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。     

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。     

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。     

内通道式磁流变阻尼器研究

设计制作了一种新型的内通道式磁流变液阻尼器,该阻尼器的流场通道位于线圈内部,磁流变效应发生在两层固定的平板区域之间,并能保持磁流变液的流向与磁感应方向垂直,即保证磁流变液的大面积成链,产生大的可控阻尼力,又具备失效安全性;根据宾汉模型,建立了阻尼器的准静态力学模型;最后对研制的阻尼器进行动力学实验研究,并与理论结果进行比较,二者吻合较好。     

永磁激励下磁流变安全联轴器物理场数值仿真计算与分析

设计了一种新型磁流变安全联轴器,该联轴器以磁流变胶作为传力介质,以Halbach永磁排列作为磁场激励源.在COMSOL Multiphysics环境下建立了三维有限元分析模型;利用Bingham磁致本构模型对自行研制的磁流变胶的剪切屈服应力和粘度进行最小二乘5次多项式拟合,得到了剪切屈服应力和粘度随磁通密度的变化曲线参...     

履带车辆磁流变减振器响应时间研究

磁流变减振器是应用磁流变液在强磁场下的快速可逆变特性而制造的一种新型振动控制装置,是履带车辆主动悬挂系统的核心部分。本文从理论上分析磁流变减振器响应时间的组成,在磁流变液的Bingham模型和流体力学的基础上,分析了流体流变响应时间的影响因素,提出了近似算法,并在磁流变减振器的实验基础上,作出阻尼力与位移的曲线图,求出阻尼力突变并达到稳定的响应时间。     

悬浮体屈服应力的测量方法和影响因素

本文着力推荐了两种静态屈服应力测量方法——叶片法和斜面平衡法，对实验方法的可靠性及实验结果进行了比较，并对三种悬浮体在全浓度范围内屈服应力与颗粒体积分率的关系，应用四参数模型方程进行了拟合，方程的预示值能与实验结果较好地吻合，并对颗粒体积分率和形貌对屈服应力的影响进行了探讨     

电流变液研究进展及最新动态——第5届国际电流变液,磁悬浮体及相关技…

通过评介第５届国际电流变液、磁悬浮体以及相关技术研讨会，指出电流变液和磁流变液的应用研究有重要进展，磁流变液的剪切应力比电流变液大一个数量级，近来又受到重视，有机理研究中，要注意电流变液的表面效应，损耗对电流变效应的影响，本文还归纳了一些电流为液材料设计的思路。     

电流变液研究进展及最新动态──第5届国际电流变液、磁悬浮体 及相关技术研讨会评介

通过评介第５届国际电流变液、磁悬浮体以及相关技术研讨会，指出电流变液和磁流变液的应用研究有重要进展；磁流变液的剪切应力比电流变液大一个数量级，近来又受到重视；在机理研究中，要注意电流变液的表面效应、损耗对电流变效应的影响．本文还归纳了一些电流变液材料设计的思路．     

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。     

Magnetorheology in an aging, yield stress matrix fluid

Field-induced static and dynamic yield stresses are explored for magnetorheological (MR) suspensions in an aging, yield stress matrix fluid composed of an aqueous dispersion of Laponite? clay. Using a custom-built magnetorheometry fixture, the MR response is studied for magnetic field strengths up to 1?T and magnetic particle concentrations up to 30?v%. The yield stress of the matrix fluid, which serves to inhibit sedimentation of dispersed carbonyl iron magnetic microparticles, is found to have a negligible effect on the field-induced static yield stress for sufficient applied fields, and good agreement is observed between field-induced static and dynamic yield stresses for all but the lowest field strengths and particle concentrations. These results, which generally imply a dominance of inter-particle dipolar interactions over the matrix fluid yield stress, are analyzed by considering a dimensionless magnetic yield parameter that quantifies the balance of stresses on particles. By characterizing the applied magnetic field in terms of the average particle magnetization, a rheological master curve is generated for the field-induced static yield stress that indicates a concentration–magnetization superposition. The results presented herein will provide guidance to formulators of MR fluids and designers of MR devices who require a field-induced static yield stress and a dispersion that is essentially indefinitely stable to sedimentation.     

Measuring static yield stress of electrorheological fluids using the slotted plate device

An electrorheological (ER) response is defined as the dramatic change in rheological properties of a suspension of small particles due to the application of a large electric field transverse to the direction of flow. ER fluids are typically composed of nonconducting or semiconducting particles dispersed in a nonconducting continuous phase. A sufficiently large electric field will cause ER fluids to solidify, giving rising to a yield stress. Many applications in torque and stress transfer devices were proposed employing the reversible yielding behavior of ER fluids. Successful applications depend on a large yield stress of ER fluids and therefore accurate measurements of the yield stress of ER fluids are required. Reported experimental yield stresses of ER fluids have been dynamic yield stresses obtained by extrapolating the shear stress–shear rate data to zero-shear rate. It would be very helpful to the understanding of ER behaviors and the applications of ER fluids to be able to measure the static yield stress of ER fluids accurately. The slotted plate technique has been shown to be a successful method to determine the static yield stress of suspensions. The values obtained via the slotted plate method are static yield stress as the platform is designed for extremely low-speed motion. In this study, we modified the slotted plate device for the application of large electric fields and measured the static yield stress of TiO₂ ER fluids under various electric fields. The measured static yield stress values are also compared with the static yield stress values from a commercial rheometer.     

Aging, rejuvenation, and thixotropy in yielding magnetorheological fluids

The yielding behavior of dilute magnetorheological (MR) fluids has been investigated using creep–recovery tests. At very low stress levels, MR fluids behave in the linear viscoelastic regime as demonstrated by the fact that the instantaneous strain equals the instantaneous (elastic) recovery. In this region, gap-spanning field-induced structures support the stress levels applied. Upon increasing the stress value, the MR fluid evolves towards a nonlinear viscoelastic response. Here, the retarded elastic and viscous strain decrease, and the plastic contribution to the instantaneous strain grows probably due to the appearance of unattached field-induced structures. A larger stress value results in a viscoplastic solid behavior with negligible retarded and viscous strain and a fully plastic instantaneous strain. Finally, a plastic fluid behavior is found when the stress value is larger than the so-called yield stress. MR fluids exhibit an intermediate behavior between non-thixotropic (simple) and highly thixotropic model yield stress fluids.     

Load and rheometric unit for the test of magnetorheological fluid

The article describes the original design of a slit flow rheometer. The behavioral problems of MR fluids and the main deviation from the commonly introduced Bingham model, which describes the MR fluid, are explained at the beginning of this article. Furthermore, the usual operating parameters of the MR fluids and devices are defined, such as: shear rate, viscosity in the off-state, yield stress, their dependence on temperature and the expected life of the filling. In the absence of a suitable rheometer a new design of rheometer was proposed, which allows long-term testing of MR fluids under real loading conditions. This article describes the important details of the construction. Particular attention is devoted to temperature measurement, calibration of the rheometer and data analysis, including mathematical tools and statistical evaluation of a set of measured points.     

Rheometry for large-particulated fluids: analysis of the ball measuring system and comparison to debris flow rheometry

For large-particulated fluids encountered in natural debris flow, building materials, and sewage treatment, only a few rheometers exist that allow the determination of yield stress and viscosity. In the present investigation, we focus on the rheometrical analysis of the ball measuring system as a suitable tool to measure the rheology of particulated fluids up to grain sizes of 10 mm. The ball measuring system consists of a sphere that is dragged through a sample volume of approximately 0.5 l. Implemented in a standard rheometer, torques exerted on the sphere and the corresponding rotational speeds are recorded within a wide measuring range. In the second part of this investigation, six rheometric devices to determine flow curve and yield stress of fluids containing large particles with maximum grain sizes of 1 to 25 mm are compared, considering both rheological data and application in practical use. The large-scale rheometer of Coussot and Piau, the building material learning viscometer of Wallevik and Gjorv, and the ball measuring system were used for the flow curve determination and a capillary rheometer, the inclined plane test, and the slump test were used for the yield stress determination. For different coarse and concentrated sediment–water mixtures, the flow curves and the yield stresses agree well, except for the capillary rheometer, which exhibits much larger yield stress values. Differences are also noted in the measuring range of the different devices, as well as for the required sample volume that is crucial for application.     

A numerical method for determining the shear stress of magnetorheological fluids using the parallel-plate measuring system

This work proposes a new numerical method for determining the shear stress, which does not need any preassumption about the exact behavior of the fluid to achieve absolute data using a parallel-plate measuring system. The ability for representing different behaviors along the entire shear-rate range makes this method particularly interesting for the study of magnetorheological (MR) fluids. In this work, the conversion factors used by the rheometer for concentric-cylinder, cone-plate, and parallel-plate measuring systems are first analyzed. This analysis shows that the software used by the rheometer is not appropriate for the quantitative characterization of non-Newtonian fluids using the parallel-plate measuring system. Therefore, a new method for conversion of the parameters measured by the rheometer to the rheological parameters of the fluid is proposed; simultaneously, this new method is compared with other correction methods proposed in the literature: the Rabinowitsch-type method and the single-point method. Finally, the proposed method is applied for the quantitative characterization of an MR fluid.     

Static yield stress of ferrofluid-based magnetorheological fluids

In this work, the yield stress of ferrofluid-based magnetorheological fluids (F-MRF) was investigated. The fluids are composed of a ferrofluid as the liquid carrier and micro-sized iron particles as magnetic particles. The physical and magnetorheological properties of the F-MRF have been investigated and compared with a commercial mineral oil-based MR fluid. With the addition of a ferrofluid, the anti-sedimentation property of the commercial MR fluids could be significantly improved. The static yield stress of the F-MRF samples with four different weight fractions (ϕ) of micro-sized iron particles were measured using three different testing modes under various magnetic fields. The effects of weight fraction, magnetic strength, and test mode on the yielding stress have been systematically studied. Finally, a scaling relation, , was proposed for the yield stress modeling of the F-MRF system.