The influence of solid-phase concentration on the performance of electrorheological fluids in dynamic squeeze flow

Materials and Design 32 (2011) 1420–1426

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The influence of solid-phase concentration on the performance of electrorheological fluids in dynamic squeeze flow A.K. El Wahed ⇑ Department of Mechanical Engineering & Mechatronics, University of Dundee, Dundee DD1 4HN, UK

a r t i c l e

i n f o

Article history: Received 11 June 2010 Accepted 2 September 2010 Available online 7 September 2010 Keywords: A. Electrorheological fluids C. Squeeze flow mode G. Particle concentration

a b s t r a c t The yield stress of electrorheological (ER) fluids increases by orders of magnitude when electric field is applied across them. In the absence of the field, ER fluids behave as Newtonian fluids. This paper is concerned with an experimental investigation to determine the rheological performance of ER fluids, consisting of a dielectric liquid carrier with a range of solid-phase concentration. The ER fluid was contained in a squeeze cell, which during motion subjects the fluid to both compressive and tensile loading. The results were analysed in terms of the capacity for the transmission of imposed forces across the fluid and showed a great dependence on the applied D.C voltage and the weight fraction of the dispersed solid-phase. In addition, the implications of the results to vibration control, where the ER fluid is employed in an engine mount, are discussed. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction Winslow [1] was credited with the first observation of the ER effect after his investigations into the application of electrical fields across oil dispersions of certain powders such as silica, and he was also the first to suggest industrial applications based on this phenomena. Since these early days and in particular during the last two decades, there has been much activity directed towards the design and development of ER devices. One such promising application is the variable damper for use in vibration control [2]. The first practical application of ER fluids in vibration control was reported in 1978 [3] in which a valve-operated vibration damper was described. Recently, the area of vibration control has seen a surge in the number of reported investigations involving ER fluids particularly in engine mounts [4,5], primary shock absorbers [6,7] and rotor support systems [8,9] in addition to adaptive structures [10,11]. The majority of ER fluid damping devices have employed the fluid in either flow or shear mode of operation in which the fluid is deformed in a direction orthogonal to the chains, and where the gap between the electrodes remains constant. An alternative arrangement, called squeeze mode, in which the fluid is subjected to oscillatory compression and subsequent tensile stresses (resulting in a variable fluid layer thickness) has been identified and investigated [12]. It was quoted that tensile/compressive forces are greater than those available in shear typically by a factor of ⇑ Tel.: +44 (0) 1382 384496; fax: +44 (0) 1382 385508. E-mail address: [email protected] 0261-3069/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2010.09.003

ten [13]. This increased fluid strength prompted systematic studies by the author and others to exploit the mechanical and electrical properties of ER fluids in squeeze [14–16]. However, a survey paper [17] provides a more substantial overview of the classification of the modes of operation of ER fluids and their potential applications in vibration control. In parallel with device investigation, during the last two decades, numerous studies have been carried out aimed at improving the mechanical and electrical characteristics of ER fluids. For example, the influence of the shape and configuration of the particulates [18,19] and their electrical properties [20,21] in addition to the solid-phase size [22,23] on the magnitude of the ER effect were assessed. Also, the effect of the dielectric properties of the base oil [24] on the performance of ER fluids was investigated. In order to obtain a clearer understanding of the mechanisms, which control ER fluid strength, it is necessary to understand the role of solid-phase concentration. This paper is concerned with the assessment of ER fluids in dynamic squeeze flow and the influence of the solid-phase weight fraction on their rheological performance. The relevance of this work in the application to vibration control in a short-stroke damper is discussed.

2. Experimental arrangement 2.1. Experimental facility The experimental rig (Fig. 1) consists of a Ling Dynamic Systems electromagnetic shaker (Model No. V450), which is capable of

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Fig. 1. Experimental arrangement.

providing vertical oscillatory motion with maximum amplitude of 19 mm (peak–peak) over a frequency in the range D.C to 7.5 kHz. The shaker head is attached rigidly to a Kistler (Model No. 9311A) piezoelectric force link and an earthed brass electrode having a recessed cylindrical cavity of diameter 95 mm which provides the reservoir for the ER fluid. The high voltage upper electrode is a circular brass disc of diameter 56 mm, its circumferential edge and rear face surrounded and supported by a PTFE collar. This is rigidly attached to a second identical force link and positioning assembly to the supporting frame. The instantaneous displacement, velocity and acceleration of the lower electrode are determined using an RDP (Type GTX 2500) LVDT, an RDP (Type 240A0500) self-induced velocity sensor and an Endevco (Type 7254-100) accelerometer, respectively, all of which are attached to the upper surface of the lower electrode. Electrical excitation of the ER fluid is achieved by means of a Trek (Model 664) high voltage amplifier, driven by a Thander (Model TG102) function generator. Data acquisition and processing was achieved using a Measurement Group (Type ESAM) analogue to digital converter that is controlled by a Pascal program running on an IBM compatible personal computer. Feedback control was not imposed in these tests but could be achieved, if required, using a Ling Dynamic Systems (Model DSC4) digital sine controller employing as input the signal from the accelerometer. In order that meaningful comparisons could be made between the results of the various tests, the ER fluid temperature was controlled by re-circulating water through a second closed cavity in the lower electrode using a Grant Instruments (Model LTD6) temperature controller.

2.2. Calibration of sensors The two force links were calibrated statically by sequential loading using small weights, while the LVDT was calibrated using a dial gauge. The accelerometer, pre-calibrated by the manufacturer, was found to function as specified to within ±0.6% when its maximum transverse sensitivity was checked using the digital sine controller. As an additional check, the displacement signal from the LVDT was differentiated twice using central differences and the resulting signal was found to compare well with that from the accelerometer. Finally the data acquisition system was checked against a DC signal supplied by a millivolt calibration unit (Time Electronics Ltd., Model 404S) and was found to be accurate to within ±0.5%.

2.3. ER fluid The ER fluid used in this investigation is a suspension of agglomerated calcium alumina silicate in silicone oil. The solidphase was supplied with an average diameter of 90 lm and was subsequently ground and sieved, using an Endecots Ltd., laboratory test sieve system in conjunction with a Fritsch analysette (Type 03502) mechanical vibrator, to produce particulates in the range 10–28 lm. The kinematic viscosity of the oil at 20 °C was 20 cSt and the weight fraction of the solid-phase was varied between 57% and 97%. The electrical conductivity of the solid-phase was measured in a dedicated cell and found to be 3.36  10 10 S/m.

3. Results and discussion An extensive testing programme was conducted in order to assess the influence of solid-phase weight fraction on the performance of ER fluids, which are employed in a squeeze cell simulating a short-stroke damper, under a range of electrical and mechanical input conditions. The tests carried out consisted of the simultaneous measurement of the input force delivered by the shaker, the transmitted force across the fluid, the displacement, velocity and acceleration of the lower electrode together with the applied voltage and the current passing through the fluid. These measurements were collected at a sampling frequency of 5 kHz for a set of mechanical frequencies in the range 2–20 Hz. The input displacement amplitude of the lower electrode, for the electrically unstressed fluid, was chosen as 1.27 mm (peak to peak) at the resonant frequency of the system. The mean separation of the electrodes was set at 2.0 mm. In this investigation, silicone-oil based ER fluids with various amounts of calcium alumina silicate particulates, namely 57%, 77% and 97% by weight, were tested under D.C excitation in the range 0–7 kV. The temperature of the ER fluids was maintained at 30 °C throughout the tests. It can be seen in Fig. 2 that, for a frequency of 20 Hz and zero applied voltage, the input displacement is almost independent of the solid-phase weight fraction. When the voltage is increased to 3 kV, Fig. 3, the fluid with 97% solid-phase weight fraction appears to provide the largest transmitted force since it was capable of producing a transmitted force of about 27 N peak to peak which was about 1.35 and 2 times greater than that developed by fluids with 77% and 57% solid-phase weight fractions, respectively. Moreover, this fluid was seen to provide the largest suppression in vibration,

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0.6

Displacement (mm)

0.2

0 -0.2 -0.4 -0.6

20 Hz, 3.0 kV 57%

0.25

77% 97%

0.4

Displacement (mm)

0.35

20 Hz, 0.0 kV 57%

77% 97%

0.15 0.05 -0.05 -0.15 -0.25

0

0.0125 0.025 0.0375

0.05

0.0625 0.075 0.0875

-0.35

0.1

0

0.0125 0.025 0.0375

1.4

18

0.1

57% V = 0.0 kV

20 Hz, 3.0 kV 57%

V = 0.5 kV

Input Displacement P-P (mm)

1.2

77% 97%

12

Transmitted Force (N)

0.0625 0.075 0.0875

Fig. 4. Variation of input displacement with time.

Fig. 2. Variation of input displacement with time.

6 0 -6 -12 -18

0.05

Time (sec)

Time (sec)

V = 1.0 kV V = 2.0 kV V = 3.0 kV

1

V = 4.0 kV V = 5.0 kV V = 6.0 kV V = 7.0 kV

0.8 0.6

`

0.4 0.2

0

0.0125 0.025 0.0375

0.05

0.0625 0.075 0.0875

0

0.1

2

4

6

8

Time (sec)

12

14

16

18

20

22

24

Frequency (Hz)

Fig. 3. Variation of transmitted force with time.

Fig. 5. Input displacement versus frequency.

57% V=0.0 kV

1.2

V=0.5 kV V=1.0 kV V=2.0 kV

1

Force Transmissibility

Fig. 4, reducing the input displacement to about 0.29 mm peak to peak in comparison with 0.38 mm and 0.52 mm produced by fluids with 77% and 57% solid-phase weight fractions, respectively. The input displacement (peak to peak) of the lower electrode and its dependence on the mechanical frequency is shown in Fig. 5 for ER fluid with 57% solid-phase weight fraction and for nine applied voltages between 0 and 7.0 kV. The displacement characteristics for the case of zero voltage essentially describe the frequency response of the shaker. As the voltage is increased the displacement is reduced. It can be seen that the input displacement was eventually reduced to a small value, causing the shaker to be almost arrested, when the fluid was excited by 7.0 kV. Fig. 6 shows the force transmissibility, defined as the ratio of transmitted force to input force plotted against frequency for the fluid with the same solid-phase weight fraction and the applied voltage range. The force transmissibility increases with the applied voltage and tends asymptotically to a value close to unity, at 7.0 kV, exhibiting the solid-body characteristics of the fluid. The performance of ER fluid with a 77% solid-phase weight fraction is summarised in Figs. 7 and 8 in terms of the input displacement and force transmissibility, respectively. The same trend can

10

V=3.0 kV V=4.0 kV V=5.0 kV V=6.0 kV

0.8

V=7.0 kV

0.6

0.4

0.2

0

2

4

6

8

10

12

14

16

18

Frequency (Hz) Fig. 6. Force transmissibility versus frequency.

20

22

24

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1.4

77%

1.2

V = 0.0 kV V = 0.5 kV V = 1.0 kV

1

V = 2.0 kV V = 3.0 kV

1

Force Transmissibility

Input Displacement P-P (mm)

1.2

V = 4.0 kV V = 5.0 kV

0.8 0.6 0.4

0.8 0.6 97%

0.4

V = 0.0 kV V = 0.5 kV

0.2

0.2

0

0

V = 1.0 kV V = 2.0 kV V = 3.0 kV V = 4.0 kV

2

4

6

8

10

12

14

16

18

20

22

24

2

4

6

8

1.2

Force Transmissibility

1 0.8 0.6

0

77 % V=0.0 kV V=0.5 kV V=1.0 kV V=2.0 kV V=3.0 kV V=4.0 kV V=5.0 kV

2

4

6

8

10

12

14

16

14

16

18

20

22

24

Fig. 10. Force transmissibility versus frequency.

Fig. 7. Input displacement versus frequency.

0.2

12

Frequency (Hz)

Frequency (Hz)

0.4

10

18

20

22

24

Frequency (Hz) Fig. 8. Force transmissibility versus frequency.

be seen in the behaviour of this fluid but the solid-body characteristics was achieved at less voltage (5.0 kV) suggesting that the added amount of particles created more chains between the electrodes and hence enhancing the yield strength of the fluid. Similarly, the transition from a fluid-like behaviour to a solid-like behaviour of ER fluid was found to occur at lower electrical fields with higher solid-phase concentration [25]. When the solid-phase weight fraction was increased to 97%, better fluid characteristics were exhibited, Figs. 9 and 10, at less applied voltages, since force transmissibilities close to unity value were achieved under a D.C voltage of 4.0 kV. This increased fluid strength with increasing solid-phase concentration was attributed to the increasing number of particle chains as well as to an enhancement effect of chains grouped in bundles [26]. Furthermore, this increased fluid performance was found to be caused by an increase in the polarisation forces among the increased number of particles [27]. Figs. 11 and 12 show a comparison between the performance of ER fluids with the three mentioned solid-phase weight fractions, which is shown only for four applied voltages, namely 1, 2, 3 and 4 kV, for clarity. It can be seen, Fig. 11, that the input displacement

1.4 97%

1.4

V = 0.0 kV V = 1.0 kV V = 2.0 kV

1

V = 3.0 kV V = 4.0 kV

0.8 0.6 0.4 0.2 0

57% 77% 97% 1.0kV 2.0kV 3.0kV 4.0kV

V = 0.5 kV

2

4

6

8

10

12

14

16

18

Frequency (Hz) Fig. 9. Input displacement versus frequency.

20

22

24

Input Displacement (P-P) (mm)

Input Displacement P-P (mm)

1.2

1.2 1 0.8 0.6 0.4 0.2 0

2

4

6

8

10

12

14

16

18

Frequency (Hz) Fig. 11. Input displacement versus frequency.

20

22

24

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1.2

20 97%, 20 Hz

15

Transmitted Force (N)

Force Transmissibility

1

0.8

0.6

0.4 57% 77% 97% 1.0kV 2.0kV 3.0kV 4.0kV

0.2

0

2

4

6

10 5

0.0kV 0.5kV 1.0kV 2.0kV 3.0kV 4.0kV

0 -5 -10 -15 -20 -3

8

10

12

14

16

18

20

22

-2

-1

0

1

2

3

Velocity (cm/s)

24

Fig. 14. Transmitted force versus velocity.

Frequency (Hz) Fig. 12. Force transmissibility versus frequency.

was reduced by a factor of 5.2 about the resonant frequency when a 4 kV was applied across the ER fluid with 97% solid-phase weight fraction. However, the reduction factors were only 3.4 and 1.5 in the cases involving fluids with 77% and 57% solid-phase weight fractions, respectively. The same effect is shown in Fig. 12 when force transmissibilities with values very close to the value of unity were produced, under 4 kV excitation, by the fluid with 97% solidphase weight fraction, whilst the fluid with 57% particle weight fraction produced the least damping characteristics under the same level of excitation. The variations of the transmitted force with displacement and velocity, for a mechanical frequency of 20 Hz and applied voltages between 0 and 4 kV, are shown in Figs. 13 and 14, respectively for the fluid with 97% solid-phase weight fraction. The whirl orbits, Fig. 13, are plotted over about one cycle and illustrate the change in the level of the transmitted force, due to the development of higher yield stresses, with increasing field strength in a rather more dramatic fashion than do the previous figures. The force versus velocity hysteresis cycles, Fig. 14, seem to reach asymptotic values which when projected back to the force axis may result into

the yield force values [28]. The yield forces are also seen to increase with increasing applied voltages. This response corresponds to the behaviour of Bingham Plastics that combine the yield properties of solids with the Newtonian flow properties of fluids. Yield stresses were also found to increase with the solid-phase volume fraction that may have a maximum at which the ER effect is expected to peak [27]. Figs. 15 and 16 show the variations of the transmitted force with displacement and velocity, respectively for the fluids with the three solid-phase weight fractions and for a mechanical frequency of 20 Hz and an applied voltage of 3 kV. The force–displacement hysteresis cycles, Fig. 15, show clearly the effect of increasing the solid-phase weight fraction of the ER fluid as larger forces are transmitted causing less vibration amplitudes. Fig. 16 shows steeper gradient of the force–velocity curves as the solid-phase weight fraction increases, emphasising the more distant departure of the electrically stressed fluid from the Newtonian behaviour [16]. This directly implies that the energy dissipation is increased with increasing solid-phase concentration [29]. Of interest also is the effect of the solid-phase weight fraction on the basic rheological characteristics of the fluids when stressed

15

20

57%

10

Transmitted Force (N)

Transmitted Force (N)

15 10 5 0 -5

97%, 20 Hz 0.0kV

-10 -15

3 kV, 20 Hz 77% 97%

5

0

-5

0.5kV 1.0kV

-10

2.0kV 3.0kV 4.0kV

-20 -0.5

-0.3

-0.1

0.1

0.3

Input Displacement (mm) Fig. 13. Transmitted force versus displacement.

0.5

-15 -0.6

-0.4

-0.2

0

0.2

Input Displacement (mm) Fig. 15. Transmitted force versus displacement.

0.4

0.6

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15

7000

3 kV, 20 Hz 57% 97%

Normal Stress (N/m2)

Transmitted Force (N)

6000

77%

10

5

0

-5

5000 4000 3000 2000 3 kV, 20 Hz 57% 77% 97%

1000 0

-10

0

100

200

300

400

500

600

700

800

Strain Rate (1/sec) -15

-2

-1.5

-1

-0.5

0

0.5

1

1.5

Fig. 18. Normal stress versus strain rate (3 kV).

2

Velocity (cm/s) 9000

Fig. 16. Transmitted force versus velocity.

4500

Normal Stress (N/m2)

4000 3500 3000 2500 2000 1500 1000

2 kV, 20 Hz 57% 77% 97%

500 0

0

100

200

300

400

500

600

Strain Rate (1/sec) Fig. 17. Normal stress versus strain rate (2 kV).

700

800

Normal Stress (N/m2)

8000

electrically, particularly for relatively small vibration frequencies. For this reason, the experimental data, for the tests involving a vibration frequency of 20 Hz and for applied voltages between 2 and 4 kV, were converted into the more conventional rheological parameters of dynamic normal stress, determined as the ratio of instantaneous transmitted force to electrode common area and instantaneous strain rate, defined as the gradient of the normal velocity of the lower electrode with respect to displacement [22]. The dependence of the normal stress on strain rate for 2 kV, Fig. 17, exhibits a gradually increasing level of stress with rate asymptoting to maximum values of normal stress of about 2.03, 3.06 and 3.9 kPa for fluids with 57%, 77% and 97% solid-phase weight fractions, respectively. When the applied voltage was raised to 3 kV, Fig. 18, it is apparent that the maximum stresses are now about 1.44, 1.55 and 1.61 times greater than those developed by fluids, with the above weight fractions, respectively. When the applied voltage is further raised to 4 kV, Fig. 19, it appears that whilst the maximum stresses for fluids with 57% and 77% solidphase weight fractions have almost the same rate of increase, the stress for the fluid with 97% solid-phase weight fraction increases at a slower rate. This is ascribed to the fact that the latter fluid exhibits solid-body characteristics almost around this excitation level (see Fig. 10) whilst 5 kV and 7 kV are required for such char-

7000 6000 5000 4000 3000 2000

4 kV, 20 Hz 57% 77% 97%

1000 0

0

100

200

300

400

500

600

700

800

Strain Rate (1/sec) Fig. 19. Normal stress versus strain rate (4 kV).

acteristics to be exhibited by the former fluids, respectively (Figs. 6 and 8). 4. Conclusions From the results of this experimental investigation into the influence of solid-phase weight fraction on the effectiveness of ER fluids in squeeze flow, it appears that for the range considered here, the fluid with the largest solid-weight fraction (97%) is generally the most effective in terms of the level of transmitted force. The significance of the work in relation to the application of ER fluids to vibration control lies in the requirement of an effective solidphase the amount of which would be chosen to suit the level of the input displacement. In the control of short stroke vibrations such as those associated with automotive engine mount applications, the results obtained in this paper would provide sufficient information on the solid-phase weight fraction to be employed. However, the increased concentration of the solid-phase may cause problems such as viscous heating when the fluid is utilised for example in torque transmission devices. Therefore, more work would be required to determine the effect of the solid-phase weight fraction on ER torque transmission devices. This is to be the subject of a future study. In general, this paper highlights the requirement for more fundamental level research on smart fluids, which would be advantageous for a host of future industrial applications.

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