A state-of-the-art review on magnetorheological elastomer devices

TLDR: In this paper, the authors present a state-of-the-art review on the recent progress of magnetorheological elastomer technology, with special emphasis on the research and development of MR elastomers and their applications.

Abstract: During the last few decades, magnetorheological (MR) elastomers have attracted a significant amount of attention for their enormous potential in engineering applications. Because they are a solid counterpart to MR fluids, MR elastomers exhibit a unique field-dependent material property when exposed to a magnetic field, and they overcome major issues faced in magnetorheological fluids, e.g. the deposition of iron particles, sealing problems and environmental contamination. Such advantages offer great potential for designing intelligent devices to be used in various engineering fields, especially in fields that involve vibration reduction and isolation. This paper presents a state of the art review on the recent progress of MR elastomer technology, with special emphasis on the research and development of MR elastomer devices and their applications. To keep the integrity of the knowledge, this review includes a brief introduction of MR elastomer materials and follows with a discussion of critical issues involved in designing magnetorheological elastomer devices, i.e. operation modes, coil placements and principle fundamentals. A comprehensive review has been presented on the research and development of MR elastomer devices, including vibration absorbers, vibration isolators, base isolators, sensing devices, and so on. A summary of the research on the modeling mechanical behavior for both the material and the devices is presented. Finally, the challenges and the potential facing magnetorheological elastomer technology are discussed, and suggestions have been made based on the authors’ knowledge and experience.

Figures

Figure 4. Three configurations for magnetic circuit design: The shadow areas indicate the field-sensitive materials. The active areas are embraced within solid line. Magnetic flux is indicated with dash line. The motion directions of the smart devices are also labeled with arrows.

Figure 8. MR elastomer TVA designed by Ginder et al [30] from Ford Research Laboratory

Figure 10. A compact MR elastomer ATVA designed by Deng and Gong [62]:1. cover; 2. guide rod; 3. linear bearing; 4. magnetic conductor; 5. shear plate;6.MREs; 7. base; 8. electromagnet; 9. mounting shell. (Acknowledgement: Original figure provided by corresponding author, Prof. X L Gong, from University of Science and Technology of China)

Figure 9. Adaptive tuned vibration absorber proposed by Deng et al [19] and its performance: (a) Sketch: 1. Oscillator; 2. MR elastomers; 3. Magnetic conductor; 4. Coils; (b) Photograph; (c) The transfer function versus frequency at various magnetic fields. (Acknowledgement: Original figures provided by corresponding author, Prof. X L Gong, from University of Science and Technology of China)

Figure 37. Bouc-Wen models for MR elastomer base isolators: (a) model proposed by Yang et al [133]; [b] model proposed by Behrooz et al [93].

Figure 22. Stiffness-variable differential mount proposed by Jeong et al [117]: (a) schematic geometry and (b) engineering drawing

Full text

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A state-of-the-art review on magnetorheological elastomer devices
Yancheng Li
1
, Jianchun Li
1
, Weihua Li
2
and Haiping Du
3
1. Centre for Built Infrastructure Research, School of Civil and Environmental Engineering, Faculty of Engineering and
Information Technology, University of Technology Sydney, NSW 2007, Australia
2. School of Mechanical, Materials and Mechatronic Engineering, University of Wollongong, Wollongong, NSW 2522, Australia
3.
School of Electrical, Computer and Telecommunications Engineering, University of Wollongong, NSW 2522, Australia
Abstract:
During last decades, magnetorheological (MR) elastomer has attracted significant amount of
attention for its enormous potentials in engineering applications. Being a solid counterpart of
MR fluids, MR elastomers exhibit unique field-dependent material property when being
exposed to magnetic field, in meanwhile overcoming major issues faced in
magnetorheological fluids, e.g. deposition of iron particles, sealing problem and
environmental contamination. Such advantages offer great potentials for designing intelligent
devices to be used in various engineering fields, especially for vibration reduction and
isolation. This paper presents a state-of-the-art review on the recent progress of MR
elastomer technology, with special emphasis on research and development of MR elastomer
devices and their applications. To keep the integrity of the knowledge, the review includes a
brief introduction of the MR elastomer materials and follows by discussion of critical issues
in designing magnetorheological elastomer devices, i.e. operation modes, coil placements and
principle fundamentals. A comprehensive review has been presented research and
development of MR elastomer devices, including vibration absorbers, vibration isolators,
base isolators, sensing devices and so on. Summary of research on modeling mechanical
behavior for both material and devices are presented. Finally, challenges and potentials facing
magnetorheological elastomer technology are discussed and suggestions have been made
based on authors’ knowledge and experience.
Keywords: magnetorheological elastomer, operation modes, vibration absorber, vibration
isolator, base isolator, sensing.
1 Introduction
Magnetorheological (MR) elastomer, belonging to MR material family, is a composite
material with magnetic-sensitive particles suspended or arranged within non-magnetic
elastomer matrix [1]. With presence of magnetic field, it exhibits MR effect providing field-
dependent material property to the material, e.g. controllable modulus and damping. The
material reclaims its original property after the magnetic field is removed. Physical status of
the material can be tuned between soft elastomer and semi-solid, depending on the external
magnetic field applied to the material [2]. Compared with another member in MR material
family - MR fluids, MR elastomers overcome the problems accompanying for applications of
MR fluids, such as deposition, sealing issue and environmental contamination, etc., which
makes the MR elastomer a favorable candidate for various vibration control application [3].
Although MR effect was firstly discovered by Rabinow in 1940s [4-5], it was not until 1985
the pioneering work on magnetically sensitive elastomer was firstly reported by Rigbi and
Jilken [6] with unfulfilled intention for the design of certain medical and measuring

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appliances. A decade later, Shiga et al [7] reported electroviscoelastic effect of a polymer
based on silicon elastomer and semiconducting particles under applied electric field. The first
comprehensive investigations on MR elastomer was conducted by Jolly and coworkers [8] in
1996. In their work, a quasi-static dipolar model was developed to explain the modulus
change inside the MR elastomer. Experimental testing was then conducted to examine the
material property of elastomer composite with ferrous particles embedded in a polymer
matrix under magnetic field. Since then, research on MR elastomer has attracted increasing
attention and become an emerging research topic.
MR elastomers consist of three basic components: polarized magnetic particles,
elastomer/rubber matrix and additives (usually silicon oil) [9-10]. The components are mixed
together to form a compound with large density magnetic particle randomly dispersed or pre-
arranged in low density matrix. The functionality of the material is based on the magnetic
interaction between magnetic particles [9]. Due to the existence of magnetically permeable
particles, the composite material exhibits field-dependent material property subjected to
external magnetic field where three-dimensional crosslink network is formed between
adjacent magnetic particles [11-12]. Such crosslink network tends to retain its original state
when external mechanical loading is applied and the tendency is proportional to the magnetic
field intensity.
Figure 1. Example of fabrication of both isotropic and anisotropic MR elastomers [17] (note: curing time may vary from case to case)
Generally, high permeability, low remnant magnetization and high saturation magnetization
are key features required for the magnetic particles [13]. High permeability of the particles is
to easily attract small magnetic leakage fields in the material compound and thus induce
maximum possible MR effect [9]. A low remnant magnetization is recommended because the
highly remnant particles will stick together after removal of magnetic field due to magnetic
residual [14], which therefore makes completely reversible MR effect impossible. The size of
the magnetic particles ranges from several micrometers to hundreds of micrometers [11, 14].
The conventional matrixes of MR elastomers are usually natural rubber or silicone rubber,
adjusted according to the needs. To prevent particle aggregation as happened to MR fluids as
well as to improve the compatibility with polymeric matrix, magnetic particles usually
undertake special treatment to remove the moisture from particle surface before curing
process [11], which makes the particles to be hydrophobic. Silicone oil is usually used as an
additive in MR elastomer material fabrication. When the molecules of silicone oil enter the

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matrix, the gaps between the matrix molecules are increased and the conglutination of
molecules is decreased. Apart from increasing the plasticity and fluidity of the matrix, the
additives can average the distribution of internal stress in the materials, which makes stable
material property for MR elastomer materials [12].
MR elastomers can be categorized into two groups, i.e. isotropic MR elastomers and
anisotropic MR elastomers (also termed aligned MR elastomers), which attribute to different
ways of curing processes. Firstly, three basic components, namely, silicon rubber, silicon oil
and iron particles, are mixed thoroughly into a homogenous mixture. Air bubbles inside the
mixture needs to be removed using special treatment, i.e. placed in a vacuum chamber [9, 15]
or heat treatment [16]. Such treatment maintains the high permeability and uniformity of the
material. Isotropic MR elastomer is cured without presence of magnetic field while
anisotropic MR elastomer is cured with action of magnetic field, shown in figure 1 [17]. The
curing process for anisotropic materials requires strong magnetic field, usually above 0.8 T
[16, 18-19], to form chained structures of magnetic particles in the matrix along the direction
of the magnetic field. A constant temperature (usually above 120 ) is also required to
maintain the flexibility of the magnetic particle for both isotropic and anisotropic materials
[16, 18] during curing. Note that some rubber matrix can be cured in room temperature [15,
20]. Prior the curing process, the magnetic particles in the rubber matrix have certain freedom
to move around. After curing, the magnetic particles are locked in the matrix [17] and only
extra work can force the particles moving away from their original positions. Following
curing process, the cured compound undertakes a further chemical process called as
vulcanization [16] or polymerization [9]. This is to modify the polymer by forming cross-
links between individual polymer chains and thus creating more durable material. The
vulcanizing processes vary from less than 1 hour [16, 18-19] to several days [20], relying on
the matrix properties. In general, soft rubber matrix needs more time to stabilize. The
vulcanization process makes the material less sticky and having superior mechanical property
for engineering applications. In some research, the vulcanization accompanies with
application of low magnetic field to further consolidate the chain structure of the magnetic
particle [21]. Figure 2 shows the scanning electron micrographs of a freeze-fractured surface
of isotropic and anisotropic MR elastomer with 30 %vol Fe [9]. As observed, Fe particles are
randomly dispersed in the isotropic MR elastomer while Fe particles are linked into chain
structures in anisotropic MR elastomer.
(a)
(b)
Figure 2. Scanning electron micrographs of a fracture surface of (a) an isotropic MRE with 30 vol% of Fe and (b) an anisotropic MRE with
30 vol% of Fe, magnification 1500×. The arrow points to the alignment direction. [22]

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The most distinctive change in material property of MR elastomer under magnetic field is its
modulus [9]. Yet in some studies it is stated that damping property of the material also
changes due to magnetic field, other studies concluded that magnetic field does not have any
remarkable effect on damping property [14, 20, 23- 28]. Majority of the damping may come
from the frictional sliding at the interfaces between the free rubber and the particles [29]. As
such, the property of MR elastomer is normally evaluated by the MR effect induced, which is
the ratio of modulus increase ∆G at measured magnetic field to the initial storage modulus
G0 at zero magnetic field.
In the pilot work by Ginder et al [30], it is suggested that MR elastomer can be used to
develop tuned vibration absorbers (TVAs). Since then, research on MR elastomer has
progressed to a new level where significant amount activities have been reported, including
material empirical modeling [18, 31-40], material development and property testing [2-3, 10-
12,14-15, 18, 21,41-60], new device design and characterization [61-84], and performance
evaluation [85-105] and aimed for applications in aeronautic engineering, mechanical
engineering, automobile engineering and civil engineering. In general, MR elastomer has
attracted broad attention with its unique property and its potential in active or semi-active
vibration control, such as for vibration or/and noise reduction and vibration isolation,
attributing to its inherent material properties, i.e. large modulus change [68], fast response
time (faster than MR fluids) [1-3], stability, compatibility to mechanical components,
reasonable low power requirement. In particularly, wide range of adjustable modulus offers
more effective means than semi-active damping associate with MR fluids devices to mitigate
vibration disturbances [20]. Creative designs of devices incorporating MR elastomer have
been reported for vibration absorbers [62-71], vibration isolators [16,72,88-89], adaptive base
isolators [20,78-79,93], vibration mount [76] and sandwich beam [82-84, 95-103], aimed for
various applications, i.e. vehicle seat vibration suspension, adaptive base isolation, and so on.
There have been efforts in reviewing the technological developments in MR elastomer
technology [17, 106], which mainly focused on material development rather than MR
elastomer device development due to the early stage of related developments when published.
To accommodate the fast growing demand and to identify the challenges in this research area
and to identify the challenges in the field, this paper attempts to provide a timely state-of-the-
art review on the MR elastomer technology with special focus on material preparation and
property, device design and analysis, experimental testing techniques, and nonlinear
performance modeling of the devices. The paper will review basic design and related issues
of the MR elastomer devices, summarize and comment on developments and applications of
creative design of MR elastomer devices to date and then discuss on challenges and
opportunities to be addressed for the future research in the area.
2 Basic design issues of MR elastomer devices
2.1 Operation modes
Understanding operation modes of MR elastomer material is essential step towards design of
devices. Operation modes of MR fluids have been discussed extensively and understood well
[1, 107-108]. They refer to combinations of deformations of material and chain structure
directions. For MR fluids, iron particles can move freely in the carrier oil and hence the

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direction of the chain structures aligns with direction of magnetic field. However, for MR
elastomers, iron particles are locked within the polymeric matrix and external excitations, i.e.
deformation and magnetic field, and only leads to local movement around original locations.
Therefore, the direction of the chain structures no longer coincides with magnetic field.
Besides, as soft elastic material with magneto-active property, MR elastomer deforms along
magnetic field [75]. Above properties make operation modes of MR elastomers quite
different from these of MR fluids.
Figure 3. Basic operation modes for MR elastomers: (a) shear mode; (b) squeeze/elongation mode; and (c) field-active mode;
Figure 3 illustrates three basic operation modes of MR elastomers: shear mode,
squeeze/elongation mode and field-active mode. Note that as solid material, pressure mode
(or flow mode) which works for MR fluids is not applicable for MR elastomer. However, in
field-active mode, MR elastomers can change their shapes, i.e. being stretched by magnetic
field. This property is called as magnetostriction [27,109]. For shear mode and
squeeze/elongation mode MR elastomers work similarly as MR fluids. Both isotropic and
anisotropic MR elastomers can be used for the three basic operation modes. Therefore,
different configurations can be found for devices with isotropic and anisotropic MR
elastomers, i.e. combinations of field directions and aligned chain directions. MR elastomer
working in field-active mode can be used to design various actuators [75, 77]. Examples of
shear mode devices include vibration absorbers [62-70], vibration isolators [16, 71, 89-90]
and base isolators [20, 78-79, 93]. Examples of squeeze/elongation mode devices are
vibration absorbers [64-65], engineering mounts [76] and compressive spring elements [22].
2.2 Magnetic circuit design
To provide a controllable magnetic field, electromagnetic coil or solenoid, is normally used
for MR fluid devices and MR elastomer devices. Altering the applied current can change the
magnetic induction through the field-sensitive materials. An effective magnetic circuit design
provides MR elastomer devices the optimal MR effect. Ideally, the magnetic field flux should
be perpendicular to the motion of the MR fluids and MR elastomers. In such way, the MR
effect can be fully utilized.
The most effective and efficient magnetic circuit design is the C-shape magnetic circuit since
it creates an enclosed field path for the magnetic flux and thus has minimum energy losses.
Example of C-shape magnetic circuits can be found in [16-17, 64]. To eliminate the energy
losses in the air as well as maintain uniform and strong magnetic field, the gap between two
poles of the C-shape magnetic circuit maintains as small as possible. Such requirement leads
H
field-active
shear
H
H
Squeeze/elongation
(a)
(b) (c)

Frequently asked questions

Q1. What have the authors contributed in "A state-of-the-art review on magnetorheological elastomer devices" ?

This paper presents a state-of-the-art review on the recent progress of MR elastomer technology, with special emphasis on research and development of MR elastomer devices and their applications. To keep the integrity of the knowledge, the review includes a brief introduction of the MR elastomer materials and follows by discussion of critical issues in designing magnetorheological elastomer devices, i. e. operation modes, coil placements and principle fundamentals. Finally, challenges and potentials facing magnetorheological elastomer technology are discussed and suggestions have been made based on authors ’ knowledge and experience. 

Q2. What is the typical design of MR elastomer?

MR elastomer is a typical nonlinear viscoelastic material with field-dependent mechanical properties, i.e. stiffness and damping. 

Q3. What is the main purpose of high permeability of the particles?

High permeability of the particles is to easily attract small magnetic leakage fields in the material compound and thus induce maximum possible MR effect [9]. 

Q4. What is the main advantage of additives in MR elastomers?

Apart from increasing the plasticity and fluidity of the matrix, the additives can average the distribution of internal stress in the materials, which makes stable material property for MR elastomer materials [12]. 

Q5. Why is the magnetic field in MR fluid devices considered only related to applied current?

Due to the independence of magnetic field with motion, magnetic field in MR fluid devices is considered only related to applied current during modeling process. 

Q6. What is the design of the MR elastomer mount?

It consists of eight parts: base, magnetic coils, magnetic conductor, shear plate, iron core MR elastomer, voice coil motor and mounting plate. 

Q7. What is the hysteresis property of MR elastomer?

With hysteresis property from the material medium, MR elastomer devices inevitably possess nonlinear hysteresis, evident by [20, 93,111]. 

Q8. How much increase in shear modulus does the MR elastomer exhibit when?

MR elastomer used in the new device exhibits an increase of more than 13 folds in shear modulus when subjected to a magnetic field of 0.44 T. 

Q9. Why is the active area of a MR fluid narrow?

The active area is normally very narrow in the way of flux line due to the low magnetic permeability of MR fluids and large force requirement of the device design [110]. 

Q10. What is the effect of external loading on the conductivity of MR elastomer?

Ginder et al [27] discovered that the electric constant and conductance of MR elastomer decrease when the material is subjected to external loading, i.e. shear or stretch. 

Q11. What is the main challenge on the potential applications of MR elastomer devices?

Another challenge on the potential applications of MR elastomer devices is to develop appropriate control strategies for the various applications to utilize their uniqueness and potentials. 

Q12. What is the main reason for the use of MR elastomer?

In the pilot work by Ginder et al [30], it is suggested that MR elastomer can be used to develop tuned vibration absorbers (TVAs). 

Q13. What could be done to improve the performance of MR elastomer devices?

If sufficient information was available, the design of compact MR elastomer devices with lower energy consumption could be achieved with help of numerical simulation in design. 

Q14. Why have very few modeling efforts been reported on MR elastomer devices?

Because of limited cycling characterization test of MR elastomer devices [20, 78-79, 93,111], very few modeling efforts have been reported till now. 

Q15. What is the reason for the drawback of MR elastomer devices?

The cause of such drawback of MR elastomer device is that usually thick MR elastomers with considerable size (consequently over-size devices) are placed in the devices to meet the performance requirements.