Compression and thermal conductivity characteristics of magnetorheological fluid

Abstract

There have been many reported applications of magnetorheological fluids but very few in combinations with the textile structures. Increasing the stiffness of magnetorheological fluid contained in a fabric by applying a magnetic field has considerable potential in engineering applications. In this work, we have used a spacer (three dimensional) fabric to contain the magnetorheological fluid and then investigated the changes in the stiffness value of the composite material by applying magnetic field using neodymium-iron-boron permanent magnets. Compression tests were performed at different temperatures with and without the application of a magnetic field. It was observed that the stiffness properties of the magnetorheological fluid–filled spacer fabric were increased significantly upon the application of the magnetic field at ambient temperature; however, a decrease in the stiffness value of the magnetorheological–spacer fabric composite was observed when the tests were carried out at higher temperatures. A considerable increase in the thermal conductivity of the magnetorheological fluid–containing spacer fabric was obtained under the influence of an applied magnetic field.

Keywords

magnetorheological fluid spacer fabric smart structures compression thermal conductivity

Introduction

Magnetorheological fluid (MRF) is a non-colloidal solution consisted of ferromagnetic particles (20–50 µm in diameter) dispersed in a non-conductive synthetic oil. Viscosity of MRF increases under the influence of an applied magnetic field, and the fluid exhibits a solid-like behaviour by offering higher resistance to the exerted stresses. These fluids are the magnetic equivalent of electrorheological fluids. Rheological properties of MRFs are changed within milliseconds by applying or removing an external magnetic field (Jolly et al., 1999; Nam and Park, 2009; Phule, 2001). Electromagnetic circuits for controlling MRFs have been designed that require relatively low voltages and exhibit fast response times (El Wahed et al., 2002; Yang et al., 2002). Since the discovery of MRFs by Rabinow (1948) and electrorheological fluids by Winslow (1949) in the 1940s, these materials have been used in numerous applications. In recent years, MRFs have received high attention for use in various systems such as shock absorbers, vibration insulators, brakes, clutches, machine mounts and prosthetic devices in mechanical and civil engineering (Carlson and Weiss, 1994; Gudmundsson et al., 2011). MRFs respond to an applied magnetic field by changing their rheological behaviour; thus, the yield stress of the material increases with the application of a magnetic field. MRFs provide simple, quiet and fast response between electronic controls and mechanical devices. The response of MRFs results from the polarization induced in the suspended particles by application of an external magnetic field. The interaction between the resulting induced dipoles causes the particles to form chain-like structures, parallel to the applied field (Facey et al., 2005; Jolly et al., 1999; Si et al., 2008).

An interesting use of MRF has been made in elastomeric matrix composites that were embedded with iron particles. A strong magnetic field was applied during the thermal curing of the elastomer in order to align the iron particles. The chains of iron particles were formed, and these were locked into place within the composite through the cross-linked network of the cured elastomer. The resistance of the composite to compressive deformation was enhanced by more than 60% in a magnetic field, and its magnetic field–induced modulus was an order of magnitude higher than its modulus in a zero magnetic field (Dagani, 1995; Harvey, 1998). Although there are numerous designs in MRF devices, however, using MRF with textile structures is quite a new concept, and only a few experimental studies have been reported on this aspect, but none have used spacer fabric as the MR carrier substrate. In a recent study, Glaser et al. (2011) have used MR liquids and carrier materials based on fabric and sponge structures for the development of composites with ‘rigidification’ characteristics. In order to improve mechanical performance of MR elastomers, Zhang et al. (2011) have reported the fabrication and evaluation of two novel hybrid MR elastomers that were embedded with MRFs and MR gels.

The work reported in this article relates to the development of smart composite materials based on MRFs and spacer fabrics. The spacer fabrics are regarded as environmentally friendly textile materials because unlike polyurethane foam, they can be recycled. Combination of MRF and spacer fabric is new in our study. Combination of textile structures with MRF is a new area for the MRF. These structures can also be defined as technical textiles or smart textiles. In this study, compression and heat conduction properties of the MRF-filled spacer fabrics were investigated with and without the application of a magnetic field. It was observed that the stiffness characteristics of the MRF-filled spacer fabrics were enhanced significantly under the influence of an applied magnetic field. Furthermore, higher stiffness values of the MR-spacer fabric composites (SFCs) were obtained at lower temperatures.

Experimental details

Materials

The spacer fabric(three-dimensional (3D) fabric) used in this study is a 3D knitted structure consisting of two outer textile substrates, which are joined together and kept apart by spacer yarns. These are warp knitted structures that are made of 100% polyester in the form of a strip of 14 cm × 4 cm × 0.5 cm dimension. The 3D spacer fabric structure (Figure 1) was used to contain the MRF. Commercial MRF-122EG MRF was obtained from Lord Corporation, UK. The properties of MRF and the spacer fabric are given in Tables 1 and 2, respectively. To generate magnetic field, neodymium-iron-boron (NdFeB) permanent magnets having a BH_max of 45 × 10⁶ GOe and dimensions of 2 cm × 2 cm × 0.25 cm were used. A constant magnetic field was used by maintaining a constant number of NdFeB magnets in all experiments involving the application of external magnetic field. In order to cool the specimen below room temperature, Peltier modules (62 mm × 62 mm × 5.8 mm) and dry ice were employed. The Peltier modules were purchased from European Thermodynamics Ltd, UK, and the dry ice was supplied by BOC, UK.

Table 1.

Properties of the commercial MRF.

Viscosity at 40°C (Pa s)	Density (g cm⁻³)	Solids content weight (%)	Flash point (°C)	Operating temperature (°C)
0.042 ± 0.020	2.28	72	>150	−40 to −130

MRF: magnetorheological fluid.

Table 2.

Basic properties of spacer fabric used.

Production method	Area density (g m⁻²)	Thickness (mm)	Count of outer yarn (dtex)	Count of spacer yarn (dtex)	Polymer type of outer yarn	Polymer type of spacer yarn
Warp knitted	95	5	400 multifilament	300 monofilament	100% polyester	100% polyester

Compression test set-up

Compression test was performed on the MRF-filled spacer fabric with and without the application of external magnetic field, using Instron 4303 (Instron Corporation, University of Bolton, UK) testing machine equipped with a plastic indenter and 100-N load cell (Figure 2). The tests were conducted at different temperatures in the range of −15°C to 70°C. In order to achieve the specimen temperatures above room temperature (30°C–70°C), tests were carried out in an environmental chamber attached to the Instron machine, and the temperature of the specimen was controlled by using thermocouples. Two Peltier modules were placed under the specimen to reach 5°C, and cooling fans were used to cool the reverse side of the Peltiers. Thermal compound paste was applied to the upper surface of the cooling fan to conduct the heat of the Peltiers to the cooling fan. In order to reach sub-zero temperature values (−15°C), dry ice was used, which was placed around the specimen during the compression test, and the temperature of the specimen was controlled using Raynger ST (Raytek Corporation, University of Bolton, UK) non-contact infrared thermometer. Compression force was applied to achieve specimen displacement values of up to 4 mm. The aim of choosing these temperatures is to determine the behaviour of MRF in a spacer fabric. And these temperatures are close to the application temperatures of MRF and textile structures.

Thermal conductivity tests

The objective of this part of the investigation was to determine the effect of the change in the magnetization and rigidity of the MRF-filled spacer fabric on its thermal conductivity. The spacer fabric was cut into a circular shape such that it fits exactly into heat source cup as shown in Figure 3. The spacer fabric was then coated with silicone paste and dried to close the fabric structure. Small gap was left in the spacer fabric surface in order to introduce the MRF. The fabric was then filled with the MRF, and the gap was sealed with the silicone and dried. The heat source was switched on and allowed to reach a temperature of 100°C, and once the temperature had stabilized, it was maintained at 100°C for 1 h. Heat source was then turned off, the sample was placed in the heat source cup, and a thermocouple was placed on the top of the sample. Sample was covered by 2-mm-thick insulating wool. Temperature was recorded at the heat source and on the top of the sample continuously using a software – five readings were taken every second. Figures 8 to 10 illustrate the pattern of temperature decrease for the heat source and the fabric samples with respect to time. Six NdFeB permanent magnets of each having a BH_max of 45 × 10⁶ GOe and dimensions of 2 cm × 2 cm × 0.25 cm were placed above the MRF-filled spacer fabric in order to apply a constant magnetic field to the specimens. The above procedure was repeated, and the temperatures of the heat source and MRF-filled spacer fabric were recorded as a function of time. Heat conductivity of the sample was calculated using equation (1) below

K = \frac{Q \cdot l}{A \cdot Δ T}

(1)

where ‘K’ is the thermal conductivity, ‘Q’ is the amount of heat transferred per second, ‘l’ is the thickness of the sample, and ‘A’ is the surface area of the sample. Q is substituted by product of heat capacity and the slope of the cooling curve (dT/dt).

Results and discussion

Spacer fabrics are 3D structures consisting of two outer layers and a connection interlayer, which joins the two outer layers. The 3D fabrics can be made by different processes, including weaving (woven spacer fabrics), circular knitting machines or by double needle bar warp knitting technology. These fabrics are used in many applications ranging from medical bandages to protective clothing. The 3D structure of spacer fabrics makes them suitable for many special applications. The compression-resistant characteristic of spacer fabrics is one of their main desirable characteristic. In this study, we have investigated the compression behaviour of MRF-filled spacer fabrics (warp knitted). The objectives of the work carried out are twofold; first to investigate the response of MR-SFCs to compressive forces and temperature under the influence of an applied magnetic field. Second, to determine the thermal conductivity characteristics of the MRF-filled spacer (3D) fabrics, both in the absence and presence of a magnetic field. The temperatures used for the compression test are not as high as to affect the polyester fibre. Polyester fibre is affected over 70°C, and melting temperature of polyester fibre is around 250°C–260°C.

Compression test

Spacer fabrics have characteristics that are different from the conventional textiles due to their special 3D structure, and therefore, characterization of these fabrics with existing methods is not appropriate (Mecit and Roy, 2009). Compression resistance of spacer fabrics provided by spacer yarns in the structure is one of their main features. But this has not been investigated in any detail. The testing method involves the measurement of forces that are required to compress test samples to a certain thickness. By using the plastic indenter and the load cell on the Instron machine, and using a constant speed for lowering the indenter onto the MR-SFCs specimen, the force needed to compress the composite to the predetermined thicknesses was measured.

Compression test results of the MR-SFCs are given in two sets. The results of the compression tests carried out at temperatures between 30°C and 70°C are given in Figures 4 and 5, and the results obtained at temperatures between −15°C and 30°C are given in Figures 6 and 7. Figure 4 shows the results of the compression tests carried out on the MRF-filled spacer composite in the absence of any magnetic field. These results show that the compression curves of all samples in the temperature range investigated exhibit a similar pattern of behaviour – an increase in the load applied was observed with increase in the displacement value. It was also observed that lower loads were required in order to achieve the same displacement (compression) values at the higher temperatures. This is obviously associated with the lower viscosity values of the MRF at higher temperatures, thus resulting in MR-SFC of lower rigidity. There may be some contribution from the softening of the polyester-connecting filaments in the spacer fabric, which has a glass transition temperature (T_g) of around 70°C. Furthermore, the load–displacement curves show a linear relationship, especially beyond displacement values of 1 mm. These results clearly show that the compressive properties of the MR-SFCs are highly temperature dependent, and at moderate temperatures, they show a controllable uniform compressive behaviour.

Figure 5 illustrates the response of MR-SFC to compressive force in the presence of externally applied magnetic field at temperatures in the range of 30°C–70°C. It is seen from Figure 5 that the compression load of the specimen was increased more than 50% when the tests were carried out under the influence of an applied magnetic field. Furthermore, shape of the load–displacement curve became non-linear at the lower temperatures with a significant increase in the compressive force at higher displacement values. The results evidently show that at temperature below 50°C, more than twofold increase in the load value was obtained at high displacement values.

The results presented in Figures 6 and 7 are for the compression tests carried out at subambient temperature values without and with an applied magnetic field, respectively. Figure 5 shows that at the lower temperatures, much higher compressive forces are required to achieve the equivalent displacement values corresponding to the above ambient temperatures without an applied magnetic field. These curves are in essence an extension of the results presented in Figure 4, which show that compression load increases as the temperature is lowered. This is mainly due to the increase in viscosity of the MRF at lower temperatures.

The results illustrated in Figure 7 show the effect of an applied magnetic field on the compression load of the MR-SFC structures. The highest compression load values were obtained at the lowest temperatures with an applied magnetic field. Compression load of the specimen was increased by nearly 350% at −15°C. However, most of the specimens reached load value of 100 N (load cell limit) at all temperatures, with the exception of room temperature test, before the displacement value of 4 mm could be achieved. The lowest compression load values were obtained at room temperature.

It can be seen from Figure 7 that the effectiveness of the MRF increased at lower temperatures as it shows higher stiffness properties at lower temperatures in combination with the spacer fabric. In the lower temperature range, under the influence of an applied magnetic field, load–displacement relationship for all specimens tested changed from near linear to exponential form, especially above displacement values of 3 mm. The results of our study on the MRF-filled spacer fabrics support the finding of other researchers who used different MRF containment media. Sahin et al. (2009) have reported that the yield stress of the MRF is decreased by increasing the temperature. This view is supported by Zhang et al. (2011) who studied MR elastomers that were composed of magnetizable iron particles and a soft rubber-like matrix. These workers have also reported that the mechanical properties, including modulus and damping capability, depend on both an externally applied magnetic field and the environmental temperature.

Thermal conductivity

Thermal conductivity of MRF-based systems is important for many applications, since this affects the performance of these systems under different environmental conditions. Although thermal conductivity of MRFs have been studied previously (Cha et al., 2010; Reinecke et al., 2008), there is very little published information about thermal conductivity of MR-liquid–textile-based composite materials. In this study, we have investigated the thermal conductivity of MRF-filled spacer fabrics in the absence and presence of a constant applied magnetic field as described in the experimental section. Cooling curves of the spacer fabric and MR-SFC sample with and without magnetic field are given in Figures 8 to 10, respectively. The calculated thermal conductivity values of the MR-SFC with and without applied magnetic field are given in Table 3. The results show that the encapsulation of the MRF has a considerable effect on the thermal conductivity of the spacer fabric. Nearly fivefold increase in the thermal conductivity of the spacer fabric was achieved with the addition of the MRF, even in the absence of an applied magnetic field.

Table 3.

Thermal conductivity of the MRF-filled spacer fabrics.

MRF-filled spacer fabric	Thermal conductivity (W m⁻¹ K⁻¹)
Without MR – silicone coated – no magnetic field	0.132
With MR – no magnetic field	0.548
With MR – with applied magnetic field	0.750

MRF: magnetorheological fluid.

The thermal conductivity of spacer fabrics is quite different from the normal homogeneous fabrics because of the different 3D characteristics of their structure. The apertures in the spacer fabric have a considerable influence on the thermal conductivity of the fabric. For a homogeneous non-woven fabric of specific fabric density, the thermal conductivity is independent of the pore size (Stark and Fricke, 1993). The heat transfer increases with an increase in aperture size in aperture fabrics (Gibson and Ashby, 1988) due to a significant increase in both radiation and convection effects. The existence of a large number of fibres aligned perpendicular to the fabric surface can also contribute to high thermal conductivity in spacer fabrics due to the thermal anisotropy of a single fibre (Kawabata, 1986). The inclusion of MRF in the spacer fabric structure further enhances the thermal conductivity of the material as a consequence of the high thermal conductivity of the metallic particles dispersed in the MRF.

The cooling curves and the thermal conductivity value for MR-SFC obtained in the presence of the applied magnetic field are presented in Figure 10 and Table 3, respectively. The results show that a significant increase in the thermal conductivity was observed when the test was performed under the influence of a magnetic field. This enhancement in the thermal conductivity of MR-SFC is mainly due to the formation of chain-like structures by the magnetizable particles in the MRF.

There is strong evidence that under a magnetic field, the magnetizable particles within MRFs self-assemble into columnar or chain-like structures(Climent et al., 2004; Fermigier and Gast, 1993; Ginder et al., 1999; Lim et al., 2005; Martin, 2000; Von Pfeil et al., 2003) due to magnetically enhanced interactions between particles. The columnar microstructure can impart anisotropic magnetic character as well as can alter the transport properties of the MRFs. Shulman et al. (1979) have studied the thermal conductivity of iron-based MRFs. These researchers found that the thermal conductivity component along the magnetic field direction can increase by 30%–50%. Our results show a similar increase in the thermal conductivity of the MR-SFC material in the presence of an applied magnetic field.

Conclusion

In this study, MR-SFCs were prepared and tested for their compressive behaviour and thermal conductivity characteristics. These properties were investigated with and without an applied magnetic field at different temperatures between −15°C and 70°C. Highest stiffness values for MR-SFC were obtained at −15°C under the influence of an applied magnetic field. Stiffness properties of the MRF-filled spacer fabric significantly increased under the influence of magnetic field and decreased with an increase in the temperature. Thermal conductivity of the MR-SFC structures increased upon the application of magnetic field as a result of the increase in the viscosity of the MRF due the formation of chain-like structures in the MRF. The results show that the MR-SFC properties are highly temperature dependent. These changes in MR-SFC characteristics indicate that the compressibility of these structures is proportional to the temperature, but the viscosity of the MRF is inversely related to the MRF temperature. This investigation shows that it is possible to develop MRF-filled spacer fabric structures, which are suitable for a wide range of energy-dissipating applications, where these materials can provide instantaneous and controllable yield strength and reversible rigidity. By altering the strength of the applied magnetic field, the performance of MR-SFCs can be controlled precisely and proportionally, as these materials can provide high yield strength in the presence of a magnetic field and low yield strength in the absence of a magnetic field. These MR-SFCs can also be used as sensors and actuators in smart and intelligent devices and systems.

Footnotes

Funding

This research received no specific grant from any funding agency in the public, commercial or not-for-profit sectors.

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Compression and thermal conductivity characteristics of magnetorheological fluid–spacer fabric smart structures

Abstract

Keywords

Introduction

Experimental details

Materials

Compression test set-up

Thermal conductivity tests

Results and discussion

Compression test

Thermal conductivity

Conclusion

Footnotes

Funding

References