天然橡胶基磁流变弹性体的研制与表征

磁流变弹性体又称磁敏高弹体,是一种由高分子聚合物和磁性颗粒构成的新型智能材料,它的力学、电学、磁学等诸性能可以由外加磁场来控制,因此磁流变弹性体在舰船、振动控制等领域具有广泛的应用前景.但目前国际上研制的磁流变弹性体存在机械性能不够好和磁致效应不够强的问题,这制约了基于磁流变弹性体器件的设计和应用.为了制备出实用型磁流变弹性体,本文对其制备条件进行了研究,包括基体类型、预结构化时磁场强度和温度、增塑剂和磁性颗粒含量对磁流变弹性体磁致效应的影响.结果表明,以天然橡胶为基体的磁流变弹性体,在高于600mT外加磁感应强度下,当磁性颗粒含量为80%(质量比)时,剪切模量的相对增量达133%;而当磁性颗粒含量为90%时,剪切模量的绝对增量达4.5MPa.本文还对磁流变弹性体应用环境进行了实验研究,结果表明磁流变弹性体在小应变下显示出更强的磁致效应,而激励频率不改变材料的磁致模量.     

天然橡胶基磁流变弹性体的研制与表征

磁流变弹性体又称磁敏高弹体，是一种由高分子聚合物和磁性颗粒构成的新型智能材料，它的力学、电学、磁学等诸性能可以由外加磁场来控制，因此磁流变弹性体在舰船、振动控制等领域具有广泛的应用前景。但目前国际上研制的磁流变弹性体存在机械性能不够好和磁致效应不够强的问题，这制约了基于磁流变弹性体器件的设计和应用。为了制备出实用型磁流变弹性体，本文对其制备条件进行了研究，包括基体类型、预结构化时磁场强度和温度、增塑剂和磁性颗粒含量对磁流变弹性体磁致效应的影响。结果表明，以天然橡胶为基体的磁流变弹性体，在高于600mT外加磁感应强度下，当磁性颗粒含量为80％（质量比）时，剪切模量的相对增量达133％；而当磁性颗粒含量为90％时，剪切模量的绝对增量达4．5MPa。本文还对磁流变弹性体应用环境进行了实验研究，结果表明磁流变弹性体在小应变下显示出更强的磁致效应，而激励频率不改变材料的磁致模量。     

磁流变弹性体剪切性能的动态实验研究

简述了一种磁流变弹性体(MRE)剪切性能的动态测试装置,其主要结构是由一块磁流变弹性体和一个质量块组成的单自由度振动系统。在磁场作用下,磁流变弹性体的力学性能发生相应的改变。通过该装置测出有场下的MRE受剪切激励后的自由衰减特性,并建立理论模型,从而间接得到剪切模量G(BMRE)和材料损耗因子βm(BMRE)。这样的处理方法使得实验条件得到简化且不依赖于初始激励状态。所得到的剪切模量的变化性能与文献中报道的一致(在没有达到磁饱和的条件下,剪切模量随外加磁场的变化而改变),但剪切模量的改变量是零场下剪切模量的60%,磁控性能也有所提高,并得到了材料损耗因子。     

磁场作用下磁流变液结构演化的实验研究

本文显微观察了工程应用中使用的高体积比磁流变液在磁场作用下的结构演化过程,实验结果指出：工程应用中高体积比浓度的磁流变液在磁场作用下将形成一种复杂的三维结构斧磁颗粒与母液油将一起形成空间结构,在形成的类固体物质内部将含有大量的裂纹等缺陷,在磁场的作用下内部的缺陷将扩展,这对磁流变液机理研究和建模提供了重要的实验依据。     

几何非线性软铁磁导电梁式板在磁场中的动力响应

本文提出一套描述静磁场中软铁磁导电梁式板大挠度自由振动的基本方程,在这组方程中,磁化、涡电流和几何非线性梁式板的力学行为之间的相互耦合被考虑。对两端铰支（不可移）的梁式板,详细讨论了电导率、磁导率、外加磁场的大小和倾角以及板的几何非线性对其自由振动的周期（或频率）和振幅的影响,数值结果显示,板的几何非线性引起的面内张力使板的固有频率上升,导电性和磁化则使频率下降。在磁弱性失稳临界磁场值两侧,板的频率随磁场变化的规律明显不同。另外,随着磁场倾角的增加或者磁导率的增大,电磁阻尼效应明显增强,振动被显著抑制。     

考虑强结合界面的磁流变弹性体力学性能分析

磁流变弹性体是将铁磁性颗粒填充到非磁性的聚合物基体中,通过固化作用形成的柱状或链状结构。目前,研究磁流变弹性体的力学模型主要是磁偶极子模型以及修正的磁偶极子模型。这些模型考虑了颗粒之间的相互作用,但尚未涉及颗粒和基体之间的相互作用。本文在考虑颗粒和基体相互作用的基础上,基于剪滞法理论计算出强结合界面磁流变弹性体模量和阻尼特性。通过实验制备硅橡胶基的磁流变弹性体,并在应变幅值较小时测试其剪切储能模量和阻尼因子,详细分析不同的应变幅值和磁场强度对磁流变弹性体性能的影响。理论结果与实验结果相符,验证了本文关于强结合界面性能分析的正确性。     

磁流变弹性体自调谐式吸振器及其优化控制

本文研制了一种基于磁流变弹性体的自调谐式吸振器,它利用磁流变弹性体这种新型智能材料作为吸振器的弹性元件和阻尼元件,通过外加磁场控制磁流变弹性体的剪切模量来改变吸振器的固有频率,实现吸振器的移频.并将遗传算法改进移植到吸振器,对其进行优化控制.实验结果表明,这种遗传算法具有全局搜索和快速收敛的特点,它能使吸振器快速找到吸振器减振效果最佳点,并且经过优化控制的磁流变弹性体自调谐式吸振器在移频范围内具有很好的减振效果,减振效果最高可达25dB.     

磁流变弹性体自调谐式吸振器及其优化控制

本文研制了一种基于磁流变弹性体的自调谐式吸振器，它利用磁流变弹性体这种新型智能材料作为吸振器的弹性元件和阻尼元件，通过外加磁场控制磁流变弹性体的剪切模量来改变吸振器的固有频率，实现吸振器的移频。并将遗传算法改进移植到吸振器，对其进行优化控制。实验结果表明，这种遗传算法具有全局搜索和快速收敛的特点，它能使吸振器快速找到吸振器减振效果最佳点，并且经过优化控制的磁流变弹性体自调谐式吸振器在移频范围内具有很好的减振效果，减振效果最高可达25dB。     

磁场下2种羰基铁磁流变液的摩擦磨损特性

本文采用改进后的四球摩擦磨损试验机,考察了羰基铁磁流变液在外加磁场条件下的摩擦磨损行为,并比较了有场和无场条件下磁流变液的摩擦系数变化情况。结果表明：外加磁场能使磁流变液的摩擦系数显著增大,且磁流变液的摩擦系数随磁场强度的增加呈现出增大的趋势;钢球磨斑形貌由圆形或椭圆形变为近似矩形,磨痕变浅;无场条件下,系统的摩擦磨损形式是钢球、磁性颗粒、钢球之间的三体磨损;外加磁场时,磁流变液的摩擦磨损形式发生改变。     

界面及热残余应力对超磁致伸缩复合材料有效性能的影响

本文基于Mori-Tanaka理论，考虑了界面相对超磁致伸缩复合材料的有效性能的影响，得到了具有界面相的超磁致伸缩复合材料的有效性能的一般解析表达。考虑到固化过程中热残余应力对超磁致伸缩复合材料有效性能的影响，通过数值计算，给出超磁致伸缩复合材料有效弹性模量、有效磁致伸缩应变及有效热膨胀系数随夹杂物长径比、体分比、界面参数和固化热残余应力的变化特征曲线，数值结果表明：界面和固化热残余应力对于超磁致复合材料有效性能的影响是显著的。     

Nonlinear viscoelastic properties of MR fluids under large-amplitude-oscillatory-shear

Nonlinear viscoelastic properties of the MR fluid, MRF-132LD, under large-amplitude oscillatory shear were investigated. This was accomplished by carrying out the experiments under the amplitude sweep mode and the frequency sweep mode, using a rheometer with parallel-plate geometry. Investigations under the influence of various magnetic field strength and temperatures were also conducted. MR fluids behave as nonlinear viscoelastic or viscoplastic materials when they are subjected to large-amplitude shear, where the storage modulus decreases rapidly with increasing strain amplitude. Hence, MR fluid behaviour ranges from predominantly elastic at small strain amplitudes to viscous at high strain amplitudes. Large-amplitude oscillatory shear measurements with frequency sweep mode reveal that the storage modulus is independent of oscillation frequency and approaches plateau values at low frequencies. With increasing frequency, the storage modulus shows a decreasing trend before increasing again. This trend may be explained by micro-structural variation. In addition, the storage modulus increases gradually with increasing field strength but it shows a slightly decreasing trend with temperature.     

Rheological properties and dispersion stability of magnetorheological (MR) suspensions

In the present article, the rheological responses and dispersion stability of magnetorheological (MR) fluids were investigated experimentally. Suspensions of magnetite and carbonyl iron particles were prepared as model MR fluids. Under an external magnetic field (H ₀) and a steady shear flow, the yield stress depends upon H ₀ ^3/2. The Yield stress depended on the volume fraction of the particle (φ) linearly only at low concentration and increased faster at high fraction. Rheological behavior of MR fluids subjected to a small-strain oscillatory shear flow was investigated as a function of the strain amplitude, frequency, and the external magnetic field. In order to improve the stability of MR fluid, ferromagnetic Co-γ-Fe₂O₃ and CrO₂ particles were added as the stabilizing and thickening agent in the carbonyl iron suspension. Such needle-like particles seem to play a role in the steric repulsion between the relatively large carbonyl iron particles, resulting in improved stability against rapid sedimentation of dense iron particles. Furthermore, the additive-containing MR suspensions exhibited larger yield stress, especially at higher magnetic field strength. Received: 4 April 2000 Accepted: 6 November 2000     

Microstructure and magnetorheology of graphite-based MR elastomers

This study focuses on the magnetorheology of graphite-based magnetorheological elastomers (Gr MREs). By introducing graphite to conventional MREs, the Gr MREs with various graphite weight fractions are fabricated. Both steady-state and dynamic tests were conducted to study rheological properties of the samples. For dynamic tests, the effects of magnetic field, strain amplitude and frequency on both storage modulus and loss modulus were measured. The influence of graphite weight fraction on mechanical performances of these samples was summarized. Also, the microstructures of isotropic and anisotropic Gr MREs were observed. In anisotropic MREs, the graphite powders disperse in matrix randomly. The graphite particles lead to an increment of initial mechanical properties and a decrement of the MR effect.     

Viscoelastic properties of magnetorheological fluids

We have studied the rheological properties of some magnetorheological fluids (MRF). MRF are known to exhibit original rheological properties when an external magnetic field is applied, useful in many applications such as clutches, damping devices, pumps, antiseismic protections, etc. While exploiting parameters such as magnetic field intensity, particle concentration and the viscosity of the suspending fluid, we highlighted the importance of each one of these parameters on rheology in the presence of a magnetic field. We made this study by conducting rheological experiments in dynamic mode at very low strain which facilitates the comprehension of the influence of the structure on MRF rheology. Our results confirmed the link between the magnetic forces which ensure the cohesion of the particles in aggregates, and the elastic modulus. Moreover, we found that the loss modulus varies with the frequency in a similar manner than the elastic modulus. The system, even with the smallest deformations, was thus not purely elastic but dissipates also much energy. Moreover, we demonstrated that this dissipation of energy was not due to the matrix viscosity. Actually, we attributed viscous losses to particle movements within aggregates.     

Field-stiffening effect of magneto-rheological elastomers

Magneto-rheological elastomers (MREs) are a class of soft active materials known for their tunable stiffness. Dispersed with magnetic particles, these polymer-based composites tend to be stiffer under a magnetic field. Such a stiffening effect is often attributed to the magnetic interaction among filler particles, but the well-acknowledged dipole-interaction model fails to explain the stiffening effect in tension/compression, which was observed in experiments. Other mechanisms, such as the effect of non-affine deformation, have been proposed, but there is no conclusive evidence on the dominating mechanism for the field-stiffening effect. This paper investigates various filler-chain structures, and seeks to identify the ultimate origin of the field-stiffening effect in MREs. Two different methods are used for cross verification: a dipole-interaction model and a finite-element simulation based on continuum field theories. This paper studies both the shear and axial deformation of the material, with a magnetic field applied in the particle-chain direction. It is found that while the magnetic interaction between particles is indeed the major cause of the stiffening effect, the wavy chain structure is the key to the modulus increase. Besides, chain–chain interaction and non-affine deformation are shown to be insignificant. In addition, the dependence of the stiffening effect on filler concentration is calculated, and the results qualitatively agree with experimental observations. The models also predict some interesting results that could be easily verified by future experiments.     

On transient layers as new phase domains in composite materials

A model describing the development of transient layers as new phase domains in compositematerials is constructed under the assumption that the transient layers around (nano)particles are layers of the matrix material changed by the phase transformation and increase the effective volume of inclusions which become compound and consist of the nucleus (original particle) and the shell (transient layer of the new phase). As a result, the inclusion volume fraction increases, which, in turn, increases the particle influence efficiency. An example of spherical particles is used to consider the new phase development around an isolated particle and then, in the effective field approximation, around interacting particles in the composite material. The dependence of the compound inclusion radius on the external (averaged) strain is obtained for isotropic phases. Stability of the interphase boundaries depending on the parameters of the original inclusion material and the matrix phase materials is studied. The energy variations and the stress redistribution owing to the development of the new phase domains are considered in detail. It is shown that, in the case of an isolated inclusion, the development of a new phase may lead to a local energy decrease near the inclusions and, as a consequence, to a decrease in the stress concentration. At the same time, the formation of transient layers due to the phase transformation can result in an increase in the bulk modulus of elasticity as the effective shear modulus decreases.     

Magnetorheology in viscoplastic media

Suspensions of iron particles in media with yield stresses were investigated to determine the effect of the continuous phase yield stress on the magnetorheological (MR) response. The steady-shear MR response was independent of the continuous phase yield stress for yield stresses in the range 0.9–37 Pa. The field-induced suspension yield stress increased sub-quadratically with the flux density. The small amplitude oscillatory shear response exhibited history dependence. The storage modulus depended not only on the magnitude of the applied magnetic field, but also on its history. This history dependence can be explained in terms of the field-dependent evolution of the suspension microstructure. Received: 24 February 1999 Accepted: 12 July 1999     

Multi-frequency excitation of magnetorheological elastomer-based sandwich beam with conductive skins

The present work deals with the dynamic stability of a symmetric sandwich beam with magnetorheological elastomer (MRE) embedded viscoelastic core and conductive skins subjected to time varying axial force and magnetic field. The conductive skins induce magnetic loads and moments under the application of magnetic field during vibration. The MRE part works in shear mode and hence the dynamic properties of the sandwich beam can be controlled by magnetic fields due to the field dependent shear modulus of MRE material. Considering the core to be incompressible in transverse direction, classical sandwich beam theory has been used along with extended Hamilton's principle and Galarkin's method to derive the governing equation of motion. The resulting equation reduces to that of a multi-frequency parametrically excited system. Second order method of multiple scales has been used to study the stability of the system for simply supported and clamped free sandwich beams. Here the experimentally obtained properties of magnetorheological elastomers based on natural rubber have been considered in the numerical simulation. The results suggest that the stability of the MRE embedded sandwich beam can be improved by using magnetic field.     

A comparison of the magnetorheology of two ferrofluids with different magnetic field-dependent chaining behavior

The effect of magnetic field-induced particle chaining on the magnetorheology of commercial iron oxide-based ferrofluids was investigated by comparison of a ferrofluid with particles that resist chaining and a ferrofluid with particles that interact when a field is applied, forming chain-like aggregates. This difference between the two ferrofluids was confirmed by optical microscopy and dynamic light scattering in an applied magnetic field. Both fluids had similar magnetic particle fraction, but showed different magnetorheological behavior. Chaining resulted in a stronger magnetic field-dependent viscosity enhancement and the appearance of an elastic modulus. The magnetorheology of these two fluids was described using the Mason number (Mn), resulting in two distinct Mn power law slopes at intermediate and small Mn values for the ferrofluid with magnetic field-induced aggregation. The commonly used magnetic coupling parameter failed to distinguish the behavior of the two ferrofluids.     

Yield stress and flow behavior of concentrated ferrofluid-based magnetorheological fluids: the influence of composition

In this paper, the magnetorheological (MR) and magnetoviscous properties of ferrofluid-based iron particle suspensions were investigated. The 2.1-µm mean size Fe particles were dispersed in high-concentration transformer oil-based ferrofluid, the iron particle volume fraction in the resulting nano-micro composite magnetorheological fluid samples varying from Φ _Fe = 5 to 40 %. The ferrofluid carrier has φ _p = 23 % solid volume fraction of magnetic nanoparticles stabilized with chemisorbed oleic acid monolayer and without any excess surfactant. In the absence of the field, the ferrofluid has a quasi-Newtonian behavior with a weak shear thinning tendency. The static yield stress shows an increase of about 3 orders of magnitude for an iron particle content of approx. Φ _Fe = 25 % (Φ _tot = 42.25 %), while above this value, a saturation tendency is observed. The dynamic yield stress (Bingham model) also increases with the magnetic induction and the particle volume fraction; however, the saturation of the MR effect is less pronounced. The relative viscosity change has a maximum at Φ _Fe = (10–15) % due to the accelerated increase of the effective viscosity of the composite for higher Fe content. Addition of micrometer-sized iron particles to a concentrated ferrofluid without any supplementary stabilizing agent proved to be a direct and simple way to control the magnetorheological and magnetoviscous behavior, as well as the saturation magnetization of the resulting nano-micro composite fluid to fulfill the requirements of their use in various MR control and rotating seal devices.