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Magnetic circuit design for the squeeze mode experiments on magnetorheological fluids
Materials and Design 30 (2009) 1985–1993
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Magnetic circuit design for the squeeze mode experiments on magnetorheological fluids S.A. Mazlan *, A. Issa, H.A. Chowdhury, A.G. Olabi School of Mechanical and Manufacturing Engineering, Dublin City University, Glasnevin, Dublin 9, Ireland
a r t i c l e
i n f o
Article history: Received 18 June 2008 Accepted 2 September 2008 Available online 17 September 2008 Keywords: Magnetorheological fluid (A) Squeeze mode (C) Finite element method magnetics (G)
a b s t r a c t Magnetorheological (MR) fluid is a manageable fluid that exhibits drastic changes in rheological properties and interchangeable depending on the applied magnetic field strength. The fluid is potentially advantageous to be employed in many applications. This paper presents the design of test equipment for the performance of compression and tension tests. Finite element method magnetics (FEMM) was used to analyze the magnetic field distribution through the MR fluid. The test equipment was constructed and modified according to the operational principles, conditions and simulation results. The tests were performed in a vertical direction to the DC magnetic field generated by a coil. Experimental results showed that the compressive/tensile stresses of MR fluids increased as the applied current increased. The test equipment was utilized to investigate the performance of the MR fluid in squeeze mode. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction A Magnetorheological (MR) fluid consists of iron particles dispersed in a carrier medium. Rheological properties of the fluid change drastically to become hard (solid-like state) in the presence of a magnetic field. In the absence of the magnetic field, the fluid returns to its original (free-flowing) state. Under field-responsive effect, particles inside the MR fluid are excited by the applied magnetic field to form strong chains along the direction of the magnetic flux lines [1]. The response time for the formation of new structures is in milliseconds. After a few seconds, a complete structure with columns or thick columns is formed depending on the size of the MR system [2]. The interaction between the particles acts as a resistance to the applied force. The degree of deformation resistance is contingent upon the applied magnetic field strength. In practice, MR fluid devices operate using; a shear mode, a valve mode, a squeeze mode or a combination of all [3]. In all cases, MR fluid is placed between two surfaces/plates but having different working conditions. The shear mode as an operational mode has only one surface that slides or rotates in relation to the other, with the magnetic fluxes perpendicular to the direction of motion of the shear surfaces. On the contrary, the valve mode is described as a flow of the MR fluid between motionless plates by a pressure drop. The squeeze mode is a geometric arrangement where two flat parallel surfaces, facing each other, are either pushed towards or pulled away from each other by an external force, operating
* Corresponding author. E-mail address: [email protected] (S.A. Mazlan). 0261-3069/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2008.09.009
orthogonally at the surfaces as shown in Fig. 1. Amongst all modes, the squeeze mode, particularly in compression, can produce the highest stress. By analogy, the achievement of the highest stress was also found by employing the squeeze mode on electrorheological (ER) fluids [4–6]. Similar rheological properties changes were observed but under different external fields, where the ER fluids responded to an electric field strength. The advantages of MR fluids in squeeze mode have been demonstrated by their applications in many functioning parts. For instance, damper-rotor systems showed the capability in controlling vibrations [7]. Vieira et al. [8] constructed a test equipment to study the behaviour of MR fluids under oscillatory conditions. This design consisted of two parallel plates surrounded by a rubber enclosure. A coil with 2200 turns and 143 X was wound on the steel plate to provide the magnetic field. It was seen that the effectiveness of the squeeze mode was most depended on varying the current or the magnetic field strength as agreed by other researchers. Accordingly, many of them have investigated the performance of MR fluids in squeeze mode. Forte et al. [9] designed an MR squeeze damper and used numerical simulation to evaluate the dynamic behaviour of the damped rotor. Wang et al. [10] performed an experimental study to analyze the characteristics of an MR squeeze film damper-rigid rotor system. Moreover, Ahn et al. [11] improved the function of this conventional method by introducing a new type of the squeeze film damper. However, the quasi-static properties of the MR fluid in squeeze mode have not been investigated thoroughly yet. Therefore, in this study, a simple test equipment was constructed, which had controllable movements in a vertical direction to obtain compressive and tensile stresses under variable factors.
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2ED. MRF-241ES is a water-based MR fluid, which is developed for general use in sealed systems of controllable, energy-dissipating applications such as shocks, dampers and brakes. MRF-132DG and MRF-122-2ED are hydrocarbon-based MR fluids, which are formulated for general use in controllable, energy-dissipating
Force Magnetic Field Force Upper plate MR fluid Lower plate Compression
Tension
Fig. 1. Basic principle of the squeeze mode of MR fluids.
2. Experimental 2.1. Materials Three different types of commercially available MR fluids produced by Lord Corporation were chosen in order to characterize the compressive/tensile stress under various conditions. These materials were namely MRF-241ES, MRF-132DG and MRF-122-
2.0
Magnetic flux density (Tesla)
1.5 1.0 0.5 0.0 -800
-600
-400
-200
0
200
400
600
800
-0.5 Fig. 4. Sketch of the test equipment showing the black hatched lines in the middle half of the test equipment and red hatched line between the upper and lower cylinders.
-1.0 MRF-241ES MRF-132DG
-1.5
MRF-122-2ED
-2.0
Magnetic field intensity (kA/m) Fig. 2. Typical magnetic properties of the tested MR fluids.
5.0
12000
4.5 10000
4.0
Curve B
3.5
8000
3.0 2.5
6000
2.0 4000
Current (Amps)
No. of Turns
Curve A
1.5 1.0
2000
0.5 0
0.0 18
20
22
24
26
28
30
Wire Gauge (SWG) Fig. 3. Number of coil’s turns and current as functions of wire gauge.
Fig. 5. Simulated magnetic flux density distribution in the test equipment.
S.A. Mazlan et al. / Materials and Design 30 (2009) 1985–1993
applications such as shocks, dampers and brakes. The selection of the carrier liquid determines the temperature ranges in which the MR fluid can be utilized. Even though silicone oil is the most frequently used carrier liquid, hydrocarbon oil has some advantages due to its low viscosity, better lubrication properties and suitability for high shear-rate applications. Moreover, a hydrocarbon oil-based MR fluid has lower zero field viscosity, which is about 0.6 times less than the silicone oil-based MR fluid [12]. On the other hand, a water-based MR fluid can minimize waste disposal problems and allows the particles to be easily recycled from the material [13]. Fig. 2 shows the magnetic induction curves or B–H curves, of the three tested commercial MR fluid materials [14]. MR fluids exhibit approximately linear magnetic properties up to 200 kA/m of applied field intensity. The magnetic flux density of MRF-241ES demonstrates greater values over those of the other MR fluids.
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The magnetic properties of MR fluids vary significantly due to a higher particle density in MRF-241ES (3.80–3.92 g/cm3) than MRF-132DG (2.98–3.18 g/cm3) and MRF-122-2ED (2.32–2.44 g/ cm3). 2.2. Apparatus A design concept of the experimental set-up similar to that used for studying ER fluids by Tian et al. [15] was utilized in this study. In order to carry out experimental trials on the MR fluids, the external field had to be changed from an electric field to a magnetic field. Therefore, new test equipment needed to be designed to accommodate these differences. Basically, the test equipment consisted of two parts; an upper and a lower cylinder. A measured amount of the MR fluid was sandwiched in between the two cylinders, so that the upper cylinder could move towards or pulls
Fig. 6. Simulated magnetic flux density distribution in (a) air, (b) MRF-241ES, (c) MRF-132DG and (d) MRF-122-2ED.
Fig. 7. Simulated magnetic flux lines across the (a) air gap, (b) MRF-241ES, (c) MRF-132DG and (d) MRF-122-2ED.
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against the lower cylinder, and simultaneously compress or decompress the MR fluid. An axial–symmetrical model was selected in the Finite element method magnetics (FEMM) software package [16]. A number of design changes were simulated in the FEMM software package. Analysis and optimization of magnetic behaviours of different devices were carried out using FEMM by other researchers [17–19]. This software package is suitable for the type of materials of each component of the test equipment, type of coil, number of turns of the
coils wrapping around the core and values of the electric current that runs in the coils. These parameters were important to produce the best value for the magnetic field intensity H, which was correlated with the magnetic flux density B. In these changes, the number of turns of the coil, the initial gap size and the current value supplied to the coil were kept constant. For the geometry of the coil, 60, 92 and 100 mm were used for the dimensions of inner diameter, outer diameter and width of the coil, respectively. These dimensions were selected based on the
a
b Magnetic flux density (Tesla)
Magnetic flux density (Tesla)
0.70 0.65 0.60 0.55 0.50 0.45 MRF-241ES
0.40
MRF-132DG
0.35
MRF-122-2ED Air gap
0.01 0.00 -0.01 -0.02 MRF-241ES MRF-132DG MRF-122-2ED
-0.03
Air gap
-0.04
0.30 0
5
10
15
20
0
25
5
10
Distance from centre (mm)
15
20
25
Distance from centre (mm)
Fig. 8. Simulated magnetic flux density distribution inside the air gap and MR fluids from centre line to the edge between the upper and the lower cylinders. (a) Normal magnetic flux density (B.n) and (b) tangential magnetic flux density (B.t).
a
b 0.8
Magnetic flux density (Tesla)
0.7 0.6 0.5 0.4 0.3
MRF-241ES MRF-132DG MRF-122-2ED Air Gap
0.2 0.1 0.0 0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
0.7 0.6 0.5 0.4 0.3
MRF-241ES MRF-132DG MRF-122-2ED Air Gap
0.2 0.1 0.0 0.0
1.6
0.2
0.4
0.6
0.8
1.0
1.2
Current (Amps)
Current (Amps)
c 0.8
Magnetic flux density (Tesla)
Magnetic flux density (Tesla)
0.8
0.7 0.6 0.5 0.4 0.3
MRF-241ES MRF-132DG MRF-122-2ED Air Gap
0.2 0.1 0.0 0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
Current (Amps) Fig. 9. Simulated magnetic flux density versus applied current for initial gap sizes of (a) 2.0, (b) 1.2 and (c) 0.4 mm.
1.4
1.6
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J¼
Im ; Aw
ð1Þ
where J is the current density (Amps/mm2), Im is the maximum current supplied to the coil (Amps) and Aw is the surface area of the uncoated wire (mm2). For instance, 22SWG copper wire has a diameter of 0.71 mm, thus, the maximum current value allowed in this wire was 1.6 Amps. Furthermore, the wire can be wound around the specified core’s dimensions with 128 turns in each layer of the width of coil, 20 layers between inner and outer diameters of the coil, and gave the total number of 2560 turns. Fig. 3 shows the range of some parameters to be compromised in order to achieve the desired results. Curve A is a function of number of turns and wire gauge and curve B is a function of current and wire gauge. After the most adequate design of the test equipment was predicted by FEMM, some modifications have been made to the original concept of the test equipment because of the manufacturing processes and material limitations. In the final design, the test equipment consisted of seven parts as shown in Fig. 4. The materials of the test equipment could be divided into three categories; magnetic materials, non-magnetic materials and a coil. Some advantages of this test equipment included a small required volume of tested samples, the ability to vary the working gap height,
Magnetic flux density (Tesla)
a
1.0
I = 1.6 A I = 1.2 A I = 0.8 A I = 0.4 A
0.8
0.6
0.4
0.2
0.0 0.0
0.4
0.8
1.2
1.6
and the simplicity and ease of use. The upper cylinder, lower cylinder, lower ring and lower base were made from magnetic materials, while the upper ring and support cylinder were made from non-magnetic materials. A length of copper wire type 22SWG having a resistance of 29 X was chosen to be wound around the lower cylinder forming 2750 turns. The support cylinder was attached on the top of the lower cylinder and acted as a container so as to refrain the MR fluid during the testing. 2.3. Magnetic field distributions The main objective of the magnetic circuit was to produce the correct magnetic flux density across the MR fluid. From the electromagnetic point of view, the test equipment could be treated as a circuit concentrating the magnetic field generated by a coil and guiding it from the core to and across the fluid. An analysis of the magnetic field between the upper cylinder and the lower cylinder (in the gap or across the MR fluids) was performed using FEMM. The magnetic properties of the non-magnetic materials were assumed to be linear such as stainless steel, copper wire and air. The magnetic properties of the magnetic materials such as low carbon steel and MR fluid materials were assumed to follow the B–H curves given in the software package and provided by the manufacturer. Simulation results were based on the middle half of the test equipment (axial–symmetrical model), where was indicated by black hatched line, while the average values of the magnetic flux density were pointed out by the red hatched line as shown in Fig. 4. The simulated magnetic flux density distribution in the test equipment is shown in Fig. 5. The simulated magnetic flux density
b Magnetic flux density (Tesla)
various values of magnetic flux density that could be achieved by varying the applied current, and furthermore, ease of manufacture and assembly. These parameters limited the area in which the copper wire can be wound. Current density was limited to 4 Amm2 in connection with the appropriate wire diameter [20]. Therefore, the maximum current density is given by
1.0
0.8
0.6
0.4
0.2
0.0 0.0
2.0
I = 1.6 A I = 1.2 A I = 0.8 A I = 0.4 A
0.4
Initial gap size (mm)
1.0
I = 1.6 A I = 1.2 A I = 0.8 A I = 0.4 A
0.8
0.6
0.4
0.2
0.0 0.0
0.4
0.8
1.2
Initial gap size (mm)
1.6
2.0
d Magnetic flux density (Tesla)
Magnetic flux density (Tesla)
c
0.8
1.2
1.6
2.0
Initial gap size (mm)
1.0
I = 1.6 A I = 1.2 A I = 0.8 A I = 0.4 A
0.8
0.6
0.4
0.2
0.0 0.0
0.4
0.8
1.2
1.6
2.0
Initial gap size (mm)
Fig. 10. Simulated magnetic flux density versus initial gap size for different values of applied current for (a) Air, (b) MRF-241ES, (c) MRF-231DG and (d) MRF-122-2ED.
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distribution within the air gap and the MR fluids are shown in Fig. 6, whereas simulated magnetic flux lines across the air gap and MR fluids are shown in Fig. 7. Figs. 5–7 show the simulated results when the applied current to the coil was set to 1.6 Amps and the initial gap size was set to 2.0 mm. The magnetic flux density distribution in the air gap and MR fluid materials can be considered to be evenly distributed. Magnetic field results of the FEMM software package for the air gap in terms of the magnetic field intensity generated by the coil were validated using a DC magnetometer (gauss-meter) supplied by AlphaLab Inc. The magnetic field strength increased when the applied current increased. The values of the magnetic field strength obtained by direct measurement and simulation were in a good agreement as presented by Mazlan et al. in previous work [21]. Fig. 8a shows that the average values of the magnetic flux density at the middle line between the upper and lower cylinders starting from the centre of the test equipment to the inner diameter of the support cylinder were 0.47, 0.70, 0.67 and 0.61 Tesla for the air gap, MRF-241ES, MRF-132DG and MRF-122-2ED, respectively. As shown in Fig. 7, the magnetic flux lines seemed to penetrate the air gap and MR fluid materials and were aligned in straight lines with the direction of the magnetic field. However, after about 24 mm distance from the centre of the test equipment, the values of normal magnetic flux density tended to reduce, while the values of tangential magnetic flux density become higher as shown in Fig. 8a and b. The values of the tangential magnetic flux density contributed to the shear mode during the testing [22], which is not the interest of this study. Furthermore, the tangential values are very small (less than 0.04 Tesla in the negative direction) and can be neglected.
b
0.8
Magnetic flux density (Tesla)
Magnetic flux density (Tesla)
a
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0
Fig. 9 shows the simulated values of the average magnetic flux density as the applied current increases at different initial gap sizes starting from 2.0 mm down to 0.4 mm. The magnetic flux density increased with increasing the applied current. The average values of the magnetic flux density were taken from centre of the test equipment through the middle line between the upper and the lower cylinders for Figs. 9–12. MRF-241ES always showed the highest values of magnetic flux density at any initial gap size, followed by MRF-132DG, MRF-1222ED and air. As the initial gap size decreased, the curves variation became smaller in the air and the MR fluids. The curves tended to slowly closely match each other as the initial gap size was decreased. This indicated a small effect on the magnetic flux density. The simulated effects of the applied current on magnetic flux density when increasing the initial gap sizes are depicted in Fig. 10. As the initial gap size increased, the values of magnetic flux density decreased. When the initial gap size increased, MRF-241ES, in Fig. 10b, showed the least variation in magnetic flux density, while for the air case, in Fig. 10a, showed the largest variation of the magnetic flux density. Fig. 11 shows the simulated values of magnetic flux density as the initial gap size is increased at different applied currents starting from 1.6 Amps down to 0.4 Amps. The values of magnetic flux density decreased with increasing the initial gap size. The curves of the magnetic flux density versus initial gap size started at the same point at zero value of the initial gap size. But the initial magnetic flux density depended on the current supplied to the coil, for example 0.79, 0.61, 0.42 and 0.21 Tesla were initial magnetic flux values corresponding to 1.6, 1.2, 0.8 and 0.4 Amps, respectively before proceeding to decrease as the initial gap size increased.
MRF-241ES MRF-132DG MRF-122-2ED Air Gap 0.4
0.8
1.2
1.6
0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0
2.0
MRF-241ES MRF-132DG MRF-122-2ED Air Gap 0.4
Initial gap size (mm) 0.8
MRF-241ES MRF-132DG MRF-122-2ED Air Gap
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0
0.4
0.8
1.2
Initial gap size (mm)
1.6
2.0
d Magnetic flux density (Tesla)
Magnetic flux density (Tesla)
c
0.8
1.2
1.6
2.0
Initial gap size (mm) 0.8
MRF-241ES MRF-132DG MRF-122-2ED Air Gap
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0
0.4
0.8
1.2
1.6
2.0
Initial gap size (mm)
Fig. 11. Simulated magnetic flux density versus initial gap size for the applied currents of (a) 1.6, (b) 1.2, (c) 0.8 and (d) 0.4 Amps.
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Magnetic flux density (Tesla)
0.8 0.7 0.6
b
h = 0.4 mm h = 0.8 mm h = 1.2 mm h = 1.6 mm h = 2.0 mm
0.8
Magnetic flux density (Tesla)
a
0.5 0.4 0.3 0.2
0.6 0.5 0.4 0.3 0.2 0.1
0.1 0.0 0.0
0.7
h = 0.4 mm h = 0.8 mm h = 1.2 mm h = 1.6 mm h = 2.0 mm
0.2
0.4
0.6
0.8
1.0
1.2
1.4
0.0 0.0
1.6
0.2
0.4
Current (Amps)
Magnetic flux density (Tesla)
0.8 0.7 0.6
d
h = 0.4 mm h = 0.8 mm h = 1.2 mm h = 1.6 mm h = 2.0 mm
0.8
0.5 0.4 0.3 0.2
1.0
1.2
1.4
1.6
0.7 0.6
1.2
1.4
1.6
h = 0.4 mm h = 0.8 mm h = 1.2 mm h = 1.6 mm h = 2.0 mm
0.5 0.4 0.3 0.2 0.1
0.1 0.0 0.0
0.8
Current (Amps)
Magnetic flux density (Tesla)
c
0.6
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
Current (Amps)
0.0 0.0
0.2
0.4
0.6
0.8
1.0
Current (Amps)
Fig. 12. Simulated magnetic flux density versus current for different values of the initial gap sizes for (a) Air, (b) MRF-241ES, (c) MRF-231DG and (d) MRF-122-2ED.
Step
Step 0.4
1.0 0.8
1.2
Constant currents were maintained manually throughout the tests
2.0 1.6 Initial gap size (mm)
Current (Amps) Fig. 13. Combinations of the test parameters.
The simulated effects of initial gap size on magnetic flux density when increasing the applied currents are shown in Fig. 12. As the applied current increased, the values of magnetic flux density increased. When increasing the current values in the coil, MRF241ES in Fig. 12b, showed the least variation in the magnetic flux density, while air in the gap in Fig. 12a, showed the largest variation of the magnetic flux density. The effects of the applied current and initial gap size on the magnetic flux density were interdependent. In general, the values of the magnetic flux density turned into greater values with
increasing the value of applied current or decreasing the initial gap size. However, these values depended on the type of carrier liquid in the MR fluid. MRF-241ES showed the highest values of magnetic flux density as compared to the other MR fluids (MRF132DG and MRF-122-2ED). The reason behind this was simply, due to the relatively higher values of magnetic properties in MRF-241ES as shown in Fig. 2. 3. Experimental set-up A schematic diagram of the experimental set-up was disclosed in an earlier work [21]. The experimental set-up consisted of five main components, namely the Instron Machine, the test equipment, the MR fluids, the power supply and the control computer. The test equipment was placed in the testing area of the Instron Universal Electromechanical Testing (UTS) machine (model 4202). The communication with the machine was achieved via a built-in data acquisition board in the computer. The Instron machine was operated in the vertical direction to obtain displacements and forces under compression and tension modes. A measured amount of the MR fluid materials was filled in the gap between the upper and the lower cylinders. The current was supplied by the XantrexTM XFR 150 V, 18 Amps DC power supply. The magnetic field strength was adjusted by manually controlling the value of the current supplied to the electromagnetic coil. All experiments were carried out in a displacement control mode at a room temperature of 20 °C. In this study, for each type of MR fluid tested, three sets of parameters were prepared to perform the compression and the
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Fig. 14. Stress–strain curves relationship in compression mode under various parameters for (a) MRF-241ES, (b) MRF-132DG and (c) MRF-122-2ED.
tension tests. Various parameters including different initial gap sizes (step 1) and applied currents (step 2) were set before performing any tests, shown in Fig. 13. Two types of experiments were performed; namely compression and tension tests. The compression tests were carried out by lowering the upper cylinder towards the lower cylinder. While, the tension tests were carried out by pulling up the upper cylinder away from the lower cylinder. Both tests were done using a computer-controlled movement with a constant speed of 1.0 mm/min. The temperature in the gap be-
Fig. 15. Stress–strain curves relationship in tension mode under various parameters for (a) MRF-241ES, (b) MRF-132DG and (c) MRF-122-2ED.
tween the upper and the lower cylinders was measured using a Type K thermocouple, which was capable of operating over a very wide temperature range. The maximum temperature obtained in the test equipment when the current was supplied continuously for 20 min was 65 °C with the maximum voltage reached 54 V. Therefore, in every test, the power supply was withdrawn after running every test and the maximum voltage was limited to 54 V.
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Initially, one of the parameters in step 1 was set accurately to the specific gap distance between the two flat parallel surfaces. Then, different current values were supplied to the coil (0.4, 0.8, 1.2 and 1.6 Amps). The compression/tension processes began when the upper cylinder was either lowered or raised at a constant speed of 1.0 mm/min, and simultaneously the loads and displacements were recorded continuously. The maximum strain values of both compression and tension tests were set to 0.75. 4. Experimental results Fig. 14 illustrates the stress–strain curves of the MR fluids under compression at various applied currents and different initial gap sizes. At high values of applied current, the compressive stress increased with increasing in compressive strain. The stress–strain curve of MRF-241ES looked steeper at a large initial gap size indicating that the compressive stress increased as the initial gap size increased for the same value of compressive strain. However, the effect of applied current and initial gap size were difficult to illustrate for fluids with poorer solids-to-liquid ratio (MRF-132DG and MRF-122-2ED) at initial gap size of 1.0 mm, due to their low magnetic properties (Fig. 2). It was assumed that the same curves pattern of the effect of initial gap size of MRF-241ES could be achieved on MRF-132DG and MRF-122-2ED, if the maximum values of compressive strain were set to higher values. Eventually, under the same strain value, the compressive stresses were strongly affected by the current magnitude and the initial gap size [21–23]. Similar results were obtained in other external field dependent rheological fluids, where the employment of the ER fluids was carried out under the influence of electrical field. These observations were comparable to those reported by Tian et al. [15]. The tensile stress–strain curves of the MR fluids at a constant tensile speed of 1.0 mm/min, different applied currents and different initial gap sizes are shown in Fig. 15. The behaviour of the MR fluids in tension mode was discussed by Mazlan et al. [24]. As can be seen, higher values of the tensile stress were observed at lower initial gap sizes when a constant current was applied to the MR fluids. The effect of the initial gap size under tension tests contradicted with the same effect under compression tests in terms of the stress–strain relationships. In sight of this, one might have expected that a higher tensile stress would occur where the applied magnetic field strength was greatest. Therefore, higher values of tensile stresses were required when the initial gap size was set to 1.0 mm in comparison to the situation when the initial gap size of 2.0 mm has been set. In addition, under the same value of the tensile strain, the value of tensile stress was higher at higher values of applied currents. Lower values of tensile stress could be achieved as the applied current is reduced. Higher values of the tensile stress could be achieved with increases in the applied current. Similar results were also obtained by other researchers on the employment of ER fluids [25,26]. 5. Conclusion A test equipment was designed and built to carry out investigations on the behaviour of the MR fluids in squeeze mode. The test equipment was utilized to perform compression and tension tests on the MR fluids, which were placed between two parallel surfaces
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(upper and lower cylinders). A selection range of the coil size/design within the test equipment was obtained based on the given dimensions of the lower cylinder. The FEMM software package helped finalizing the coil selection in order to produce the best range of the magnetic field strength. Consequently, the best values of the magnetic field strength generated by the optimized coil design were used. As a result, the effects of varying the applied current and initial gap size on the stress–strain relationships were investigated. Comparable with other experiments, similar results were observed either with different types of MR fluids (under the influence of magnetic field) or with ER fluids (under the influence of electric field). References [1] Hagenbuchle M, Liu J. Chain formation and chain dynamics in a dilute magnetorheological fluid. Appl Opt 1997;36:7664–71. [2] Tao R. Super-strong magnetorheological fluids. J Phys: Condens Mat 2001;13:R979–99. [3] Carlson JD, Jolly MR. MR fluid, foam and elastomer devices. Mechatronics 2000;10:555–69. [4] Lukkarinen A, Kaski K. Computational studies of compressed and sheared electrorheological fluid. J Phys D: Appl Phys 1996;29:2729–32. [5] Monkman GJ. The electrorheological effect under compressive stress. J Phys D: Appl Phys 1995;28:588–93. [6] Tian Y, Meng Y, Mao H, Wen S. Electrorheological fluid under elongation, compression, and shearing. Phys Rev E 2002;65:031507. [7] Wang J, Meng G, Feng N, Hahn EJ. Dynamic performance and control of squeeze mode MR fluid damper-rotor system. Smart Mater Struct 2005;14:529–39. [8] Vieira SL, Ciocanel C, Kulkarni P, Agrawal A, Naganathan N. Behaviour of MR fluids in squeeze mode. Int J Vehicle Des 2003;33:36–49. [9] Forte P, Paternò M, Rustighi E. A magnetorheological fluid damper for rotor applications. Int J Rotating Mach 2004;10:175–82. [10] Wang J, Feng N, Meng G, Hahn EJ. Vibration control of rotor by squeeze film damper with magnetorheological fluid. J Intell Mater Syst Struct 2006;17:353–7. [11] Ahn YK, Ha J-Y, Yuk B-S. A new type controllable squeeze film damper using an electromagnet. J Vib Acoust 2004;126:380–3. [12] Shen C, Wen W, Yang S, Sheng P. Wetting-induced electrorheological effect. J Appl Phys 2006;99:106104. [13] Carlson JD, JonesGuion JC. Aqueous magnetorheological materials. US Patent 1997;5:5670077. [14] Lord Corporation.; 2007. [15] Tian Y, Wen S, Meng Y. Compressions of electrorheological fluids under different initial gap distances. Phys Rev E 2003;67:051501. [16] Meeker DC. FEMM. ; 2006. [17] Grossingger R, Kupferling M, Kasperkovitz P, et al. Eddy currents in pulsed field measurements. J Magn Magn Mater 2002;242–245:911–4. [18] Bohlmark J, Ostbye M, Lattemann M, Ljungcrantz H, Rosell T, Helmersson U. Guiding the deposition flux in an ionized magnetron discharge. Thin Solid Films 2006;515:1928–31. [19] Vasic D, Bilas V, Ambrus D. Pulsed eddy-current nondestructive testing of ferromagnetic tubes. IEEE T Instr Measure 2004;53:1289–94. [20] Sassi S, Cherif K, Mezghani L, Thomas M, Kotrane A. An innovative magnetorheological damper for automotive suspension: from design to experimental characterization. Smart Mater Struct 2005;14:811–22. [21] Mazlan SA, Ekreem NB, Olabi AG. The performance of a magnetorheological fluid in squeeze mode. Smart Mater Struct 2007;16:1678–82. [22] Mazlan SA, Ekreem NB, Olabi AG. An investigation of the behaviour of magnetorheological fluids in compression mode. J Mater Process Tech 2008;241:780–5. [23] Mazlan SA, Ekreem NB, Olabi AG. Apparent stress–strain relationships in experimental equipment where magnetorheological fluids operate under compression mode. J Phys D: Appl Phys 2008;41:095002. [24] Mazlan SA, Issa A, Olabi AG. Magnetorheological fluids behaviour in tension loading mode. Adv Mater Research 2008;47–50:242–5. [25] Tian Y, Zou Q. Normalized method for comparing tensile behaviors of electrorheological fluids. Appl Phys Lett 2003;82:4836–8. [26] Tian Y, Zou Q, Meng Y, Wen S. Tensile behavior of electrorheological fluids under direct current electric fields. J Appl Phys 2003;94:6939–44.
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Magnetic circuit design for the squeeze mode experiments on magnetorheological fluids S.A. Mazlan *, A. Issa, H.A. Chowdhury, A.G. Olabi School of Mechanical and Manufacturing Engineering, Dublin City University, Glasnevin, Dublin 9, Ireland
a r t i c l e
i n f o
Article history: Received 18 June 2008 Accepted 2 September 2008 Available online 17 September 2008 Keywords: Magnetorheological fluid (A) Squeeze mode (C) Finite element method magnetics (G)
a b s t r a c t Magnetorheological (MR) fluid is a manageable fluid that exhibits drastic changes in rheological properties and interchangeable depending on the applied magnetic field strength. The fluid is potentially advantageous to be employed in many applications. This paper presents the design of test equipment for the performance of compression and tension tests. Finite element method magnetics (FEMM) was used to analyze the magnetic field distribution through the MR fluid. The test equipment was constructed and modified according to the operational principles, conditions and simulation results. The tests were performed in a vertical direction to the DC magnetic field generated by a coil. Experimental results showed that the compressive/tensile stresses of MR fluids increased as the applied current increased. The test equipment was utilized to investigate the performance of the MR fluid in squeeze mode. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction A Magnetorheological (MR) fluid consists of iron particles dispersed in a carrier medium. Rheological properties of the fluid change drastically to become hard (solid-like state) in the presence of a magnetic field. In the absence of the magnetic field, the fluid returns to its original (free-flowing) state. Under field-responsive effect, particles inside the MR fluid are excited by the applied magnetic field to form strong chains along the direction of the magnetic flux lines [1]. The response time for the formation of new structures is in milliseconds. After a few seconds, a complete structure with columns or thick columns is formed depending on the size of the MR system [2]. The interaction between the particles acts as a resistance to the applied force. The degree of deformation resistance is contingent upon the applied magnetic field strength. In practice, MR fluid devices operate using; a shear mode, a valve mode, a squeeze mode or a combination of all [3]. In all cases, MR fluid is placed between two surfaces/plates but having different working conditions. The shear mode as an operational mode has only one surface that slides or rotates in relation to the other, with the magnetic fluxes perpendicular to the direction of motion of the shear surfaces. On the contrary, the valve mode is described as a flow of the MR fluid between motionless plates by a pressure drop. The squeeze mode is a geometric arrangement where two flat parallel surfaces, facing each other, are either pushed towards or pulled away from each other by an external force, operating
* Corresponding author. E-mail address: [email protected] (S.A. Mazlan). 0261-3069/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2008.09.009
orthogonally at the surfaces as shown in Fig. 1. Amongst all modes, the squeeze mode, particularly in compression, can produce the highest stress. By analogy, the achievement of the highest stress was also found by employing the squeeze mode on electrorheological (ER) fluids [4–6]. Similar rheological properties changes were observed but under different external fields, where the ER fluids responded to an electric field strength. The advantages of MR fluids in squeeze mode have been demonstrated by their applications in many functioning parts. For instance, damper-rotor systems showed the capability in controlling vibrations [7]. Vieira et al. [8] constructed a test equipment to study the behaviour of MR fluids under oscillatory conditions. This design consisted of two parallel plates surrounded by a rubber enclosure. A coil with 2200 turns and 143 X was wound on the steel plate to provide the magnetic field. It was seen that the effectiveness of the squeeze mode was most depended on varying the current or the magnetic field strength as agreed by other researchers. Accordingly, many of them have investigated the performance of MR fluids in squeeze mode. Forte et al. [9] designed an MR squeeze damper and used numerical simulation to evaluate the dynamic behaviour of the damped rotor. Wang et al. [10] performed an experimental study to analyze the characteristics of an MR squeeze film damper-rigid rotor system. Moreover, Ahn et al. [11] improved the function of this conventional method by introducing a new type of the squeeze film damper. However, the quasi-static properties of the MR fluid in squeeze mode have not been investigated thoroughly yet. Therefore, in this study, a simple test equipment was constructed, which had controllable movements in a vertical direction to obtain compressive and tensile stresses under variable factors.
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2ED. MRF-241ES is a water-based MR fluid, which is developed for general use in sealed systems of controllable, energy-dissipating applications such as shocks, dampers and brakes. MRF-132DG and MRF-122-2ED are hydrocarbon-based MR fluids, which are formulated for general use in controllable, energy-dissipating
Force Magnetic Field Force Upper plate MR fluid Lower plate Compression
Tension
Fig. 1. Basic principle of the squeeze mode of MR fluids.
2. Experimental 2.1. Materials Three different types of commercially available MR fluids produced by Lord Corporation were chosen in order to characterize the compressive/tensile stress under various conditions. These materials were namely MRF-241ES, MRF-132DG and MRF-122-
2.0
Magnetic flux density (Tesla)
1.5 1.0 0.5 0.0 -800
-600
-400
-200
0
200
400
600
800
-0.5 Fig. 4. Sketch of the test equipment showing the black hatched lines in the middle half of the test equipment and red hatched line between the upper and lower cylinders.
-1.0 MRF-241ES MRF-132DG
-1.5
MRF-122-2ED
-2.0
Magnetic field intensity (kA/m) Fig. 2. Typical magnetic properties of the tested MR fluids.
5.0
12000
4.5 10000
4.0
Curve B
3.5
8000
3.0 2.5
6000
2.0 4000
Current (Amps)
No. of Turns
Curve A
1.5 1.0
2000
0.5 0
0.0 18
20
22
24
26
28
30
Wire Gauge (SWG) Fig. 3. Number of coil’s turns and current as functions of wire gauge.
Fig. 5. Simulated magnetic flux density distribution in the test equipment.
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applications such as shocks, dampers and brakes. The selection of the carrier liquid determines the temperature ranges in which the MR fluid can be utilized. Even though silicone oil is the most frequently used carrier liquid, hydrocarbon oil has some advantages due to its low viscosity, better lubrication properties and suitability for high shear-rate applications. Moreover, a hydrocarbon oil-based MR fluid has lower zero field viscosity, which is about 0.6 times less than the silicone oil-based MR fluid [12]. On the other hand, a water-based MR fluid can minimize waste disposal problems and allows the particles to be easily recycled from the material [13]. Fig. 2 shows the magnetic induction curves or B–H curves, of the three tested commercial MR fluid materials [14]. MR fluids exhibit approximately linear magnetic properties up to 200 kA/m of applied field intensity. The magnetic flux density of MRF-241ES demonstrates greater values over those of the other MR fluids.
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The magnetic properties of MR fluids vary significantly due to a higher particle density in MRF-241ES (3.80–3.92 g/cm3) than MRF-132DG (2.98–3.18 g/cm3) and MRF-122-2ED (2.32–2.44 g/ cm3). 2.2. Apparatus A design concept of the experimental set-up similar to that used for studying ER fluids by Tian et al. [15] was utilized in this study. In order to carry out experimental trials on the MR fluids, the external field had to be changed from an electric field to a magnetic field. Therefore, new test equipment needed to be designed to accommodate these differences. Basically, the test equipment consisted of two parts; an upper and a lower cylinder. A measured amount of the MR fluid was sandwiched in between the two cylinders, so that the upper cylinder could move towards or pulls
Fig. 6. Simulated magnetic flux density distribution in (a) air, (b) MRF-241ES, (c) MRF-132DG and (d) MRF-122-2ED.
Fig. 7. Simulated magnetic flux lines across the (a) air gap, (b) MRF-241ES, (c) MRF-132DG and (d) MRF-122-2ED.
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against the lower cylinder, and simultaneously compress or decompress the MR fluid. An axial–symmetrical model was selected in the Finite element method magnetics (FEMM) software package [16]. A number of design changes were simulated in the FEMM software package. Analysis and optimization of magnetic behaviours of different devices were carried out using FEMM by other researchers [17–19]. This software package is suitable for the type of materials of each component of the test equipment, type of coil, number of turns of the
coils wrapping around the core and values of the electric current that runs in the coils. These parameters were important to produce the best value for the magnetic field intensity H, which was correlated with the magnetic flux density B. In these changes, the number of turns of the coil, the initial gap size and the current value supplied to the coil were kept constant. For the geometry of the coil, 60, 92 and 100 mm were used for the dimensions of inner diameter, outer diameter and width of the coil, respectively. These dimensions were selected based on the
a
b Magnetic flux density (Tesla)
Magnetic flux density (Tesla)
0.70 0.65 0.60 0.55 0.50 0.45 MRF-241ES
0.40
MRF-132DG
0.35
MRF-122-2ED Air gap
0.01 0.00 -0.01 -0.02 MRF-241ES MRF-132DG MRF-122-2ED
-0.03
Air gap
-0.04
0.30 0
5
10
15
20
0
25
5
10
Distance from centre (mm)
15
20
25
Distance from centre (mm)
Fig. 8. Simulated magnetic flux density distribution inside the air gap and MR fluids from centre line to the edge between the upper and the lower cylinders. (a) Normal magnetic flux density (B.n) and (b) tangential magnetic flux density (B.t).
a
b 0.8
Magnetic flux density (Tesla)
0.7 0.6 0.5 0.4 0.3
MRF-241ES MRF-132DG MRF-122-2ED Air Gap
0.2 0.1 0.0 0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
0.7 0.6 0.5 0.4 0.3
MRF-241ES MRF-132DG MRF-122-2ED Air Gap
0.2 0.1 0.0 0.0
1.6
0.2
0.4
0.6
0.8
1.0
1.2
Current (Amps)
Current (Amps)
c 0.8
Magnetic flux density (Tesla)
Magnetic flux density (Tesla)
0.8
0.7 0.6 0.5 0.4 0.3
MRF-241ES MRF-132DG MRF-122-2ED Air Gap
0.2 0.1 0.0 0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
Current (Amps) Fig. 9. Simulated magnetic flux density versus applied current for initial gap sizes of (a) 2.0, (b) 1.2 and (c) 0.4 mm.
1.4
1.6
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J¼
Im ; Aw
ð1Þ
where J is the current density (Amps/mm2), Im is the maximum current supplied to the coil (Amps) and Aw is the surface area of the uncoated wire (mm2). For instance, 22SWG copper wire has a diameter of 0.71 mm, thus, the maximum current value allowed in this wire was 1.6 Amps. Furthermore, the wire can be wound around the specified core’s dimensions with 128 turns in each layer of the width of coil, 20 layers between inner and outer diameters of the coil, and gave the total number of 2560 turns. Fig. 3 shows the range of some parameters to be compromised in order to achieve the desired results. Curve A is a function of number of turns and wire gauge and curve B is a function of current and wire gauge. After the most adequate design of the test equipment was predicted by FEMM, some modifications have been made to the original concept of the test equipment because of the manufacturing processes and material limitations. In the final design, the test equipment consisted of seven parts as shown in Fig. 4. The materials of the test equipment could be divided into three categories; magnetic materials, non-magnetic materials and a coil. Some advantages of this test equipment included a small required volume of tested samples, the ability to vary the working gap height,
Magnetic flux density (Tesla)
a
1.0
I = 1.6 A I = 1.2 A I = 0.8 A I = 0.4 A
0.8
0.6
0.4
0.2
0.0 0.0
0.4
0.8
1.2
1.6
and the simplicity and ease of use. The upper cylinder, lower cylinder, lower ring and lower base were made from magnetic materials, while the upper ring and support cylinder were made from non-magnetic materials. A length of copper wire type 22SWG having a resistance of 29 X was chosen to be wound around the lower cylinder forming 2750 turns. The support cylinder was attached on the top of the lower cylinder and acted as a container so as to refrain the MR fluid during the testing. 2.3. Magnetic field distributions The main objective of the magnetic circuit was to produce the correct magnetic flux density across the MR fluid. From the electromagnetic point of view, the test equipment could be treated as a circuit concentrating the magnetic field generated by a coil and guiding it from the core to and across the fluid. An analysis of the magnetic field between the upper cylinder and the lower cylinder (in the gap or across the MR fluids) was performed using FEMM. The magnetic properties of the non-magnetic materials were assumed to be linear such as stainless steel, copper wire and air. The magnetic properties of the magnetic materials such as low carbon steel and MR fluid materials were assumed to follow the B–H curves given in the software package and provided by the manufacturer. Simulation results were based on the middle half of the test equipment (axial–symmetrical model), where was indicated by black hatched line, while the average values of the magnetic flux density were pointed out by the red hatched line as shown in Fig. 4. The simulated magnetic flux density distribution in the test equipment is shown in Fig. 5. The simulated magnetic flux density
b Magnetic flux density (Tesla)
various values of magnetic flux density that could be achieved by varying the applied current, and furthermore, ease of manufacture and assembly. These parameters limited the area in which the copper wire can be wound. Current density was limited to 4 Amm2 in connection with the appropriate wire diameter [20]. Therefore, the maximum current density is given by
1.0
0.8
0.6
0.4
0.2
0.0 0.0
2.0
I = 1.6 A I = 1.2 A I = 0.8 A I = 0.4 A
0.4
Initial gap size (mm)
1.0
I = 1.6 A I = 1.2 A I = 0.8 A I = 0.4 A
0.8
0.6
0.4
0.2
0.0 0.0
0.4
0.8
1.2
Initial gap size (mm)
1.6
2.0
d Magnetic flux density (Tesla)
Magnetic flux density (Tesla)
c
0.8
1.2
1.6
2.0
Initial gap size (mm)
1.0
I = 1.6 A I = 1.2 A I = 0.8 A I = 0.4 A
0.8
0.6
0.4
0.2
0.0 0.0
0.4
0.8
1.2
1.6
2.0
Initial gap size (mm)
Fig. 10. Simulated magnetic flux density versus initial gap size for different values of applied current for (a) Air, (b) MRF-241ES, (c) MRF-231DG and (d) MRF-122-2ED.
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distribution within the air gap and the MR fluids are shown in Fig. 6, whereas simulated magnetic flux lines across the air gap and MR fluids are shown in Fig. 7. Figs. 5–7 show the simulated results when the applied current to the coil was set to 1.6 Amps and the initial gap size was set to 2.0 mm. The magnetic flux density distribution in the air gap and MR fluid materials can be considered to be evenly distributed. Magnetic field results of the FEMM software package for the air gap in terms of the magnetic field intensity generated by the coil were validated using a DC magnetometer (gauss-meter) supplied by AlphaLab Inc. The magnetic field strength increased when the applied current increased. The values of the magnetic field strength obtained by direct measurement and simulation were in a good agreement as presented by Mazlan et al. in previous work [21]. Fig. 8a shows that the average values of the magnetic flux density at the middle line between the upper and lower cylinders starting from the centre of the test equipment to the inner diameter of the support cylinder were 0.47, 0.70, 0.67 and 0.61 Tesla for the air gap, MRF-241ES, MRF-132DG and MRF-122-2ED, respectively. As shown in Fig. 7, the magnetic flux lines seemed to penetrate the air gap and MR fluid materials and were aligned in straight lines with the direction of the magnetic field. However, after about 24 mm distance from the centre of the test equipment, the values of normal magnetic flux density tended to reduce, while the values of tangential magnetic flux density become higher as shown in Fig. 8a and b. The values of the tangential magnetic flux density contributed to the shear mode during the testing [22], which is not the interest of this study. Furthermore, the tangential values are very small (less than 0.04 Tesla in the negative direction) and can be neglected.
b
0.8
Magnetic flux density (Tesla)
Magnetic flux density (Tesla)
a
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0
Fig. 9 shows the simulated values of the average magnetic flux density as the applied current increases at different initial gap sizes starting from 2.0 mm down to 0.4 mm. The magnetic flux density increased with increasing the applied current. The average values of the magnetic flux density were taken from centre of the test equipment through the middle line between the upper and the lower cylinders for Figs. 9–12. MRF-241ES always showed the highest values of magnetic flux density at any initial gap size, followed by MRF-132DG, MRF-1222ED and air. As the initial gap size decreased, the curves variation became smaller in the air and the MR fluids. The curves tended to slowly closely match each other as the initial gap size was decreased. This indicated a small effect on the magnetic flux density. The simulated effects of the applied current on magnetic flux density when increasing the initial gap sizes are depicted in Fig. 10. As the initial gap size increased, the values of magnetic flux density decreased. When the initial gap size increased, MRF-241ES, in Fig. 10b, showed the least variation in magnetic flux density, while for the air case, in Fig. 10a, showed the largest variation of the magnetic flux density. Fig. 11 shows the simulated values of magnetic flux density as the initial gap size is increased at different applied currents starting from 1.6 Amps down to 0.4 Amps. The values of magnetic flux density decreased with increasing the initial gap size. The curves of the magnetic flux density versus initial gap size started at the same point at zero value of the initial gap size. But the initial magnetic flux density depended on the current supplied to the coil, for example 0.79, 0.61, 0.42 and 0.21 Tesla were initial magnetic flux values corresponding to 1.6, 1.2, 0.8 and 0.4 Amps, respectively before proceeding to decrease as the initial gap size increased.
MRF-241ES MRF-132DG MRF-122-2ED Air Gap 0.4
0.8
1.2
1.6
0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0
2.0
MRF-241ES MRF-132DG MRF-122-2ED Air Gap 0.4
Initial gap size (mm) 0.8
MRF-241ES MRF-132DG MRF-122-2ED Air Gap
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0
0.4
0.8
1.2
Initial gap size (mm)
1.6
2.0
d Magnetic flux density (Tesla)
Magnetic flux density (Tesla)
c
0.8
1.2
1.6
2.0
Initial gap size (mm) 0.8
MRF-241ES MRF-132DG MRF-122-2ED Air Gap
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0
0.4
0.8
1.2
1.6
2.0
Initial gap size (mm)
Fig. 11. Simulated magnetic flux density versus initial gap size for the applied currents of (a) 1.6, (b) 1.2, (c) 0.8 and (d) 0.4 Amps.
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Magnetic flux density (Tesla)
0.8 0.7 0.6
b
h = 0.4 mm h = 0.8 mm h = 1.2 mm h = 1.6 mm h = 2.0 mm
0.8
Magnetic flux density (Tesla)
a
0.5 0.4 0.3 0.2
0.6 0.5 0.4 0.3 0.2 0.1
0.1 0.0 0.0
0.7
h = 0.4 mm h = 0.8 mm h = 1.2 mm h = 1.6 mm h = 2.0 mm
0.2
0.4
0.6
0.8
1.0
1.2
1.4
0.0 0.0
1.6
0.2
0.4
Current (Amps)
Magnetic flux density (Tesla)
0.8 0.7 0.6
d
h = 0.4 mm h = 0.8 mm h = 1.2 mm h = 1.6 mm h = 2.0 mm
0.8
0.5 0.4 0.3 0.2
1.0
1.2
1.4
1.6
0.7 0.6
1.2
1.4
1.6
h = 0.4 mm h = 0.8 mm h = 1.2 mm h = 1.6 mm h = 2.0 mm
0.5 0.4 0.3 0.2 0.1
0.1 0.0 0.0
0.8
Current (Amps)
Magnetic flux density (Tesla)
c
0.6
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
Current (Amps)
0.0 0.0
0.2
0.4
0.6
0.8
1.0
Current (Amps)
Fig. 12. Simulated magnetic flux density versus current for different values of the initial gap sizes for (a) Air, (b) MRF-241ES, (c) MRF-231DG and (d) MRF-122-2ED.
Step
Step 0.4
1.0 0.8
1.2
Constant currents were maintained manually throughout the tests
2.0 1.6 Initial gap size (mm)
Current (Amps) Fig. 13. Combinations of the test parameters.
The simulated effects of initial gap size on magnetic flux density when increasing the applied currents are shown in Fig. 12. As the applied current increased, the values of magnetic flux density increased. When increasing the current values in the coil, MRF241ES in Fig. 12b, showed the least variation in the magnetic flux density, while air in the gap in Fig. 12a, showed the largest variation of the magnetic flux density. The effects of the applied current and initial gap size on the magnetic flux density were interdependent. In general, the values of the magnetic flux density turned into greater values with
increasing the value of applied current or decreasing the initial gap size. However, these values depended on the type of carrier liquid in the MR fluid. MRF-241ES showed the highest values of magnetic flux density as compared to the other MR fluids (MRF132DG and MRF-122-2ED). The reason behind this was simply, due to the relatively higher values of magnetic properties in MRF-241ES as shown in Fig. 2. 3. Experimental set-up A schematic diagram of the experimental set-up was disclosed in an earlier work [21]. The experimental set-up consisted of five main components, namely the Instron Machine, the test equipment, the MR fluids, the power supply and the control computer. The test equipment was placed in the testing area of the Instron Universal Electromechanical Testing (UTS) machine (model 4202). The communication with the machine was achieved via a built-in data acquisition board in the computer. The Instron machine was operated in the vertical direction to obtain displacements and forces under compression and tension modes. A measured amount of the MR fluid materials was filled in the gap between the upper and the lower cylinders. The current was supplied by the XantrexTM XFR 150 V, 18 Amps DC power supply. The magnetic field strength was adjusted by manually controlling the value of the current supplied to the electromagnetic coil. All experiments were carried out in a displacement control mode at a room temperature of 20 °C. In this study, for each type of MR fluid tested, three sets of parameters were prepared to perform the compression and the
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Fig. 14. Stress–strain curves relationship in compression mode under various parameters for (a) MRF-241ES, (b) MRF-132DG and (c) MRF-122-2ED.
tension tests. Various parameters including different initial gap sizes (step 1) and applied currents (step 2) were set before performing any tests, shown in Fig. 13. Two types of experiments were performed; namely compression and tension tests. The compression tests were carried out by lowering the upper cylinder towards the lower cylinder. While, the tension tests were carried out by pulling up the upper cylinder away from the lower cylinder. Both tests were done using a computer-controlled movement with a constant speed of 1.0 mm/min. The temperature in the gap be-
Fig. 15. Stress–strain curves relationship in tension mode under various parameters for (a) MRF-241ES, (b) MRF-132DG and (c) MRF-122-2ED.
tween the upper and the lower cylinders was measured using a Type K thermocouple, which was capable of operating over a very wide temperature range. The maximum temperature obtained in the test equipment when the current was supplied continuously for 20 min was 65 °C with the maximum voltage reached 54 V. Therefore, in every test, the power supply was withdrawn after running every test and the maximum voltage was limited to 54 V.
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Initially, one of the parameters in step 1 was set accurately to the specific gap distance between the two flat parallel surfaces. Then, different current values were supplied to the coil (0.4, 0.8, 1.2 and 1.6 Amps). The compression/tension processes began when the upper cylinder was either lowered or raised at a constant speed of 1.0 mm/min, and simultaneously the loads and displacements were recorded continuously. The maximum strain values of both compression and tension tests were set to 0.75. 4. Experimental results Fig. 14 illustrates the stress–strain curves of the MR fluids under compression at various applied currents and different initial gap sizes. At high values of applied current, the compressive stress increased with increasing in compressive strain. The stress–strain curve of MRF-241ES looked steeper at a large initial gap size indicating that the compressive stress increased as the initial gap size increased for the same value of compressive strain. However, the effect of applied current and initial gap size were difficult to illustrate for fluids with poorer solids-to-liquid ratio (MRF-132DG and MRF-122-2ED) at initial gap size of 1.0 mm, due to their low magnetic properties (Fig. 2). It was assumed that the same curves pattern of the effect of initial gap size of MRF-241ES could be achieved on MRF-132DG and MRF-122-2ED, if the maximum values of compressive strain were set to higher values. Eventually, under the same strain value, the compressive stresses were strongly affected by the current magnitude and the initial gap size [21–23]. Similar results were obtained in other external field dependent rheological fluids, where the employment of the ER fluids was carried out under the influence of electrical field. These observations were comparable to those reported by Tian et al. [15]. The tensile stress–strain curves of the MR fluids at a constant tensile speed of 1.0 mm/min, different applied currents and different initial gap sizes are shown in Fig. 15. The behaviour of the MR fluids in tension mode was discussed by Mazlan et al. [24]. As can be seen, higher values of the tensile stress were observed at lower initial gap sizes when a constant current was applied to the MR fluids. The effect of the initial gap size under tension tests contradicted with the same effect under compression tests in terms of the stress–strain relationships. In sight of this, one might have expected that a higher tensile stress would occur where the applied magnetic field strength was greatest. Therefore, higher values of tensile stresses were required when the initial gap size was set to 1.0 mm in comparison to the situation when the initial gap size of 2.0 mm has been set. In addition, under the same value of the tensile strain, the value of tensile stress was higher at higher values of applied currents. Lower values of tensile stress could be achieved as the applied current is reduced. Higher values of the tensile stress could be achieved with increases in the applied current. Similar results were also obtained by other researchers on the employment of ER fluids [25,26]. 5. Conclusion A test equipment was designed and built to carry out investigations on the behaviour of the MR fluids in squeeze mode. The test equipment was utilized to perform compression and tension tests on the MR fluids, which were placed between two parallel surfaces
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(upper and lower cylinders). A selection range of the coil size/design within the test equipment was obtained based on the given dimensions of the lower cylinder. The FEMM software package helped finalizing the coil selection in order to produce the best range of the magnetic field strength. Consequently, the best values of the magnetic field strength generated by the optimized coil design were used. As a result, the effects of varying the applied current and initial gap size on the stress–strain relationships were investigated. Comparable with other experiments, similar results were observed either with different types of MR fluids (under the influence of magnetic field) or with ER fluids (under the influence of electric field). References [1] Hagenbuchle M, Liu J. Chain formation and chain dynamics in a dilute magnetorheological fluid. Appl Opt 1997;36:7664–71. [2] Tao R. Super-strong magnetorheological fluids. J Phys: Condens Mat 2001;13:R979–99. [3] Carlson JD, Jolly MR. MR fluid, foam and elastomer devices. Mechatronics 2000;10:555–69. [4] Lukkarinen A, Kaski K. Computational studies of compressed and sheared electrorheological fluid. J Phys D: Appl Phys 1996;29:2729–32. [5] Monkman GJ. The electrorheological effect under compressive stress. J Phys D: Appl Phys 1995;28:588–93. [6] Tian Y, Meng Y, Mao H, Wen S. Electrorheological fluid under elongation, compression, and shearing. Phys Rev E 2002;65:031507. [7] Wang J, Meng G, Feng N, Hahn EJ. Dynamic performance and control of squeeze mode MR fluid damper-rotor system. Smart Mater Struct 2005;14:529–39. [8] Vieira SL, Ciocanel C, Kulkarni P, Agrawal A, Naganathan N. Behaviour of MR fluids in squeeze mode. Int J Vehicle Des 2003;33:36–49. [9] Forte P, Paternò M, Rustighi E. A magnetorheological fluid damper for rotor applications. Int J Rotating Mach 2004;10:175–82. [10] Wang J, Feng N, Meng G, Hahn EJ. Vibration control of rotor by squeeze film damper with magnetorheological fluid. J Intell Mater Syst Struct 2006;17:353–7. [11] Ahn YK, Ha J-Y, Yuk B-S. A new type controllable squeeze film damper using an electromagnet. J Vib Acoust 2004;126:380–3. [12] Shen C, Wen W, Yang S, Sheng P. Wetting-induced electrorheological effect. J Appl Phys 2006;99:106104. [13] Carlson JD, JonesGuion JC. Aqueous magnetorheological materials. US Patent 1997;5:5670077. [14] Lord Corporation.