分析磁流变流体屈服应力微观力学模型

剪切屈服应力是磁流变流体固化强度的重要指标之一,具有重要的工程意义。基于磁性物理学理论,从磁流变流体在磁场作用下铁磁性固体颗粒极化成链的微观结构出发,探讨磁流变流体中固体颗粒间的相互作用机理,分析研究颗粒间的相互作用力,建立了一种微观力学模型,可用于分析磁流变流体在外加磁场作用下屈服应力及其影响因素的作用效果,揭示磁流变效应的微结构机理,为磁流变流体的性能优化、工程开发及应用磁流变流体提供理论依据。     

一种平板式磁流变体静态屈服应力测试方法

针对目前国内外磁流变体（MR流体）屈服应力测试设备较为缺乏这种状况,本文设计加工了一套平板式磁流变体静态屈服应力测试设备,并用该设备对本文实验制作的两种MR流体进行了屈服应力测试,同时探讨了磁感应强度、分散相体积浓度、分散相粒度对这两种MR流体静态屈服应力的影响。     

磁流变液屈服应力的管道流测试方法研究

设计加工了一套磁流变液测试设备，并对由不同组分的铁粉／硅油组成的磁流变液进行了一系列的实验测试，结果表明：用钴纳米粒子修饰铁粉可以提高此种磁流变液的屈服应力，而用醋酸处理铁粉会严重减弱其屈服应力。     

磁流变剪切变硬胶的“磁力”耦合效应研究

剪切变硬胶(shear stiffening gel)是一种新型智能材料,由于其在外界应力刺激下模量可以迅速增大,表现出优良的剪切变硬效应,所以在人体防护和阻尼隔振等领域具有重要的应用前景。本文将软磁性羰基铁粒子(CI)引入到剪切变硬胶体中,成功制备出多功能磁流变剪切变硬胶复合材料(MSC),通过流变测试系统分析了该复合材料的"磁力"耦合效应。研究表明,MSC具有优良的剪切变硬效应(S-ST effect),其储能模量随着剪切频率的增加而显著增大(4个数量级)。磁控测试和蠕变实验结果均表明该复合材料对于外界磁场表现出较好的响应性能。当羰基铁粉含量为60wt%时,样品的模量随着磁场的增加而迅速增大,其最大模量高达2.44MPa。在磁场和剪切应力双重刺激下,复合材料的剪切变硬效应得到极大提高,且剪切应变增大,样品模量相对降低。同时,该材料在低温下也具有良好的剪切变硬和磁流变效应,高温下性能略有下降。最后,探讨了MSC在磁场、剪切场耦合条件下性能增强原因,发现"B-O化学交联键"和CI粒子链的形成是该复合材料具有优良剪切变硬效应和磁流变性能的主要原因。     

两种磁流变液测试系统的比较研究

本文首先简单介绍了自行设计加工的两套磁流变液屈服应力的测试系统,然后利用此实验设备对美国某公司生产的商用磁流变液进行了测量,并将结果与德国生产的型号为 Ｍ Ｒ１００４５０ 的 Ｍ Ｒ Ｆ测试系统对此磁流变液的测试结果进行了比较,通过比较结果肯定了自行设计的碟片旋转式磁流变液测试系统的测量准确性和可靠性,并且发现了管道流测试方法的固有局限性。     

磁流变液体的流变力学特性试验和建模

从唯象角度讨论磁流变液体的流变力学特性。基于对磁流变液体的试验 ,讨论了磁感强度、温度和剪切应变速率等对磁流变液剪切应力的影响 ;建立了描述磁流变液剪切应力的 Bingham模型、广义 Bingham模型和非线性模型。     

体积分数对基于沸石和硅油的悬浮液的电流变性能的影响

用 HAAKE RV2 0型流变仪 ,在不同外加电场强度和不同颗粒体积分数下测试了基于沸石和硅油的电流变液的剪切应力变化 .结果表明 :随着外加电场强度升高 ,电流变液的零电场粘度急剧增加 ,电流变液的剪切屈服应力增加 ;随着电流变液中沸石颗粒体积分数升高 ,电流变液的剪切屈服强度急剧上升 .这种变化可以用颗粒间作用力与颗粒间距的关系、单位面积的颗粒链数目变化以及多体作用对电流变液性能的影响来解释     

Herschel-Bulkley模型下磁流变减振器旁路泊肃叶流动分析

制备了羰基铁粉(CIP)的质量分数为70%的硅树脂基磁流变胶。用安东帕MCR-301型流变仪对其流变特性进行了测量,并对测试结果进行了Herschel-Bulkley本构模型参数识别。最后对MRG-70在该模型下磁流变减振器旁路中的泊肃叶(Poiseuille)流动展开了分析。试验及分析结果表明,在外加磁场下,Herschel-Bulkley本构模型中非牛顿指数n1。剪切应力随半径的增大而线性增大;当τ_rτ_y形成剪切流,反之则为柱塞流动。在剪切流动区域内,流速随半径增大而非线性降低,剪切速率随半径增大而非线性增大。磁感应强度越大,柱塞流区域越大,柱塞流区域内剪切速率恒为0。体积流量随磁感应强度增强而非线性减小。在0mT～500mT之间体积流量随磁感应强度变化剧烈,进一步增加磁感应强度则变化缓慢。     

母液对磁流变液力学性质的影响

研究了磁流变液中母液分子在磁场作用下磁矩的变化以及由此引起的剪切屈服强度的变化,探讨了磁流变的微观机理。研究表明,由分子磁矩引起的剪切屈服强度与磁流变液总屈服强度的比随母液分子内电子数的增加而增加,一般情况下,单位体积内的平均分子磁矩占总磁矩的2·53%,由此引起的剪切屈服强度的减少占总剪切屈服强度的5·15%。为减少误差,可尽量选用电子数较少的分子材料作母液。     

剪切阀式磁流变减振器力学特性实验研究

本文对剪切阀式磁流变减振器的结构和工作原理作了介绍,基于平板模型对其结构进行简化,采用叠加法得出了其阻尼力的理论计算模型。利用材料试验机(MTS)分别在不同电流、振幅、速度和频率等条件下,对自行研制的剪切阀式磁流变减振器的动态特性进行了实验研究,得出了相应的实验曲线。实验结果表明剪切阀式磁流变减振器具有优良的电控阻尼力特性,且振幅、速度和频率等因素对其阻尼力有着重要影响,另外也对其阻尼力的理论计算公式作了验证。     

Study of magnetorheological fluids at high shear rates

The tunable rheological properties of magnetorheological (MR) materials at high shear rates are studied using a piston-driven flow-mode-type rheometer. The proposed method provides measurement of the apparent viscosity and yield stress of MR fluids for a shear rate range of 50 to 40,000 s⁻¹. The rheological properties of a commercial MR fluid, as well as a newly developed MR polymeric gel, and a ferrofluid-based MR fluid are investigated. The results for apparent viscosity and dynamic and static shear stresses under different applied magnetic fields are reported.     

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     

Wall material and roughness effects on transmittable shear stresses of magnetorheological fluids in plate–plate magnetorheometry

A direct comparison of plate–plate magnetorheometry results for nonmagnetic (titanium/brass) and ferromagnetic plates is presented, using a modified Anton Paar magnetocell MRD180/1T. Necessary corrections to derive the true flux density in the magnetorheological fluid (MRF) from the online Hall probe reading and to account for the gap opening effect caused by normal forces on shear stress and flux density are addressed. The measured shear stress versus magnetic flux density characteristics agree in the low flux density regime <0.1 T but yield distinctly higher transmittable shear stresses for ferromagnetic plates at elevated flux densities (49% increase at 1 T for 90% by weight carbonyl iron powder (CIP) and 84% for 85% by weight CIP). Remarkably, the normal force, if corrected for its magnetostatic part, remains independent of the type of plates up to about 0.6 T. We address the role of normal forces, of magnetic interactions between CIP and wall, as well as the role of wall roughness in a solid body friction model. A systematic variation of wall properties and materials was achieved by introducing both a modular rotor and stator, which ease the variation of the walls in contact to the MRF. The transmittable shear stress of nonmagnetic plates (e.g., brass) may be increased up to the level of ferromagnetic disks by a higher wall roughness or by grooves. No shear stress increase is obtained for grooves in ferromagnetic plates, which is explained by the different local flux density modulation at the grooves for ferromagnetic compared to nonmagnetic plates. Finally, we address the effect of ferromagnetic and nonmagnetic coatings on brass and steel disks, and show that, e.g., a layer of CIP on brass efficiently increases the transmittable shear stress.     

High-flux magnetorheology at elevated temperatures

Commercial applications of magnetorheological (MR) fluids often require operation at elevated temperatures as a result of surrounding environmental conditions or intense localized viscous heating. Previous experimental investigations of thermal effects on MR fluids have reported significant reductions in the magnetorheological stress with increasing temperature, exceeding the predictions made by considering the thermal variations in the individual physical properties of the fluid and solid constituents of a typical MR fluid. In the low-flux regime, designers of MR fluid actuators can alleviate this thermal reduction in stress by increasing the applied magnetic field strength. However, this is not possible in the high-flux regime because of magnetic saturation, and it becomes necessary to explore and understand the intrinsic limitations of the fluid at elevated temperature. We describe a new magnetorheological fixture, which was designed as an accessory to a commercial torsional shear rheometer, capable of applying magnetic flux densities up to 1 T and controlling the sample temperature up to 150°C. During the design of the instrument, close attention was given to the uniformity of the magnetic field applied to the sample by using numerical simulations. Incorporation of a custom-built magnetic flux sensor which matches the environmental capabilities of the fixture enables in situ measurement of the local magnetic field at each temperature. The numerical results are also validated by spatially resolved measurements of the local magnetic field. Finally, we explore the ability of a shift factor between fluid magnetization and yield strength to describe the measured variation in the MR fluid response at elevated temperatures.     

Linear and non-linear rheological behaviour of cement and silica suspensions. Effect of polymer addition

Oil well cement pastes and model silica suspensions demonstrate similar rheology: in oscillatory shear, beyond a critical stress, a sharp transition is ob- served between gel and liquid behaviour. In creep tests, an apparent yield stress and shear-thinning are followed by the appearance of shear thickening. The minimum viscosity measured in steady shear is close in value to the complex viscosity obtained from oscillatory measurements. The observations can be explained by the formation of liquid trapping aggregates whose compactness may be estimated by fitting the Tsenoglou model, and whose cohesion is reflected in the rigidity of the gel and in the critical strain (or stress) of gel dissolution. Substituting cement or silica particles by polymer redispersible powder causes a decrease of the storage modulus in the gel state and a lower viscosity, while leaving the general features of the flow curve unchanged. Decrease in material rigidity may be due to a weaker inter-particle attraction generated by the polymer presence. The decrease in viscosity is explained by a lessening of water entrapped within the aggregates, which now contain polymer particles which are less hydrophilic than either cement or silica.     

On the origin of super-rotating layers in magnetohydrodynamic flows

An asymptotic analysis has been performed for the magnetohydrodynamic flow between perfectly conducting concentric cylindrical shells. The flow in the model geometry exhibits all the features which had been discovered in the past for the case of differentially rotating spherical shells considered in the context of geophysical analyses. For strong magnetic fields, the flow domain splits into distinct subregions and exhibits two different types of cores which are separated from each other by a tangent shear layer. The fluid in the inner core flows similar to a solid-body rotation and the outer core is entirely stagnant. With increasing magnetic fields the shear layer becomes thinner and, since the flow rate carried by the layer asymptotes to a finite value, the velocity in the layer increases as the layer thickness decreases. Moreover, the flux carried by the layer rotates in opposite direction compared with the rotation of the body. It is shown that the rotating jet is driven by the electric potential difference between the edges of the inner and the outer core.     

Combined slit and plate–plate magnetorheometry of a magnetorheological fluid (MRF) and parameterization using the Casson model

We describe a magneto-slit die of 0.34 mm height and 4.25 mm width attached to a commercial piston capillary rheometer, enabling the measurement of apparent flow curves of a magnetorheological fluid (MRF) in the high shear rate regime (apparent shear rates 276 up to 20,700 s^???1, magnetic flux density up to 300 mT). The pressure gradient in the magnetized slit is measured via two pressure holes. While the flux density versus coil current without MRF could directly be measured by means of a Hall probe, the flux density with MRF was investigated by finite element simulations using Maxwell® 2D. The true shear stress versus shear rate is obtained by means of the Weissenberg–Rabinowitsch correction. The slit die results are compared to plate–plate measurements performed in a shear rate regime of 0.46 up to 210 s^???1. It is shown that the Casson model yields a pertinent fit of the true shear stress versus shear rate data from plate–plate geometry. Finally, a joint fit of the slit and plate–plate data covering a shear rate range of 1 up to 50,000 s^???1 is presented, again using the Casson model. The parameterization of the MRF behavior over the full shear rate regime investigated is of relevance for the design of MR devices, like, e.g., automotive dampers. In the Appendix, we demonstrate the drawbacks of the Bingham model in describing the same data. We also show the parameterization of the flow curves by applying the Herschel–Bulkley model.     

A chemo-mechanical model coupled with thermal effect on the hollow core–shell electrodes in lithium-ion batteries

Electrode is a key component to remain durability and safety of lithium-ion(Li-ion) batteries. Li-ion insertion/removal and thermal expansion mismatch may induce high stress in electrode during charging and discharging processes. In this paper, we present a continuum model based on COMSOL Multiphysics software, which involves thermal, chemical and mechanical behaviors of electrodes. The results show that,because of diffusion-induced stress and thermal mismatch, the electrode geometry plays an important role in diffusion kinetics of Li-ions. A higher local compressive stress results in a lower Li-ion concentration and thus a lower capacity when a particle is embedded another, which is in agreement with experimental observations.     

A thermodynamic model for magnetorheological fluids

An analysis of the dynamic behavior of a magnetorheological (MR) fluid is given in terms of a vectorial internal variable describing the change of the macroscopic average of the relative position vector of suspensions. Under the restriction of the second law, the constitutive equations of the MR fluid for stress, heat flux, magnetization and internal variable can be derived. The related issue of dissipative and energy transfer mechanisms is treated at some length. Studies on the steady shear flow indicate the direction of the internal variable is independent of shear rate. The Bingham-type constitutive equation for shear stress is obtained and endowed with a new meaning. The pressure-driven flow, another significant flow type for the design of MR devices, is also analyzed to study the plug flow region and the relationship between yield stress and flow rate. In addition, a criterion of flow initiated by the applied shear force is proposed based on the saturation of the internal variable and the condition of the equilibrium of forces in the fluid and solid regions. Received April 08, 1997     

Blood flow in stenosed arteries with radially variable viscosity, peripheral plasma layer thickness and magnetic field

A novel approach of combined mathematical and computational models has been developed to investigate the oscillatory two-layered flow of blood through arterial stenosis in the presence of a transverse uniform magnetic field applied. Blood in the core region and plasma fluid in the peripheral layer region are assumed to obey the law of Newtonian fluid. An analytical solution is obtained for velocity profile and volumetric flow rate in the peripheral plasma region and also wall shear stress. Finite difference method is employed to solve the momentum equation for the core region. The numerical solutions for velocity, flow rate and flow resistance are computed. The effects of various parameters associated with the present flow problem such as radially variable viscosity, hematocrit, plasma layer thickness, magnetic field and pulsatile Reynolds number on the physiologically important flow characteristics namely velocity distribution, flow rate, wall shear stress and resistance to flow have been investigated. It is observed that the velocity increases with the increase of plasma layer thickness. An increase or a decrease in the velocity and wall shear stress against the increase in the value of magnetic parameter (Hartmann number) and hematocrit is dependent on the value of t. An increase in magnetic field leads to an increase in the flow resistance and it decreases with the increase in the plasma layer thickness and pulsatile Reynolds number. The information concerning the phase lag between the flow characteristics and how it is affected by the hematocrit, plasma layer thickness and Hartmann number has, for the first time, been added to the literature.