Magnetic Circuit Simulation for Magnetorheological (MR) Fluids Testing Rig in Squeeze Mode

Izwan Ismail1, a, Saiful Amri Mazlan2,b and Abdul Ghani Olabi1,c 1School of Mechanical and Manufacturing Engineering, Dublin City University, Dublin 9, Ireland 2College of Science and Technology, Universiti Teknologi Malaysia, Kuala Lumpur, Malaysia

aismail.izwan2@mail.dcu.ie, bamri@ic.utm.my, cabdul.olabi@dcu.ie

Keywords: Magnetorheological fluid, squeeze mode, testing rig, magnetic simulation

Abstract. In this study, a testing rig in squeeze was designed and developed with the ability to conduct various tests especially for quasi-static squeezing at different values of magnetic field strength. Finite Element Method Magnetics (FEMM) was utilized to simulate the magnetic field distribution and magnetic flux lines generation from electromagnetic coil to the testing rig. Tests were conducted with two types of MR fluid. MRF-132DG was used to obtain the behaviour of MR fluid, while synthesized epoxy-based MR fluid was used for investigating the magnetic field distribution with regards to particle chains arrangement. Simulation results of the rig design showed that the magnetic flux density was well distributed across the tested materials. Magnetic flux lines were aligned with force direction to perform squeeze tests. Preliminary experimental results showed that stress-strain pattern of MR fluids were in agreement with previous results. The epoxy-based MR samples produced excellent metallographic samples for carbonyl iron particles distributions and particle chain structures investigation.

Introduction

Magnetorheological (MR) fluids are defined as suspensions of magnetic particles disperse in a carrier liquid. Rheological properties of MR fluids can be controlled depending on magnetic field strength. Generally, the design of MR devices is using one of the three basic modes of operation [1]; valve, shear and squeeze or any combination of them. MR fluids in squeeze mode were proven capable to resist a very high stress [2] but exhibit a complex behaviour and difficult to model [3]. At certain stage of compression, Mazlan et al [4] found that the magnetic particles and carrier liquid were separated and thought to be a reason of the obtained stress-strain curve. Mclyntyre et al [5] described that phenomenon as a fluid-particle separation. However, Farjoud et al [6] mentioned about a clumping effect during the process, where the particles remained in the effective gap whilst some of the carrier fluid was ejected from the compression area. The effect was obviously undesirable because it led to anisotropic of MR fluid performance.

In developing testing rig for squeeze mode, it is important to design in such way the fluid-particle separation phenomena is measurable. The phenomenon is difficult to be confirmed due to the complex flow environment of MR fluids [5]. The complexity of the phenomenon is increased when many other related factors such as gap distance, amount of supplied current through coil, squeezing frequency and particle density are taking into consideration. Furthermore, a parallel magnetic flux lines along with squeezing direction is one of important requirements [6,7]. Using low reluctance or high magnetic permeability materials will help to reduce magnetic field diversions and consequently improve the magnetic flux direction. In general, MR fluid performance is dependent on magnetic field density. Thus, distribution and density of magnetic field must be accurately predicted by analyzing the magnetic circuit. Each part of the test equipment was analyzed and optimized to make sure that the magnetic field energy in the fluid gap was maximized while minimizing the energy lost to non-working MR fluid areas. Design alterations, which were related to the parts dimension of the rig, were simulated in the FEMM software package [7].

Advanced Materials Research Vols. 123-125 (2010) pp 991-994Online available since 2010/Aug/11 at www.scientific.net© (2010) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.123-125.991

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In this paper, design development and magnetic circuit simulation for testing rig, in order to establish the fluid-particle separation phenomenon in squeeze mode, were carried out. The developed testing rig was capable of performing quasi-static squeezing using epoxy-based MR fluid. The design will provide an opportunity to study and consequently understand on how the MR fluids react to such operating condition.

Development of Testing Rig

Design. The testing rig was designed to investigate the behaviour of MR fluids in squeeze mode under various setting conditions. The testing rig was also capable of producing solidified epoxy-based MR fluids in order to study microstructural changes during fluid-particle separation phenomena. In this condition, the rig was assumed as a casting mould that could continuously supply the magnetic field during curing or solidification process. The rig comprised of detachable parts to remove the solidified sample, which could be assembled into different modules. The modular design of the rig made its features flexible yet maintaining the magnetic circuit integrity and performance. Two-dimensional model of the rig was developed and drawn in axial–symmetrical orientation for simulation process as shown in Fig. 1(a).

Fig. 1. (a) Schematic diagram of the testing rig assembly and (b) fabricated testing rig parts. In developing the testing rig, three fixed components and five detachable parts were designed.

The fixed components were coil, bobbin stopper and lower cylinder, which also act as the magnetic core. The detachable parts comprised of top cylinder, sleeve, support cylinder and sleeve cap. Fig. 1(b) shows the schematic diagram of the testing rig assembly. The parts design were assemble in two separate positions which also compatible to be mounted on the Zwick Roel 50kN universal testing machine. The upper and lower positions were separated at 0.5 to 2 mm of gap range by the testing area. The testing rig was made from three types of materials, which were steel AISI 1010 for magnetic materials, aluminium 1100 for non-magnetic materials and 22SWG coated copper wire for magnetic coil. The AISI 1010 was used due to its high permeability to guide and focus magnetic flux into the fluid gap (testing area). The top cylinder, sleeve cap, sleeve and lower cylinder were made from magnetic materials, while the support cylinder and bobbin stopper were made from non-magnetic materials. Magnetic Field Simulation. Fig. 2(a) shows the magnetic field simulation for the testing rig at the desired magnetic field distribution throughout the circuits. The magnetic field was successfully diverted, guided and concentrated at the testing area due to the application of flux guide components (the shield and shield cap). Therefore, magnetic field loss was reduced and energy efficiency was increased. To operate the testing rig in squeeze mode, the simulation was done to generate linear and parallel magnetic flux line with almost isotropic magnetic field distribution

(a) (b)

Sleeve cap

Support cylinder

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Top cylinder

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around the testing area as shown in Fig. 2(b). The isotropic magnetic field distribution inside the testing area at A-A cross section is shown in Fig. 2(c). A consistent and continuous magnetic field of 1.1 Tesla was produced at 1 Amp of the supplied current.

Fig. 2. FEMM simulation of (a) magnetic field distribution, (b) magnetic flux direction and (c)

magnetic field across testing area at 1 Amps in the testing area of the testing rig

Experimental

MRF-132DG was used to obtain the behaviour of MR fluid in squeeze mode, while an irreversible epoxy-based MR fluid was used to study the microstructures [8]. The epoxy-based MR fluid was prepared in laboratory with 20% volume fraction of CS grade carbonyl iron (CI) particles disperse in a liquid epoxy solution. The epoxy liquid resin (Trimethylolpropan Tricrylate) was mixed with Polyamine hardener to solidify the samples. In the squeeze experiments, the MRF-132DG was stirred in a container prior to testing. A sample of 2 ml was placed on the testing area with initial gap of 1 mm. A constant current of 0.8 Amps was supplied to the coil in order to generate magnetic fields. Then, the upper cylinder was either moved down or moved up to compress or decompress the MR fluid at 2 mm/min of a constant speed. Final gaps for compression and tension were set to 0.5 mm and 2.0 mm, respectively. The fluid-separation phenomena experiment was conducted using the same parameter for compression settings. At the end of the testing the epoxy-based MR fluid was left cured for 1 hour at three different gaps to measure the fluid separation phenomena. Solidified samples were sectioned, ground, polished and gold coated for metallographic study. Metallographic study was done using EVO-L15 scanning electron microscope (SEM).

Preliminary Results and Discussion

The stress-strain curves resulted from the squeeze experiments represented the behaviour of MR fluid under normal stresses. Fig. 3 shows the stress-strain curves of MRF-132DG under compression and tension loadings. The testing rig was capable of producing repeatable curves lines. The curve produced was in agreement with previous research findings [4,6], where the regions of squeeze behaviour were clearly depicted.

Fig. 4 shows the micrographs of fluid-particle separation phenomena in MR fluid under compression at three different final gaps. The MR fluid immediately formed simple chain structures of particles when the magnetic field was induced. Fig. 4(a) shows the aggregation of CI particles into chain structures at 2.0 mm of the initial gap. The chains were accumulated and produced

(b)

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greater particles distribution at 1.5 mm of the final gap as shown in Fig. 4(b). Fig. 4(c) shows a highly dense particle structures when the final gap reached 0.5 mm. At this stage, most of the carrier fluid was assumed to expel from the testing area.

Fig. 3. Stress-strain curves of hydrocarbon-based MR fluid

Fig. 4. Micrographs of an epoxy-based MR fluid.

Conclusion A testing rig of MR fluid in squeeze mode was developed in conjunction with fluid-particle

separation phenomena investigation. The rig was designed and built according to specific requirements and magnetic field distributions from simulation using FEMM software. The stress-strain curves resulted from compression and tension experiments were in agreement with previous works. The fluid-particle separation phenomenon was captured by SEM micrographs. Consequently, the testing rig is significant to provide an opportunity to understand the detail behaviour of MR fluid in squeeze mode.

References

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Multi-Functional Materials and Structures III 10.4028/www.scientific.net/AMR.123-125 Magnetic Circuit Simulation for Magnetorheological (MR) Fluids Testing Rig in Squeeze Mode 10.4028/www.scientific.net/AMR.123-125.991

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