在平衡点附近具有高静态刚度、低动态刚度特性。对于小幅路面激励,该新型果蔬运输车将在全频段大幅优于对应的线性系统,随着路面激励幅值的增大,隔振装置刚度快速增加,频率跳跃线性逐渐出现,峰值逐渐出现并右移,中频段的隔振效果逐渐变差。高频段的隔振效果远优于对应的线性系统,且与路面激励幅值不敏感。结论 通过引入水平预压缩弹簧与垂向弹簧并联形式的几何非线性结构显著提高了果蔬运输隔振系统的隔振效果,减小运输过程中果蔬所受到的振动与冲击,从而避免了果蔬损伤和品质下降。     

Negative refraction in semiconductor metamaterials

An optical metamaterial is a composite in which subwavelength features, rather than the constituent materials, control the macroscopic electromagnetic properties of the material. Recently, properly designed metamaterials have garnered much interest because of their unusual interaction with electromagnetic waves. Whereas nature seems to have limits on the type of materials that exist, newly invented metamaterials are not bound by such constraints. These newly accessible electromagnetic properties make these materials an excellent platform for demonstrating unusual optical phenomena and unique applications such as subwavelength imaging and planar lens design. 'Negative-index materials', as first proposed, required the permittivity, epsilon, and permeability, mu, to be simultaneously less than zero, but such materials face limitations. Here, we demonstrate a comparatively low-loss, three-dimensional, all-semiconductor metamaterial that exhibits negative refraction for all incidence angles in the long-wave infrared region and requires only an anisotropic dielectric function with a single resonance. Using reflection and transmission measurements and a comprehensive model of the material, we demonstrate that our material exhibits negative refraction. This is furthermore confirmed through a straightforward beam optics experiment. This work will influence future metamaterial designs and their incorporation into optical semiconductor devices.     

高速动平衡机摆架振动特性分析与改进

高速动平衡机摆架的动力学特性直接影响到动平衡试验的准确性.为了改善动平衡机摆架的动力学特性,分析了动主刚度杆对平衡机摆架动刚度的影响.首先,使用有限元分析软件ANSYS建立高速动平衡机摆架模型,对其进行模态分析,得到动平衡机摆架的各阶频率和振型,除去对动平衡机摆架工作无影响的固有振型.对不同结构的摆架进行模态分析,比较计算结构,除去对动平衡机摆架固有频率没影响的结构.其次,在简化模型的基础上,提出摆架主刚度杆的5种改进方法.分别对各种改进之后的摆架进行谐响应分析,得到动刚度与频率关系曲线.结果表明,适当改变主刚度杆的结构尺寸或材料特性,都会改变摆架的动刚度.因此,在设计的过程中,可以通过适当改变主刚度杆的结构尺寸和材料特性来满足动平衡机摆架的设计要求.此方法具有较好的工程实际价值,为工程应用提供分析方法和改进参考.     

泡沫铝填充汽车车架的抗振性研究

为探讨满足汽车自身轻量化、环保、舒适和安全方向发展需要的新途径,以某汽车为例,初步设计了泡沫铝填充结构车架,并应用有限元分析软件ANSYS对其进行了静态与动态分析。结果表明,该新型结构车架在满足自身性能要求下,其动态特性明显提高,从而抗振性得到提高。     

新型准零刚度果蔬运输隔振系统设计

目的 为避免果蔬在运输过程中因振动造成损伤,导致价值下降,基于准零刚度隔振技术,提出一种采用紧凑、高效的弹簧–滚轮–滚珠负刚度机构的新型果蔬运输隔振箱装置。方法 首先,基于弹簧–滚轮–滚珠负刚度机构与垂向正刚度弹簧并联的方式设计准零刚度果蔬运输隔振系统结构;其次,分析其静力学特性,推导该非线性隔振装置的刚度特性;最后,在果蔬运输系统模型运动微分方程基础上建立Simulink仿真模型,研究在C级路面激励下,该新型运输隔振系统的动态隔振特性。结果 研究结果表明,通过合理的结构参数设计,该隔振装置在平衡点附近将具有高静态刚度、低动态刚度的准零刚度特性。在C级路面激励下,该新型运输隔振系统的振动位移峰的峰值相较于对应线性系统的衰减约29.3%,加速度均方根值衰减约97.3%。结论 设计合理的新型果蔬运输系统的隔振效率优于对应的线性隔振系统的。     

Folding at the Microscale: Enabling Multifunctional 3D Origami‐Architected Metamaterials

Mechanical metamaterials inspired by the Japanese art of paper folding have gained considerable attention because of their potential to yield deployable and highly tunable assemblies. The inherent foldability of origami structures enlarges the material design space with remarkable properties such as auxeticity and high deformation recoverability and deployability, the latter being key in applications where spatial constraints are pivotal. This work integrates the results of the design, 3D direct laser writing fabrication, and in situ scanning electron microscopic mechanical characterization of microscale origami metamaterials, based on the multimodal assembly of Miura‐Ori tubes. The origami‐architected metamaterials, achieved by means of microfabrication, display remarkable mechanical properties: stiffness and Poisson’s ratio tunable anisotropy, large degree of shape recoverability, multistability, and even reversible auxeticity whereby the metamaterial switches Poisson’s ratio sign during deformation. The findings here reported underscore the scalable and multifunctional nature of origami designs, and pave the way toward harnessing the power of origami engineering at small scales.     

双材料自由阻尼层结构拓扑优化设计方法

目的 在不减小板壳结构动刚度的前提下有效提高板壳结构的抗振性能,提出一种板壳阻尼复合结构拓扑优化设计.方法 以结构模态阻尼比为目标函数,以某阶固有频率不小于设定值为约束条件,推导目标函数和约束条件对设计变量的灵敏度表达式,采用SIMP插值函数和移动渐近线法求解优化数学模型.结果 研究表明,文中提出的双材料阻尼层结构相比传统单材料阻尼层结构的振动响应明显减小,同时适当改变设计目标和约束条件,可满足不同的工程应用需要.结论 通过在宏观上对高刚度低阻尼材料和低刚度高阻尼材料的分布进行了设计,优化后的结构兼顾低刚度高阻尼材料的高阻尼特性和高刚度低阻尼材料的高刚度特性,实现了结构高刚度高阻尼的设计.     

滚球型准零刚度隔振器的特性分析

针对工程实践中的低频隔振难题,提出一种紧凑的、带滚球装置的准零刚度隔振器。首先,对隔振器进行了静力学分析,得到了实现准零刚度的参数条件;其次,建立了隔振系统动力学模型,利用谐波平衡法分析了系统频响特性及隔振性能,讨论了激励幅值和阻尼对响应的影响;最后,利用ADAMS对系统动力学特性进行仿真及隔振效果评估。结果表明,与相应的线性隔振系统相比,准零刚度隔振器的起始隔振频率显著降低,且在低频范围内力传递率比线性系统低得多。因此,滚球装置提供的负刚度明显降低了系统竖向总刚度,使系统具有优异的低频隔振性能,更重要的是紧凑的设计使其更易工程化。