Field-dependent Yield Stress Research Articles

Temperature effects and a prediction method of field-dependent yield stress in graphite magnetorheological grease

Graphite additives have become increasingly prevalent in magnetorheological (MR) materials, as they can substantially improve the shear yield stress of such materials. However, the effect of graphite additive proportion and temperature on graphite magnetorheological grease (GMRG) has not yet been fully reported, which limits the engineering application of GMRG. This work prepared magnetorheological greases with five different graphite ratios and investigated how their steady-state shear properties responded to temperature. The shear yield stress and relative MR effect of GMRG at different magnetic field strengths were measured and calculated. The results verified that the yield stress of GMRG increased with the increase of graphite additive ratio, but the relative MR effect decreased significantly. Besides, the steady-state shear mode was employed to evaluate the rheological properties of GMRG under varying magnetic field strengths and temperatures. The yield stress was analyzed through fitting experiments conducted under different magnetic field strengths and temperatures. The results showed that the addition of graphite powder increased the shear yield stress of the GMRG. But the enhancement effect was weakened by increasing temperature. In addition, a support vector machine (SVM)-based prediction method was proposed to predict the performance of GMRG with different ratios under complex operating conditions. The SVM-based estimation results showed good agreement with actual experimental values. This method provides a new approach for proportioning, characterization, and performance prediction of other materials.

A Cylindrical Grip Type of Tactile Device Using Magneto-Responsive Materials Integrated with Surgical Robot Console: Design and Analysis.

This paper proposes a cylindrical grip type of tactile device that is effectively integrated to a surgical robot console so that a surgeon can easily touch and feel the same stiffness as the operating organs. This is possible since the yield stress (or stiffness) of magnetic-responsive materials can be tuned or controlled by the magnetic field intensity. The proposed tactile device consists of two main parts: a magnetorheological elastomer (MRE) layer and a magnetorheological fluid (MRF) core. The grip shape of the device to be positioned on the handle part of the master of the surgical robot is configured and its operating principle is discussed. Then, a couple of equations to calculate the stiffness from the gripping force and the field-dependent yield stress of MRF are derived and integrated using the finite element analysis (FEA) model. After simulating the stiffness of the proposed tactile device as a function of the magnetic field intensity (or current), the stiffnesses of various human organs, including the liver and heart, are calculated from known data of an elastic modulus. It is demonstrated from comparative data between calculated stiffness from human tissues and simulated stiffness from FEA that the proposed tactile device can generate sufficient stiffness with a low current level to recognize various human organs which are significantly required in the surgical robot system.

Magnetoviscosity of a Magnetic Fluid Based on Barium Hexaferrite Nanoplates.

This paper presents the results of an experimental study of the influence of an external magnetic field on the shear flow behaviour of a magnetic fluid based on barium hexaferrite nanoplates. With the use of rheometry, the magnetoviscosity and field-dependent yield-stress in the fluid are evaluated. The observed fluid behaviour is compared to that of ferrofluids with magnetic nanoparticles having high dipole interaction. The results obtained supplement the so-far poorly studied topic of the influence of magnetic nanoparticles’ shape on magnetoviscous effects. It is concluded that the parameter determining the observed magnetoviscous effects in the fluid under study is the ratio , where V is the volume of the nanoparticle and l is the size of the nanoparticle in the direction corresponding to its orientation in the externally applied magnetic field.

Core-Shell Structured Magnetite-Poly(diphenylamine) Microspheres and Their Tunable Dual Response under Magnetic and Electric Fields.

Core-shell type poly(diphenylamine)-coated magnetite (Fe3O4-PDPA) microspheres were designed and adopted as a novel actively tunable smart material which is responsive under both electric and magnetic fields. Their morphology, chemical structure, crystalline structure, and thermal properties were characterized using scanning electron microscopy, transmission electron microscope, Fourier transform-infrared spectroscopy, X-ray diffraction, and a thermal gravimetric analyzer. Their magnetic and dielectric properties were determined using vibrating-sample magnetometer and an LCR meter, respectively. They were dispersed in silicone oil and their electrorheological (ER) and magnetorheological (MR) responses under the electric and magnetic fields, respectively, were examined. The formation of chain structure of Fe3O4-PDPA based E/MR fluid under the application of electric field or magnetic field was observed by an optical microscopy and the sedimentation stability was observed by a Turbiscan optical analyzer system. It was observed that the yield stress, ER efficiency, and leakage current density increased with an increase in the particle concentration, while the slope of the electric field-dependent yield stress decreased. Several models such as the Bingham model, Herschel-Bulkley model, and Cho-Choi-Jhon equations were used to describe the shear stress curves of the ER fluid; the curves fitted well. For the dielectric properties, the two types of ER fluids tested displayed the same relaxation time and distribution; however, the one with the higher concentration had a higher dielectric constant and polarizability. The Fe3O4-PDPA based MR fluid (10 vol %) exhibited typical MR properties. In addition, the Herschel-Bulkley model matched well with the shear stress curves under a magnetic field.

Synthesis of magnetorheological fluid and its application in a twin-tube valve mode automotive damper

The change in rheological properties of smart materials like magnetorheological fluid when brought under the influence of a magnetic field can be utilized to develop magnetorheological devices where the output has to be continuously and quickly varied using electronic control interface. In the present study, magnetorheological fluid is synthesized and used as a smart fluid in a twin-tube magnetorheological damper operating in valve mode. The behavior of the magnetorheological fluid is experimentally characterized in a rheometer and mathematically modeled using Herschel–Bulkley model. The parameters of the Herschel–Bulkley model are expressed as polynomial functions of strength of the magnetic field in order to find the shear stress developed by the magnetorheological fluid at any given strength of the magnetic field applied. The magnetorheological damper, which was designed for application in a passenger van, is tested in the damper testing machine. The performance of the damper at different damper velocities and current supplied is studied. The range of values for the parameters of the experimental testing are chosen to emulate the actual conditions of operation in its intended application. Nondimensional analysis is performed, which links magnetorheological fluid rheological properties and geometrical parameters of magnetorheological damper design with the force developed by the damper. Finite element method magnetics is used to find the strength of the magnetic field at the fluid flow gap. Analytical methods are used to calculate the damper force developed due to the field-dependent yield stress and compared with experimental force values. The resulting dynamic range of the magnetorheological damper is also assessed.

New magneto-rheological fluids with high stability: Experimental study and constitutive modelling

Magneto-rheological fluids (MRF) are known as a category of smart materials because they exhibit sudden viscosity changes upon application of magnetic field. In contrast to normal fluids, MRFs can sustain shear up to a yield stress. Stability and resistance against movement are important factors which determine the extent of application of a MRF. In this work, new MRFs are developed using engine oil as carrier liquid, carbonyl iron powder as magnetic particle, stearic acid and CHRYSO® Optima100 as additives. Stability of the samples is measured over time. The samples are exposed to magneto-rheological tests with combined liquid and Peltier temperature control. Samples A, B and C are prepared with low, medium and high particle fractions respectively and tested at different temperatures (−10 °C, 5 °C, 60 °C) but for samples D, E, F and G the rheology tests are conducted in room temperature (25 °C) but at variable magnetic field and shear rate. Inherent assumption of the existing constitutive models is that the flow curve of MRF is shifted by a field-dependent yield stress. In this paper the effect of magnetic field is formulated and based on the physical properties of MRFs, a new method is introduced for identification of material parameters. This method predicts the yield stress by comparing the storage and shear moduli. Obtained results are compared with those obtained from fitting the experimental flow curves and also with those obtained from Bingham model. It is shown that, results of the proposed model are in good agreement with the experimental data. Moreover, the calculated sedimentation ratio shows that simultaneous use of stearic acid and Optima100 significantly improves stability of MRFs.

Microstructure Simulation and Constitutive Modelling of Magnetorheological Fluids Based on the Hexagonal Close-packed Structure.

This paper presents a new constitutive model of high particles concentrated magnetorheological fluids (MRFs) that is based on the hexagonal close-packed structure, which can reflect the micro-structures of the particles under the magnetic field. Firstly, the particle dynamic simulations for the forces sustained by carbonyl iron powder (CIP) particles of MRFs are performed in order to investigate the particles chain-forming process at different time nodes. Subsequently, according to the force analyses, a hexagonal close-packed structure, which differs from the existing single-chain structure and body-cantered cubic structure, is adopted to formulate a constitutive model of MRFs with high concentration of the magnetic-responsive particles. Several experiments are performed while considering crucial factors that influence on the chain-forming mechanism and, hence, change the field-dependent shear yield stress in order to validate the proposed model. These factors include the magnetic induction intensity, volume fraction and radius of CIP particles, and surfactant coating thickness. It is shown that the proposed modeling approach can predict the field-dependent shear yield stress much better than the single-chain model. In addition, it is identified that the shear yield stress is increased as the particle volume fraction increases and surfactant coating thickness decreases. It is believed that the proposed constitutive model can be effectively used to estimate the field-dependent shear yield stress of MRFs with a high concentration of iron particles.

Rheological analysis of titanium dioxide nano-whisker based electrorheological fluids

Understanding yield stress of electrorheological (ER) fluids becomes important for not only development of advanced ER materials but also their engineering applications. This study reanalyzed dynamic yield stresses of both titanium dioxide nano-whisker- and titanium dioxide nano-particle-based ER suspensions under an input electrical field, based on the universal yield stress function proposed. The experimental data was plotted well to a single line by the scaled universal yield stress function via a deduced critical electrical field value which divides electric field dependent yield stress region with a slope of 1.5 from that with 2.0 of polarization mechanism.

Effects of Silicone Oil Viscosity and Carbonyl Iron Particle Weight Fraction and Size on Yield Stress for Magnetorheological Grease Based on a New Preparation Technique.

This paper investigated the effects of silicone oil viscosity (SOV) and carbonyl iron particle (CIP) weight fraction and size on dynamic yield stress for magnetorheological (MR) grease. The MR grease samples were prepared using orthogonal array L9 on the basis of a new preparation technology. The shear rheological tests were undertaken using a rotational shear rheometer and yield stress was obtained based on the Bingham fluid model. It was found that CIP fractions ranging from 65 wt% to 75 wt% and SOV varying from 50 m2·s−1 to 1000 m2·s−1 significantly affect the magnetic field-dependent yield stress of MR grease, but the CIPs with sizes of 3.2–3.9 μm hardly had any influence based on the analysis of variance (ANOVA). In addition, the yield stress of MR grease mainly depended on the CIP fraction and SOV by comparing their percent contribution (PC). It was further confirmed that there were positive effects of CIP fraction and SOV on yield stress through response surface analysis (RSA). The results showed a high dynamic yield stress. It indicated that MR grease is an intelligent material candidate which can be applied to many different areas requiring high field-induced rheological capabilities without flow for suspension. Moreover, based upon the multivariate regression equation, a constitutive model was developed to express the function of the yield stress as the SOV and fraction of CIPs under the application of magnetic fields.

A new platform for the prediction of field-dependent yield stress and plastic viscosity of magnetorheological fluids using particle swarm optimization

The yield stress and plastic viscosity of magnetorheological (MR) fluids are identified by fitting rheological models based on a selected dataset on a certain range of shear rates. However, the datasets are often arbitrarily determined as there is no standardized procedure available. To overcome this problem, a platform that capable to minimize the fitting error while considering the classification of the shear rate regions is needed. Therefore, this work proposed a new platform for the systematic prediction of field-dependent rheological characteristics using particle swarm optimization (PSO). PSO is a meta-heuristic algorithm for solving optimization problems based on a guided search of the defined problem space, which is governed by the objective function. An intersection point of low and high shear rate regions critical shear rate is formulated as part of the objective function to standardize the characterization within the defined regions. The objective function is inspired by the modified Bingham biplastic and Papanastasiou models to predict five magnetic field dependent-rheological parameters. In the development stage, the shear stress model was first established using a previously developed extreme learning machine method. Then, the codes of the PSO, objective functions and search space identification were developed and implemented. To validate the effectiveness of the proposed procedure, the platform performance was analysed at different algorithmic parameters and compared with the existing optimization methods. The simulation results indicated that the proposed platform performed better than the existing ones with R2 of 0.943 and was able to systematically and accurately predict the rheological parameters.

Dynamic characteristics of passive and semi-active cabin mounts for vibration control of a wheel loader

In this study, the dynamic characteristics of passive mounts and a semi-active magneto-rheological (MR) mount for vibration attenuation of a wheel loader cabin are experimentally evaluated. First, the design procedures of two different passive mounts are described, and the principal design parameters such as stiffness are determined from a simple but practically feasible dynamic model. Then, passive mounts with different material characteristics are manufactured and experimentally tested. Secondly, a semi-active MR mount is designed based on the field-dependent yield stress of MR fluid. A prototype of the MR mount is then manufactured considering two flow paths: annular path and radial path. Subsequently, the dynamic characteristics of the three mounts such as the frequency-dependent stiffness and damping are evaluated, and their salient properties are discussed. In addition, it is shown from the transmissibility investigation that the semi-active MR mount can effectively isolate unwanted vibrations in a wider operating frequency range compared with the passive rubber mounts.

Dynamic characteristics of passive and semi-active cabin mounts for vibration control of a wheel loader

In this study, the dynamic characteristics of passive mounts and a semi-active magneto-rheological (MR) mount for vibration attenuation of a wheel loader cabin are experimentally evaluated. First, the design procedures of two different passive mounts are described, and the principal design parameters such as stiffness are determined from a simple but practically feasible dynamic model. Then, passive mounts with different material characteristics are manufactured and experimentally tested. Secondly, a semi-active MR mount is designed based on the field-dependent yield stress of MR fluid. A prototype of the MR mount is then manufactured considering two flow paths: annular path and radial path. Subsequently, the dynamic characteristics of the three mounts such as the frequency-dependent stiffness and damping are evaluated, and their salient properties are discussed. In addition, it is shown from the transmissibility investigation that the semi-active MR mount can effectively isolate unwanted vibrations in a wider operating frequency range compared with the passive rubber mounts.

A new constitutive model of a magneto-rheological fluid actuator using an extreme learning machine method

In this work, a new constitutive model of a magneto-rheological fluid (MRF) actuator is proposed using an extreme learning machine (ELM) technique to enhance the prediction accuracy of the field-dependent actuating force. After briefly reviewing existing rheological constitutive models of MRF actuator, ELM algorithm is formulated using a single-hidden layer feed-forward neural network. In this formulation, both the magnetic field and measured shear rates are used as inputs variables, while the shear stress predicted from the ELM training is used as an output variable. Subsequently, in order to validate the effectiveness of the proposed model, the target defined as the error between the prediction and measured data is set. Then, the fitness of the training and prediction performances is evaluated using a normalized root mean square error (NRMSE) method. It is shown that the shear stress estimation based on the ELM model using sinusoidal activation function is more accurate than conventional rheological constitutive models such as Herschel-Bulkley model. It is also demonstrated that the proposed model is capable of predicting the field-dependent yield stress which is defined as an actuating force of the MRF actuator without causing significant errors.

Electrorheological response of microporous covalent triazine-based polymeric particles

Microporous covalent triazine-based polymer (MCTP) particles were synthesized via a Friedel–Crafts reaction catalyzed by AlCl3, and their morphology and textural properties were confirmed by scanning electron microscopy and N2 adsorption isotherms, respectively. Electrorheological (ER) behavior of the MCTP particle-based ER fluid dispersed in silicone oil at a volume fraction of 5% was examined using a rotational rheometer to examine its viscoelastic properties such as shear stress, shear viscosity, yield stress, and dynamic moduli. Typical ER characteristics showed an increase with increased applied electric field strength following a polarization mechanism with the slope of 2 of the electric field-dependent yield stress, highlighting MCTP as a potential ER material. The dielectric spectra were also correlated with its ER effects using an LCR meter.

Pickering emulsion polymerized magnetite-poly(methyl methacrylate) composite particles and their magnetorheology

Magnetic poly(methyl methacrylate) (PMMA)/Fe3O4 composite nanoparticles were fabricated by Pickering emulsion polymerization using nano-sized iron oxide (Fe3O4) particles as a solid stabilizer that had been synthesized initially from a hydrothermal process. High-resolution scanning electron microscopy was applied to characterize the morphology of the materials. Fourier transformation infrared spectroscopy confirmed the chemical structure of both pure Fe3O4 and PMMA/Fe3O4 composites. The magnetic properties and atomic and molecular structure of a crystal of the materials were examined by vibration sample magnetometry and X-ray diffraction, respectively. The magnetorheological (MR) features of both pure Fe3O4 and PMMA/Fe3O4 nanoparticles dispersed in mineral oil under a magnetic field were characterized by a rotational rheometer. The dynamic yield stress was higher than the elastic stress under external magnetic field. The PMMA/Fe3O4 composite-based MR fluid showed good dispersion stability, while exhibiting typical MR characteristics for the applied magnetic field dependent yield stress with a slope of 1.5.

An investigation on the mitigation of end-stop impacts in a magnetorheological damper operated by the mixed mode

This paper presents mitigation behaviour of magnetorheological (MR) damper operated with a mixed working modes. A combination of the shear and squeeze modes is employed in the structure of MR damper to obtain the field-dependent normal yield stress as well as strengthen the squeeze effect. The experimental evaluation shows that when the piston is squeezing the bottom gap from the stroke of 25 to 26 mm, the sudden increase of squeeze force is observed confirming the existence of the mitigation effect. It is also observed that the magnitude of mitigation force is positively correlated with the magnitude of current given to the electromagnet. The measured peak mitigation forces are ranged from 722 N to 1032 N when the electromagnet currents are varied from 0.2 A to 0.8 A, respectively. The variable mitigation effect indicates that the concept can be further discussed as a potential impact protection feature in an MR damper.

Design and control of a prosthetic leg for above-knee amputees operated in semi-active and active modes

This paper proposes a new prosthesis operated in two different modes; the semi-active and active modes. The semi-active mode is achieved from a flow mode magneto-rheological (MR) damper, while the active mode is obtained from an electronically commutated (EC) motor. The knee joint part of the above knee prosthesis is equipped with the MR damper and EC motor. The MR damper generates reaction force by controlling the field-dependent yield stress of the MR fluid, while the EC motor actively controls the knee joint angle during gait cycle. In this work, the MR damper is designed as a two-end type flow mode mechanism without air chamber for compact size. On other hand, in order to predict desired knee joint angle to be controlled by EC motor, a polynomial prediction function using a statistical method is used. A nonlinear proportional-derivative controller integrated with the computed torque method is then designed and applied to both MR damper and EC motor to control the knee joint angle. It is demonstrated that the desired knee joint angle is well achieved in different walking velocities on the ground ground.

Yield Hardening of Electrorheological Fluids in Channel Flow

Understanding the physics behind electric-field-responsive fluids would open the way for advanced hydraulic components like valves, dampers, and clutches, but existing theories do not explain some observations. The authors study the increase in field-dependent yield stress of electrorheological fluids in pressure-driven flow. This increase is tied to the increase in local particle volume fraction, which is dynamically set by the balance of electrostatic forces and shear stresses. The authors' model allows quantitative design and optimization of valve parameters for robotic systems.

Numerical study of magnetic circuit response in magneto-rheological damper

Purpose – The purpose of this paper is to evaluate the performance of the semi-active fluid damper. It is recognized that the performance of such a damper depends upon the magnetic and hydraulic circuit design. These dampers are generally used to control the vibrations in various applications in machine tools and robots. The present paper deals with the design of magneto-rheological (MR) damper. A finite element model is built to analyze and understand the performance of a 2D axi-symmetric MR damper. Various configurations of damper with modified piston ends are investigated. The input current to the coil and the piston velocity are varied to evaluate the resulting change in magnetic flux density (B), magnetic field (H), field dependent yield stress and magnetic force vectors. The simulation results of the various configurations of damper show that higher magnetic force is associated with plain piston ends. The performance of filleted piston ends is superior to that of other configurations for the same magnitude of coil current and piston velocity. Design/methodology/approach – The damper design is done based on the fact that mechanical energy required for yielding of MR fluid increases with increase in applied magnetic field intensity. In the presence of magnetic field, the MR fluid follows Bingham’s plastic flow model, given by the equation τ = η γ•+τ y (H) τ > τ y . The above equation is used to design a device which works on the basis of MR fluid. The total pressure drop in the damper is evaluated by summing the viscous component and yield stress component which is approximated as ΔP = 12ηQL/g3W + CτyL/g, where the value of the parameter, C ranges from a minimum of 2 (for ΔPτ ΔPη less than approximately 1) to a maximum of 3 (for ΔPτ/ΔPη greater than approximately 100). To calculate the change in pressure on either side of the piston within the cylinder, yield stress is required which is obtained from the graph of yield stress vs magnetic field intensity provided by Lord Corporation for MR fluid −132 DG. Findings – In this work, three different finite element models of MR damper piston are analyzed. The regression equations, contour plots and surface plots are obtained for different parameters. This study can be used as a reference for selecting the parameters for meeting different requirements. It is observed from the simulation of these models that the plain ends model gave optimum magnetic force and 2D flux lines with respect to damper input current. This is due to the fact that the plain ends model has more area when compared with that of other models. It is also observed that filleted ends model gave optimum magnetic flux density and yield stress. As there is reduced pole length in the filleted ends model, the MR fluid occupies vacant area, and hence results in increased flux density and yield shear stress. The filleted ends assist the formation of dense magnetic flux lines thereby increasing the flux density and yield stress. This implies that higher load can be carried by the filleted ends damper even with a smaller size. Originality/value – This work is carried out to manufacture different capacities of the dampers. This can be applied as vibration controls.

Field-dependent characteristics of magnetorheological fluids containing corroded iron particles

Magnetorheological (MR) devices in vehicle applications and robotic systems require both consistent control performance and reliable operation. However, corrosion of iron particles in MR fluid may occur during long-time operation. This corrosion behavior can significantly affect the performance and reliability of application devices or systems utilizing MR fluids. This study experimentally investigates the effect of the MR particle corrosion on the performance of MR fluids in terms of the field-dependent yield stress magnitude and shear stress controllability. MR particles are first corroded by two different liquids; water–sodium chloride solution and water–calcium chloride solution. The resulting MR particles are examined by scanning electron microscopy and their molar ratios are analyzed by energy dispersive x-ray analysis. Using the corroded MR particles, MR fluid is synthesized for evaluation of the effects on its characteristics. A rotational viscometer is used to measure the field-dependent shear stress and response time. The measured results are compared between the original MR fluid composed of the non-corroded iron particles and the MR fluid composed of the corroded iron particles. In addition, the effect of the corroded particles in MR fluid on the shear stress controllability is investigated in the time domain.