Magnetic Dipole Theory Research Articles

The field-dependent surface roughness of magnetorheological elastomer: numerical simulation and experimental verification

Uncertainty about the effect of magnetic field on the surface of a magnetorheological elastomer (MRE) results in difficulty in controlling the surface change effectively. Thus, a mesoscopic model based on magneto-mechanical coupling analysis is proposed to investigate how a magnetic field induces interaction of the ferromagnetic particles inside a MRE, and consequently how the surface microstructures of the MRE are changed. The results show that when a magnetic field is applied, the ferromagnetic particles are magnetized and there are interactions between particles; this leads to a change in the microstructure of the MRE surface, which accounts for the controllable tribological properties of the MRE. The influence of the initial surface profile, the particle volume fraction, the magnetic field strength and the modulus of the matrix on the surface roughness are also discussed. Based on the results of simulations, the mechanism of roughness change under a magnetic field is analyzed in combination with magnetic dipole theory. Further, a roughness measuring platform is developed and samples with different initial surface profiles and particle volume fractions are tested with and without a magnetic field; the results are consistent with simulations.

Micro-analysis of slip differential heat of magnetorheological fluids based on micromechanics and microstructures

For describing the slip differential heat between two neighboring particles of magnetorheological fluids (MRFs) in shear and squeeze modes, a theoretical model was developed in this study based on micromechanics and microstructures. Firstly, the interaction force and the contact stress during the shear and squeeze deformation was analyzed based on the magnetic dipole theory and the Hertzian contact theory. And then the motion and the relative velocity of particles were analyzed by assuming the incline or bending of a single chain ordered in MRFs. Furthermore, the slip differential heat flow caused by the friction of neighboring particles in MRFs was analyzed based on the tribology theory. The model takes into account the effect of the main influencing factors on the slip differential heat flow between two neighboring particles in a single chain of MRFs in shear and squeeze modes, such as the magnetic induction intensity, the size of particles, the number of the particles in a single chain, the relative velocity between the upper and lower walls in shear mode and the squeeze velocity in squeeze mode. Using the proposed model, the individual effect of typical governing parameters on slip differential heat flow under the realistic conditions of the magnetorheological clutch and damper was investigated by the numerical simulation. The results indicate that these factors have great influences on the slip differential heat flow, and the proposed model can satisfactorily describe the main micro-characteristics of the slip differential heat of MRFs in shear and squeeze modes.

Analysis on Spatial Spectrum of Magnetic Flux Leakage Using Fourier Transform

As an efficient non-destructive testing method, magnetic flux leakage (MFL) has been widely used for damage detection in ferromagnetic materials. The magnetic dipole theory is a traditional method to calculate the magnetic leakage field. In this paper, the magnetic leakage field functions of a slot in a 2-D infinity plate are obtained using the magnetic dipole theory. Equations of the magnetic leakage field are converted into the convolution of two other functions. Spatial spectral equations of the MFL are developed according to the convolution theorem using the Fourier transform (FT) method. The results are analytical functions different from other numerical methods, such as the discrete FT and fast FT methods, and the calculation speed is fast. The results show that the $y$ -component of the magnetic leakage field has 90° phase shift with the $x$ -component. This result has clear physical significance compared with the traditional MFL equations using the magnetic dipole theory. Different spatial spectral curves for different defects are presented in this paper. Since there is a sinc function for the width value in the equation, the width value can be deduced from the first zero-crossing point in the spatial spectrum curves. A new method is proposed to determine the width value using the relationship $w =1/f_{s}$ , where $f_{s}$ is the first zero cross point. Two specimens with different cracks are investigated, and the experimental results show this method can be used as an inverse MFL data interpretation technique.

Numerical Modeling of Magnetic Nanoparticle and Carrier Fluid Interactions Under Static and Double-Shear Flows

Magnetic nanoparticles are widely utilized in smart materials, the most well-known application being magnetorheological fluids. In this paper, a comprehensive numerical model of magnetic nanoparticles and the carrier fluid interactions is presented. The soft sphere model approach is adopted to simulate the nanoparticle interactions at a coarse molecular level, with the consideration of both contact and noncontact particle interactions. Magnetic dipole theory is used to simulate the particle behavior under external magnetic fields. The carrier fluid flow is solved using the computational fluid dynamics approach with a two-way coupling with the nanoparticles. Based on the present model, simulations have been conducted for both static fluid and dynamic double-shear flow cases. Without the presence of an external magnetic field, local particle clusters are formed as a result of van der Waals attraction between the particles. When the external magnetic field is present, chain-like structures of nanoparticles are observed for both the static and dynamic cases parallel to the direction of the applied magnetic field. For the static case, the formation of the chain-like structures is rapid, and “steady state” is reached within a short time (~1 ms). In the double-shear flow case, the chain-like structures are found to be much thinner and shorter than the static case. The developed model provides a fundamental understanding of the real physics of magnetic nanoparticles and carrier fluid interactions. It also enables future investigations in optimizing physical and transport properties of smart materials.

A Novel Non-destructive Testing Method by Measuring the Change Rate of Magnetic Flux Leakage

The application of magnetic sensors in the traditional magnetic flux leakage (MFL) technique has a significant influence on the detection results. The sensor is typically used to directly measure the amplitude of the magnetic leakage flux intensity as the detection signal. In view of noise effects on the detection result and the subsequent misinterpretation of defect signals, a new non-destructive testing method is proposed. The proposed method intends to measure the magnetic flux change rate using two sensors. A mathematical model is first established to present the principle of the change rate measurement. Based on the magnetic dipole theory, it is inferred that the new method is applicable and sensitive to the detection and location of defects. Moreover, this method is advantageous as it inhibits the interference of MFL noises such as the distension noise, background noise, and vibration noise. The model predictions are then verified by a series of simulations. Finally, an experimental platform is set up to practically detect the defect of a steel plate, and the results agree with the demonstrations and simulations.

磁场中磁性颗粒运动机理的研究进展及应用领域

本文从磁学理论、DLVO理论、地磁作用和等效表面电流原理、磁偶极子理论等方面综述了磁性颗粒在磁场中的运动机理的研究进展，评述了这些进展的科学性与局限性。这些研究进展主要包括在不同理论下磁场中磁性颗粒之间相互作用力的计算、被磁化的磁性颗粒间磁能的计算。重点围绕目前的热点问题，包括利用磁性物质减少药物副作用、驱动微型机器人、制作磁研磨介质、解决磁团聚问题等研究进行了介绍和讨论，旨在对近年来该领域的进展及获得的成果做一个概述和归纳，期望对将来的深入研究提供有一定借鉴。 In this paper, the progress of the motion mechanism of magnetic particles in magnetic field is re-viewed by the aspects of magnetism, DLVO, equivalent surface current principle and magnetic dipole theory. The scientificity and limitations of these progresses are reviewed. The progress of these studies mainly includes the calculation of the interaction between the magnetic particles in the magnetic field under different theories, and the calculation of the magnetic energy between the magnetized magnetic particles. Focusing on the current hot issues, the use of magnetic substances to reduce drug side effects, drive microrobot, make magnetic grinding media, solve the problem of magnetic reunification was introduced and discussed. This article provides an overview and induction of the progress and achievements of the field in recent years, hoping to provide certain reference for future in-depth research.

Research on chain-model transition identification of magnetic dipole theory for magneto-rheological fluid

Chain formation model is very useful to characterize the magneto-rheological phenomenon and prepare good magneto-rheological fluids. The single-chain model is common to explain the process of chain formation for ferromagnetic particles under magnetic field. With the increment of magnetic field and ferromagnetic particle content, the chain will transit from the single chain to multi-chains. However, there are few literatures involved in this phenomenon. This study investigates the effect of magnetic field and ferromagnetic particles content on the transition. The static yield stresses at different magnetic fields were measured under quasi-static mode for different magneto-rheological fluid samples. The results show that the transition of chain model can be identified on two parameters including the amplitude of static yield angle, [Formula: see text], and the inclined angles distribution, [Formula: see text]. The single-chain model is only effective under low magnetic field and ferromagnetic particles below content of 30% volume. With the increment of magnetic field and ferromagnetic particles content, the transition from single chain to multiple chains will be observed, which validates that there is a transition from the single chain to multi-chains.

Modeling and analysis of optical properties of a gold nanoring based on electric and magnetic dipoles.

The optical behavior of a plane-wave excited gold nanoring (NR), originated from localized surface plasmon resonance is modeled by two coupled electric- and magnetic-point dipoles. Considering the extinction cross-section spectrum, it is found that the electric-dipole effect is dominant in comparison with the magnetic-dipole effect although the magnetic-dipole signature is observable in the near-field response of the NR. In addition, the far-field electromagnetic radiation pattern of the NR verifies the corresponding radiation pattern of the point dipoles. The numerical simulation near-field results are in agreement with the proposed electric- and magnetic-dipole theory.

Weak magnetic flux leakage: A possible method for studying pipeline defects located either inside or outside the structures

Magnetic leakage distribution results from linear defects of oil–gas pipelines in a weak magnetic field, which is modeled by the magnetic dipole theory. The analysis is useful for the identification of defects located either inside or outside the pipelines. The results indicate that the radial signals of inside–outside defects can be clearly distinguished, and the axial signals are basically the same in a weak magnetic field. The theoretical and the experimental results are very consistent.

Theoretical Analyses of MFL Signal Affected by Discontinuity Orientation and Sensor-Scanning Direction

To improve the accuracy of the magnetic flux leakage (MFL) nondestructive testing in practical applications, we analyzed the MFL signal characteristics affected by discontinuity orientation and sensor-scanning direction. On the basis of magnetic dipole theory, the descriptions for the MFL field distributions of discontinuities in arbitrary orientations were established, indicating that the MFL density increases with the discontinuity orientation increasing from 0 to $\pi $ /2 and that magnetic flux always flows through the discontinuity perpendicularly, which was verified by relevant experiments. Further, the influence of the sensor-scanning direction on MFL signal features was analyzed and it was found that the MFL component signal parallel to scanning direction increases, the MFL component signal perpendicular to scanning direction falls, the MFL component signal perpendicular to discontinuity stays the same, and the widths of test signals become narrow, when the angle between the discontinuity orientation and sensor-scanning direction increases.

Numerical simulation of agglomeration process dynamics of ferromagnetic mineral particles in a weak magnetic field

Based on the magnetic dipole theory of magnetic-field distribution, the forces acting on ferromagnetic mineral particles (FMMPs) in the magnetic separation process were calculated in detail and a macro-scale dynamic model of magnetic agglomeration (MA) of particle swarms was established. A calculation method toward the dynamic model was proposed, and the two-dimensional dynamic process of the interaction between FMMPs was simulated. Furthermore, the energy conversion of single mineral particles to magnetic chains (MCs) was also analyzed. The results show that magnetic dipole–dipole attraction (MDDA) acting on FMMPs was the primary force in the magnetic separation process, and the dynamic process of MC formation from single mineral particles was closely related to time, distance between particles, and the external magnetic-field orientation. For FMMPs arranged in chain structures oriented along the external magnetic field, the time required for FMMPs to form MCs was less than 50ms. In addition, the optimum time for MC destruction was in the initial stages (less than 5ms) of the MC-formation process; either removal of the permanent magnet or discontinuation of the excitation current was the most direct and efficient way to fragment MCs once formed.

Research of Interference Field Measurement and Error Compensation for Geomagnetic Navigation

Aiming at removing interference field in the geomagnetic navigation, the technology of interference magnetic field measurement and error compensation are studied. The scheme of error measuring and compensating are designed, the compensation model is established based on the magnetic dipole theory, its calculation method is designed, experiments prove that this technology is of high accuracy and its process is simpler, it provides a new way for eliminating interference in the aeromagnetic survey.

The development of an adaptive tuned magnetorheological elastomer absorber working in squeeze mode

In the past, adaptive tuned vibration absorbers (ATVAs) based on magnetorheological elastomers (MREs) have mainly been developed in a shear working mode. The enhancing effect of MREs in squeeze mode has already been investigated, but ATVAs in squeeze mode have rarely been studied. This paper reports the development of a compact squeeze MRE absorber and its subsequent performance in various magnetic fields characterized under various frequencies by a vibration testing system. The results revealed that the natural frequency of the MRE absorber working in squeeze mode can be tuned from 37 Hz to 67 Hz. Following this, a theoretical model based on magnetic dipole theory was developed to investigate the dynamic performance of the squeeze MRE absorber, and the vibration attenuation of the squeeze MRE absorber was then verified by mounting it on a beam with supports under both ends. The results revealed that the squeeze MRE absorber extended its vibration attenuation range from 37 Hz to 67 Hz while the passive absorber was only effective around 53 Hz.

Signal Acquisition Analysis in Hi-Speed and Hi-Precision MFL Testing for Steel Pipe

Dealing with the relationship properly between the sensor scanning and signal acquisition is the base of hi-speed and hi-precision MFL (magnetic flux leakage) testing for steel pipe. Firstly, the MFL wave form characteristic was established using magnetic dipole theory. Further, on that basis, the relationship between signal frequency and sensor scanning was analyzed. Finally, sample frequency was designed according to the requirement of the automatic steel pipe MFL testing. Additionally, the MFL signal acquisition experiment was conducted to verify the influence of the signal sampling frequency. The signal acquisition analysis was of great significant to perform the MFL testing for steel pipe in hi-speed and hi-precision.

The Theoretical Analysis and Experimental Research of Wireless Power Transfer via Magnetic Resonance

Wireless power transfer through magnetic resonance is a new technology which attracts the attention of researchers widely. At present, three theories are put forward to explain the essence of it, namely coupled-mode theory, circuit-coupled theory and magnetic dipole theory. The theories have their unique advantages and disadvantages. In this paper, comparison between different theories will be put in order to find out the best area that one theory can apply to.

Modeling and analysis of magnetic dipoles in weak magnetic field

The magnetic leakage field distribution resulting from linear defects of a tube sample in the geomagnetic field is modeled according to the magnetic dipole theory. The formula to compute the normal component of the weak magnetic field is deduced based on the spatial distribution of the magnetic dipole. The shape and characteristics of the zero line (an important criterion for magnetic memory testing) of the normal field is analyzed under different longitudinal magnetizations. Results show that the characteristics of the zero line should be considered when the metal magnetic memory testing method is used to find and locate the defect.

Estimation of defects in a PWS rope by scanning magnetic flux leakage

Scanning the magnetic flux leakage { b} around a PWS rope which is magnetized to the saturated state in the longitudinal direction, an attempt has been made to prepare tomograms based on the degree of damage represented by magnetic charges { q}. They are obtained as the solution to an inverse problem expressed by a simultaneous linear equation: { b} = [ K]{ q}, where [ K] is a matrix determined by locations of the flux leakage sensors. These equations are based on the magnetic dipole theory: in this case, since the rope and the surrounding air can be regarded as a magnetically uniform medium, if a defect is produced within a rope due to a break or corrosion, it can be considered as a magnetic dipole. These equations were solved by using a least squares method with the restriction that the magnetic charge { q must not be negative. Though defects located in the superficial layer of a rope could be accurately estimated by observation in any one direction ( r θ or z), generally, it is recommended to use the z and θ directions when observation in two directions is available.

Theory of far-infrared absorption in small-metal-particle-insulator composites.

We present a theory of the far-infrared absorption coefficient for metal--alkali-halide composites assumed to consist of finite metal-particle-cluster subcomposites, a geometry very likely to exist in such systems. Three different models for the cluster composite topology are considered: highly metallic clusters, clusters with near-percolation paths of metal, and clusters with weakly allowed tunneling between isolated metallic constituents. For each, the effective dielectric properties of the cluster are first derived and the effective dielectric function for the entire composite is then determined from an application of classical magnetic or electric dipole theory. Subsequent calculations of the absorption coefficient are in reasonable agreement with a number of experimentally observed absorption features, and thus resolve the long-standing discrepancy of orders-of-magnitude difference between theory and experiment in such systems.

On a corollary to the magnetic dipole theory of the origin of spiral structure

Some observable features of our Galaxy are explained in terms of a theory of the origin of spiral structure, whose basic feature is a gradually contracting gaseous subsystem with a magnetic dipole of which the axis is very close to the galactic plane.