The rheological behaviour of MR fluids is often represented as a Bingham plastic model with a variable yield strength depending on the applied magnetic field H (Fig. 3a and 3b). The flow is governed by the equation:
Figure 5. MR fluid used in valve mode
(b) Direct shear mode剪切
The fluid is located between a pair of moving poles (translation or rotation motion). The relative displacement is parallel to the poles (Fig. 4b). The apparent viscosity, and thus the ―drag force‖ applied by the fluid to the moving surfaces can be controlled by modifying the magnetic field between the poles . Devices using this mode of operation include clutches离合器, brakes, locking devices and dampers.
Fig. 3 : (a) and (b) Bingham plastic model, (c) Operating range, (d) Hysteretic behaviour in MR fluid devices
磁滞现象 Hysteresis
磁滞现象在铁磁性材料中是被广泛认知的。是指在磁化和去磁过程 中，铁磁质的磁化强度不仅依赖于外磁场强度，还依赖于原先磁化强度 的现象。 当外加磁场施加于铁磁质时，其原子的偶极子按照外加场自行排列。 即使当外加场被撤离，部分排列仍保持：此时，该材料被磁化。在该材 料中，磁场强度（H）和磁感应强度（B）之间的关系是非线性的。如 果在增强场强条件下，此二者关系将呈曲线上升到某点，到达此点后， 即使场强H继续增加，磁感应强度B也不再增加。该情况被称为磁饱和 （magnetic saturation）。 如果此时磁场线性降低，该线性关系将以另一条曲线返回到0场强的 某点，该点的B将被初始曲线的磁感应强度量BR叫做剩磁感应强度或剩 磁（remanent flux density）相抵消。 如果绘制以外加磁场的全部强度的二者关系图，将为S形的回路。S 的中间厚度描述了磁滞量，该量与材料的矫顽力相关。 该现象的实际影响可为，例如，当通过磁芯的外加电流被撤离，由于 残留磁场继续吸引电枢，而引起滞后从而延迟磁能的释放。
Direct shear mode
Squeeze mode
Fig.4 Working modes of MR fluids
(a) Valve mode阀
The fluid is located between a pair of stationary poles. The resistance to the fluid flow is controlled by modifying the magnetic field between the poles, in a direction perpendicular to the flow (Fig. 4a). Devices using this mode of operation include servo-valves, dampers, shock absorbers and actuators.
地势
MRF-122EG Magneto-Rheological Fluid
Features and Benefits
• Fast Response Time – responds instantly and reversibly to changes in a magnetic field. • Dynamic Yield Strength – provides high yield strength in the presence of a magnetic field and very low yield strength in the absence of a magnetic field; allows for a wide range of controllability. • Temperature Resistant – performs consistently throughout a broad temperature range, meeting the requirements of demanding applications such as automotive shock absorbers. • Hard Settling Resistant – provides high resistance to hard settling; easily redispersed(重分散). • Non-Abrasive(零磨损)– formulated to not abrade(损伤) the devices in which the MR fluid is used.
Fig. 1 : Chain-like structures formation in the MR fluid under an externally applied magnetic field (from LORD corporation)
Fig. 2: Chains formation in a drop of MR fluid (from ISC Fraunhofer)
An MR fluid device is said to operate in shear mode when a thin layer ( ≈ 0.005 to 0.015 in.) of MR fluid is sandwiched between two paramagnetic顺磁性 moving surfaces. Shear mode (see Figure 6) is useful primarily for dampers that are not required to produce large forces and for clutches and br fluids?
Magnetorheological fluids belong to the group of so called controllable fluids. This means that they exhibit a significant change in their rheological behaviour when an external magnetic field(外部磁 场) is applied to them. They are indeed composed of micron-sized magnetic particles(微米级磁粒), located inside a liquid carrier, that form chain-like structures when the external magnetic field is applied, resulting in an increase of the apparent viscosity(粘性) of the fluid (Fig. 1 and 2).
磁流变阻尼器的力学模型虽然多种多样, 而且有些能 很精确模拟阻尼器的动态特性, 但是无法直接反映阻尼 器的逆向动态特性。由于磁流变阻尼器的研究涉及到电 磁学、流体力学、热力学以及机械学等多学科, 这些学 科的交叉和融合为研究带来了挑战, 但这些研究是使磁 流阻尼在工程应用中不可缺少的关键技术。
B-H loop
valve mode is the most widely used of the three modes. An MR device is said to operate in valve mode when the MR fluid is used to impede阻碍 the flow of MR fluid from one reservoir储存仓 to another.
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对于晶粒取向电材料的一组B-H环路（BR 表示剩磁，而HC为矫顽力。）
Working modes of magneto rheological fluids
Magnetorheological fluids can be operated in three distinct modes
Valve mode
where is the shear stress, is the shear strain and is the viscosity of the fluid. The operating range is the shaded area in Fig. 3c. Below the yield stress (at strains of the order of 10-3), the MR fluid behaves viscoelastically:
Figure 6. MR fluid used in shear mode