On the yield stress of magnetorheological fluids

Magnetorheological fluids (MRFs) are a category of functional materials that exhibit magneto-mechanical coupling. These materials exhibit a reversible and instantaneous change from a free-flowing Newtonian fluid to a semi-solid state upon application of a magnetic field. In contrast to ordinary fluids, MRFs can tolerate shear stresses up to a yield value in the presence of a magnetic field. The yield stress strongly depends on intensity of the applied magnetic field and volume fraction of magnetic particles. As the yield stress is the most important parameter of an MRF and must be considered in the design of MR devices, in this work, effects of magnetic field and volume fraction of particles are investigated both experimentally and theoretically. MRF samples with the same carrier fluid but different particle concentrations are analyzed, and an empirical model is proposed for the yield stress of MRFs that covers a wide field strength range and also captures magnetic saturation of the MR fluids. Though the model is mathematically simple, it also includes the effect of particle concentration such that once calibrated, it can be utilized for different particle concentrations as well. Moreover, a modified form of the magnetic dipole model is proposed to model the yield stress of MRFs where an exponential distribution function is utilized to describe the arrangement of particle chains in the presence of a magnetic field. It is shown that, though the model has a simple mathematical formulation, it leads to a reasonable distribution of chains compared to previous similar models

Data for: Application of fractional time derivatives in modeling the finite deformation viscoelastic behavior of carbon-black filled NR and SBR¬¬

The uploaded figures contain all data presented in the manuscrip

Data for: Application of fractional time derivatives in modeling the finite deformation viscoelastic behavior of carbon-black filled NR and SBR¬¬

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On the design of cylindrical magnetorheological clutches

Magnetorheological fluids (MRFs) exhibit variable mechanical properties in response to magnetic stimuli. Thanks to their rapid and reversible viscosity changes, MRFs can be utilized in a variety of applications including torque transmission devices such as clutches. In this work, the geometrical design of cylindrical MR clutches is investigated with the aim of optimizing the torque transmission capability. Effects of design parameters such as radius, gap size, effective length, and MRF volume are investigated in the presence of variable magnetic field. Magneto-mechanical behavior of some MR fluids with different particle content are investigated by means of two different constitutive models to simulate the clutch performance in a range of geometrical parameters. It is shown that the transmitted torque increases nonlinearly by inner radius of the clutch, for example, in the studied range, 150% higher torque is achieved for only 40% larger radius. The clutch’s gap size does not much affect the torque, however, since it significantly affects the required volume of MRF, a lower gap size is favorable. The torque is also calculated for constant volumes of the MRFs. At a certain volume, although a higher radius translates to a shorter length, it is still favorable. For example, a 40% increase in the design radius, almost doubles the transmitted torque for both the studied MRFs. Moreover, a clutch filled by an MRF with higher particle content can transmit higher torques. It is also concluded that increasing the clutch’s radii is an easier way to improve the mean torque while altering the applied magnetic field is a better way to adjust the range of achievable torques. The simulations also demonstrate the importance of an accurate and reliable constitutive model in the design of MR devices. It is shown that Bingham model is not reliable at high magnetic fields as it underestimates the transmitted torque though calibrated at each field intensity. However, the employed nonlinear model provides more reliable results by only being calibrated at an arbitrary field

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

A material-based model for the simulation and control of soft robot actuator

An innovative material-based model is described for a three-pneumatic channel, soft robot actuator and implemented in simulations and control. Two types of material models are investigated: a soft, hyperelastic material model and a novel visco-hyperelastic material model are presented and evaluated in simulations of one-channel operation. The advanced visco-hyperelastic model is further demonstrated in control under multi-channel actuation. Finally, a soft linear elastic material model was used in finite element analysis of the soft three-pneumatic channel actuator within SOFA, moving inside a pipe and interacting with its rigid wall or with a soft hemispherical object attached to that wall. A collision model was used for these interactions and the simulations yielded “virtual haptic” 3d-force profiles at monitored nodes at the free- and fixed-end of the actuator