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Compression properties of magnetostrictive polymer composite gels
Polymer Testing 24 (2005) 163–168 www.elsevier.com/locate/polytest
Material Properties
Compression properties of magnetostrictive polymer composite gels M. Farshad*, M. Le Roux EMPA, Materials Science and Technology, Polymers/Composites Laboratory, Ueberlandstrasse 129, 8600 Du¨bendorf, Switzerland Received 6 August 2004; accepted 15 September 2004
Abstract In this research work, compression behavior of magnetostrictive silicone gels under the influence of an external magnetic field was investigated. Magnetostrictive gel specimens were made of silicon gel with 80% weight fraction of 3.8 mm embedded carbonyl iron particles. Compression tests were performed on cylindrical samples of these materials in the absence and in the presence of a magnetic field. It was found that the presence of a magnetic field would increase both the stiffness and the loadbearing capacity, but would reduce the maximum strain of the material. At 30% strain, a 50% increase in maximum stress in the presence of 0.32 T and 100% in stress in the presence of 0.44 T magnetic field inductions were observed. The induced magnetic force on the samples was also measured. The induced force decreased nonlinearly with the distance of the electromagnet from the sample and increased nonlinearly with the magnetic induction. This research was oriented towards certain applications. These types of material characterization may provide useful information for potential applications of soft magnetostrictive elastomers in muscle type actuators and tunable stiffness elements. q 2004 Elsevier Ltd. All rights reserved. Keywords: Compression properties; Magnetostrictive gels
1. Introduction Magnetically active soft materials are polymer-based elastomeric materials which react to an external magnetic field and undergo deformation or experience mechanical stresses. These materials may be produced by chemical synthesis or through physical combination. In the latter case, magnetic components are added to the polymeric material to result in a magneto active polymer composite. Terminologies such as magnetostrictive, magnetorheological, and magneto active materials have been used as synonyms in various literatures. Magnetostrictive polymer gels are
* Corresponding author. Tel.: C41 1 823 4491; fax: C41 1 821 6244. E-mail address: [email protected] (M. Farshad). 0142-9418/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2004.09.007
a sub-class of soft elastomeric materials containing magnetizable components which have been embedded in the gel during the cross-linking process [1]. If metallic powders are used, they can be aligned. As the result of this process, a material with anisotropic mechanical and magnetic properties is produced. The relatively weak cross-linking of polymer gels makes them attractive as large deformation actuators. The material properties and potential applications of magnetic field sensitive gels have been investigated by a number of researchers. Magneto sensitive gels demonstrate actuator properties, which can be used in robotic applications as artificial muscles. The magnetoviscoelastic behavior of composite gels made of iron particles and a silicone matrix was investigated by Shiga et al. [2]. Those gels were cured for 5 h at 70 8C under an electromagnetic field of 27 kA/m. It was found that in applied field strength of
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20–100 kA/m, both the shear storage and the shear loss modules were linear with the applied field strength. An et al. [3,4] explored the production methods and the use of ordered iron filled soft silicon gels as shear actuators. The gel was cured for 24 h with a magnetic induction of more than 1 T for the first 2 h. Then, the shear storage modulus under variable applied field up to 1200 kA/m was measured. The goal of investigation was to find the field dependence of dynamic shear modulus. It was found that addition of 1% volume particles would double the shear modulus in an electric field of 2 kV mmK1. A comparative study of electric and magnetic field sensitive gels was carried out by Zrinyi et al. [5–7]. In this study, a potential application of magnetic field sensitive gels as artificial muscles was experimentally explored. A relatively limited number of studies have been devoted to the compression behavior of magnetostrictive gels. The compression behavior of barium ferrite particles mixed in polyvinyl alcohol was studied by Mitsumata [8]. In that work, in order to saturate the magnetic moment of the particles, a magnetic field of 10 kOe was applied. It was shown that the longitudinal modulus of magnetized gel was slightly higher than the gel without magnetization. It was also concluded that the magnetization direction would not have a major influence on the compression modulus. Zhou [9] used a technique to measure the dynamic compression response of RTV silicone rubber filled with carbonyl ferrous particles. The method consisted of recording two accelerations at different exciting frequencies, and different magnetic fields.
2. Sample preparation In the present work, the goal of compression experiments was to study the influence of magnetic field on the compression response of the particle filled silicone gels. Various cylindrical samples were subjected to displacement-controlled compressive loading. A number of samples were tested in the absence of any magnetic field. Other samples were subjected to permanent magnetic force applied at one end. Different levels of magnetic flux were used. For comparison, samples consisting of pure silicone were also tested. To reduce the second order effects and to prevent the potential buckling, the height to width ratio of the samples was limited to two and less. For short specimens of this type influence of the end conditions was to be expected. Moreover, due to the presence offriction, the state of pure axial compression could not be ideally achieved. This would also influence the end effects on the compression behavior of the samples. The magnetostrictive gel specimen consisted of three main components: (1) the cross-linking agent, (2) the platinum catalyst, and (3) the carbonyl iron particles. Through a trial and error procedure, for production of relatively hard gel specimens, a weight ratio of cross-linking agent to platinum catalyst equal to 1.5 was used. The amount of carbonyl iron powder in each sample was of 80% of the total weight. The carbonyl iron particles had an average diameter of 3.8 mm and a density of about 3.8 g/cc. First, the three components were mixed thoroughly; then the mixture was poured into cylindrical moulds having nominal diameter of about 13.5 mm and a height of about 13.5 mm.
Fig. 1. Stress versus strain curves of 80% weight fraction of carbonyl iron particle filled silicon gel under various permanent magnetic inductions.
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The specimens were cured at room temperature for a minimum of 24 h.
3. Compression tests
Fig. 2. Experimental set-up for compression tests and for magnetic force measurements. The lower part shows the aluminum block and the upper part shows the electromagnet attached to the upper head. For compression tests, instead of electromagnet, permanent magnets were used.
Cylindrical specimens of magnetostrictive gels were tested in a Zwick-1474 testing machine with a loading speed of 1 mm/min in the absence and the presence of a permanent magnetic field. Tests in a magnetic field were made with: (a) a single 15!15!5 mm permanent magnet of strength 0.32 T, and (b) a pair of 15!15!5 mm permanent magnets of strength 0.44 T attached to the upper steel plate. To avoid an undesired magnetic interaction between the two head plates an aluminum cylinder having a height of 42 mm and a diameter of 60 mm was placed on the lower plate. This experimental set-up was also used to measure the magnitude of the magnetic forces. For the magnetic force measurement, however, the permanent magnets were replaced by electromagnets as shown in Fig. 2. Testing was carried out in accordance with the EN ISO 604 Standard. The pretests with some specimens showed that the material would withstand strains up to more than 90%. However, in view of potential applications, requiring lower strains, the samples were compressed to a maximum strain of 30%. A sequence of five loading and unloading with a recovery period of 2 min between each loading were performed. In order to have statistically significant results, six powder filled samples were used for each level of magnetic induction. The axial load and the machine head displacement were continuously recorded. Due to very slight variability of compression results, only three unfilled
Fig. 3. Attractive magnetic force on the sample with respect to distance from the magnet.
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Fig. 4. Magnetic induction on the sample with respect to distance from the magnet.
samples were used. The results reported in Section 4 pertain to the third loading stage.
in each case. The curve related to the unfilled specimens shows the average of three samples. 4.2. Magnetic attractive force on samples
4. Experimental results 4.1. Stress–strain behavior Fig. 1 shows the stress versus strain curves of magnetostrictive gels with and without the influence of various permanent magnetic fields. It shows the average curves among six samples as well as the standard deviation
The experimental set-up used for the force measurements was similar to the one for compression tests. However, to measure the magnetic force applied to the gel samples, instead of permanent magnets, an electromagnet was used. Fig. 2 shows the experimental set-up for the tests. The attraction force was measured by the Zwick machine at a given distance of the electromagnet from the top of the cylindrical specimen. A prescribed voltage was applied and
Fig. 5. Variation of attractive force with magnetic induction.
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Fig. 6. Attractive force with respect to distance for standard steel, magnetostrictive elastomer, and magnetostrictive gel.
the magnetic induction was measured with a Teslameter. The results of magnetic force measurements are shown in Figs. 3–5. Fig. 3 shows a decrease of magnetic field with increase of the distance of the electromagnet from the gel sample. Fig. 4 shows a decrease in magnetic induction with increase of the distance. Fig. 5 shows the relationship between the attractive force and the magnetic induction. As expected, both the attractive force and the magnetic induction decrease rapidly with increase of distance between the electromagnet and the sample. The decrease would be more pronounced when the sample is placed very near to the electromagnet. The nonlinear relation between the attractive force and magnetic induction, depicted in Fig. 5, implies a rapid increase of attractive force changes with the change in the magnetic induction. For instance, for a voltage of 24 V and a distance of 1 mm, an attractive force of 168 mN corresponding to a magnetic induction of 62 mT was determined. For materials with higher magnetic susceptibility a higher magnetic force would be expected. For comparison, a standard steel (37-2 DIN grade) plate, a carbonyl iron particle filled silicone with 65% W metallic content, and a carbonyl iron particle filled silicone gel with 65% W particle content were exposed to identical force measurement tests. Fig. 6 shows the attractive force versus distance for each of these three materials.
5. Conclusions The compression tests on cylindrical specimens from particle iron-filled silicone showed a marked increase in stiffness and strength as compared with pure silicone samples. At a strain of 30%, the compressed pure silicon
experienced a stress of 13.8 kPa, while for the particle filled gel a stress of 75.2 kPa, i.e. more than five times higher was obtained. In the case of the magnetostrictive gel compressed under a BZ0.32 T magnetic field a compressive stress of 114 kPa, i.e. a 52% increase relative to the case BZ0 T and for BZ0.44 T magnetic field a stress of 148 kPa that is a 97% increase relative to the case BZ0 T was observed. At 30% strain, the tangent compressive modulus of pure silicon was 60 kPa, while for the particle-filled gel the compressive modulus amounted to 427 kPa that is 7 times the modulus of unfilled gel. Under a magnetic field of 0.23 T, the calculated modulus was 616 kPa, that is a 44% increase from the case BZ0 T. Under a magnetic field induction of 0.44 T the compression modulus reached the value 861 kPa, which marked a 100% increase from the case BZ0 T. In all cases, it was found that an increase in the magnetic induction had stiffening and strengthening effect on the magnetostrictive gels. Magnetostrictive very soft elastomers offer potential applications including variable stiffness components, large strain actuators, and electro-magnetically active damping elements. Polymer-based gels may also have potential applications as artificial muscles and damping components.
References [1] M. Farshad, A.N. Benine, Magnetoactive elastomer composites, Polymer Testing 23 (2004) 347–353. [2] T. Shiga, A. Okada, T. Kurauchi, Magnetoviscoelastic behavior of composite gels, Journal of Applied Polymer Science 58 (1995) 787–792. [3] Y. An, B. Liu, Soft gels with ordered iron particles: fabrication and electrorheological response, International Journal of Modern Physics B 16 (17 and 18) (2002) 2440–2446.
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[4] Y. An, T. Montgomery, Actuating properties of soft gels with ordered iron particles: basis for a shear actuator, Smart Material and Structures 12 (2003) 157–163. [5] M. Zrinyi, Generation of smart elastomers, 4th Annual UNESCO School and IUPAC Conference, April 7–11 (2001). [6] M. Zrinyi, Szabo, Muscular contraction mimiced by magnetic gels, International Journal of Modern Physics 15 (6–7) (2001) 557–563.
[7] M. Zrinyi, D. Szabo, J. Feher, Comparative studies of electricand magnetic field sensitive polymer gels, Proceedings of SPIE 3667 (1999) 406–413. [8] T. Mitsumata, et al., Compressive Modulus of Ferrite Containing Polymer Gels, International Journal of Modern Physics 16 (17 and 18) (2002) 2419–2425. [9] G.Y. Zhou, J.R. Li, Dynamic behavior of a magnetorheological elastomer under uniaxial deformation: I. Experiment, Smart Materials and Structures 12 (2003) 859–872.
Material Properties
Compression properties of magnetostrictive polymer composite gels M. Farshad*, M. Le Roux EMPA, Materials Science and Technology, Polymers/Composites Laboratory, Ueberlandstrasse 129, 8600 Du¨bendorf, Switzerland Received 6 August 2004; accepted 15 September 2004
Abstract In this research work, compression behavior of magnetostrictive silicone gels under the influence of an external magnetic field was investigated. Magnetostrictive gel specimens were made of silicon gel with 80% weight fraction of 3.8 mm embedded carbonyl iron particles. Compression tests were performed on cylindrical samples of these materials in the absence and in the presence of a magnetic field. It was found that the presence of a magnetic field would increase both the stiffness and the loadbearing capacity, but would reduce the maximum strain of the material. At 30% strain, a 50% increase in maximum stress in the presence of 0.32 T and 100% in stress in the presence of 0.44 T magnetic field inductions were observed. The induced magnetic force on the samples was also measured. The induced force decreased nonlinearly with the distance of the electromagnet from the sample and increased nonlinearly with the magnetic induction. This research was oriented towards certain applications. These types of material characterization may provide useful information for potential applications of soft magnetostrictive elastomers in muscle type actuators and tunable stiffness elements. q 2004 Elsevier Ltd. All rights reserved. Keywords: Compression properties; Magnetostrictive gels
1. Introduction Magnetically active soft materials are polymer-based elastomeric materials which react to an external magnetic field and undergo deformation or experience mechanical stresses. These materials may be produced by chemical synthesis or through physical combination. In the latter case, magnetic components are added to the polymeric material to result in a magneto active polymer composite. Terminologies such as magnetostrictive, magnetorheological, and magneto active materials have been used as synonyms in various literatures. Magnetostrictive polymer gels are
* Corresponding author. Tel.: C41 1 823 4491; fax: C41 1 821 6244. E-mail address: [email protected] (M. Farshad). 0142-9418/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2004.09.007
a sub-class of soft elastomeric materials containing magnetizable components which have been embedded in the gel during the cross-linking process [1]. If metallic powders are used, they can be aligned. As the result of this process, a material with anisotropic mechanical and magnetic properties is produced. The relatively weak cross-linking of polymer gels makes them attractive as large deformation actuators. The material properties and potential applications of magnetic field sensitive gels have been investigated by a number of researchers. Magneto sensitive gels demonstrate actuator properties, which can be used in robotic applications as artificial muscles. The magnetoviscoelastic behavior of composite gels made of iron particles and a silicone matrix was investigated by Shiga et al. [2]. Those gels were cured for 5 h at 70 8C under an electromagnetic field of 27 kA/m. It was found that in applied field strength of
164
M. Farshad, M. Le Roux / Polymer Testing 24 (2005) 163–168
20–100 kA/m, both the shear storage and the shear loss modules were linear with the applied field strength. An et al. [3,4] explored the production methods and the use of ordered iron filled soft silicon gels as shear actuators. The gel was cured for 24 h with a magnetic induction of more than 1 T for the first 2 h. Then, the shear storage modulus under variable applied field up to 1200 kA/m was measured. The goal of investigation was to find the field dependence of dynamic shear modulus. It was found that addition of 1% volume particles would double the shear modulus in an electric field of 2 kV mmK1. A comparative study of electric and magnetic field sensitive gels was carried out by Zrinyi et al. [5–7]. In this study, a potential application of magnetic field sensitive gels as artificial muscles was experimentally explored. A relatively limited number of studies have been devoted to the compression behavior of magnetostrictive gels. The compression behavior of barium ferrite particles mixed in polyvinyl alcohol was studied by Mitsumata [8]. In that work, in order to saturate the magnetic moment of the particles, a magnetic field of 10 kOe was applied. It was shown that the longitudinal modulus of magnetized gel was slightly higher than the gel without magnetization. It was also concluded that the magnetization direction would not have a major influence on the compression modulus. Zhou [9] used a technique to measure the dynamic compression response of RTV silicone rubber filled with carbonyl ferrous particles. The method consisted of recording two accelerations at different exciting frequencies, and different magnetic fields.
2. Sample preparation In the present work, the goal of compression experiments was to study the influence of magnetic field on the compression response of the particle filled silicone gels. Various cylindrical samples were subjected to displacement-controlled compressive loading. A number of samples were tested in the absence of any magnetic field. Other samples were subjected to permanent magnetic force applied at one end. Different levels of magnetic flux were used. For comparison, samples consisting of pure silicone were also tested. To reduce the second order effects and to prevent the potential buckling, the height to width ratio of the samples was limited to two and less. For short specimens of this type influence of the end conditions was to be expected. Moreover, due to the presence offriction, the state of pure axial compression could not be ideally achieved. This would also influence the end effects on the compression behavior of the samples. The magnetostrictive gel specimen consisted of three main components: (1) the cross-linking agent, (2) the platinum catalyst, and (3) the carbonyl iron particles. Through a trial and error procedure, for production of relatively hard gel specimens, a weight ratio of cross-linking agent to platinum catalyst equal to 1.5 was used. The amount of carbonyl iron powder in each sample was of 80% of the total weight. The carbonyl iron particles had an average diameter of 3.8 mm and a density of about 3.8 g/cc. First, the three components were mixed thoroughly; then the mixture was poured into cylindrical moulds having nominal diameter of about 13.5 mm and a height of about 13.5 mm.
Fig. 1. Stress versus strain curves of 80% weight fraction of carbonyl iron particle filled silicon gel under various permanent magnetic inductions.
M. Farshad, M. Le Roux / Polymer Testing 24 (2005) 163–168
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The specimens were cured at room temperature for a minimum of 24 h.
3. Compression tests
Fig. 2. Experimental set-up for compression tests and for magnetic force measurements. The lower part shows the aluminum block and the upper part shows the electromagnet attached to the upper head. For compression tests, instead of electromagnet, permanent magnets were used.
Cylindrical specimens of magnetostrictive gels were tested in a Zwick-1474 testing machine with a loading speed of 1 mm/min in the absence and the presence of a permanent magnetic field. Tests in a magnetic field were made with: (a) a single 15!15!5 mm permanent magnet of strength 0.32 T, and (b) a pair of 15!15!5 mm permanent magnets of strength 0.44 T attached to the upper steel plate. To avoid an undesired magnetic interaction between the two head plates an aluminum cylinder having a height of 42 mm and a diameter of 60 mm was placed on the lower plate. This experimental set-up was also used to measure the magnitude of the magnetic forces. For the magnetic force measurement, however, the permanent magnets were replaced by electromagnets as shown in Fig. 2. Testing was carried out in accordance with the EN ISO 604 Standard. The pretests with some specimens showed that the material would withstand strains up to more than 90%. However, in view of potential applications, requiring lower strains, the samples were compressed to a maximum strain of 30%. A sequence of five loading and unloading with a recovery period of 2 min between each loading were performed. In order to have statistically significant results, six powder filled samples were used for each level of magnetic induction. The axial load and the machine head displacement were continuously recorded. Due to very slight variability of compression results, only three unfilled
Fig. 3. Attractive magnetic force on the sample with respect to distance from the magnet.
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M. Farshad, M. Le Roux / Polymer Testing 24 (2005) 163–168
Fig. 4. Magnetic induction on the sample with respect to distance from the magnet.
samples were used. The results reported in Section 4 pertain to the third loading stage.
in each case. The curve related to the unfilled specimens shows the average of three samples. 4.2. Magnetic attractive force on samples
4. Experimental results 4.1. Stress–strain behavior Fig. 1 shows the stress versus strain curves of magnetostrictive gels with and without the influence of various permanent magnetic fields. It shows the average curves among six samples as well as the standard deviation
The experimental set-up used for the force measurements was similar to the one for compression tests. However, to measure the magnetic force applied to the gel samples, instead of permanent magnets, an electromagnet was used. Fig. 2 shows the experimental set-up for the tests. The attraction force was measured by the Zwick machine at a given distance of the electromagnet from the top of the cylindrical specimen. A prescribed voltage was applied and
Fig. 5. Variation of attractive force with magnetic induction.
M. Farshad, M. Le Roux / Polymer Testing 24 (2005) 163–168
167
Fig. 6. Attractive force with respect to distance for standard steel, magnetostrictive elastomer, and magnetostrictive gel.
the magnetic induction was measured with a Teslameter. The results of magnetic force measurements are shown in Figs. 3–5. Fig. 3 shows a decrease of magnetic field with increase of the distance of the electromagnet from the gel sample. Fig. 4 shows a decrease in magnetic induction with increase of the distance. Fig. 5 shows the relationship between the attractive force and the magnetic induction. As expected, both the attractive force and the magnetic induction decrease rapidly with increase of distance between the electromagnet and the sample. The decrease would be more pronounced when the sample is placed very near to the electromagnet. The nonlinear relation between the attractive force and magnetic induction, depicted in Fig. 5, implies a rapid increase of attractive force changes with the change in the magnetic induction. For instance, for a voltage of 24 V and a distance of 1 mm, an attractive force of 168 mN corresponding to a magnetic induction of 62 mT was determined. For materials with higher magnetic susceptibility a higher magnetic force would be expected. For comparison, a standard steel (37-2 DIN grade) plate, a carbonyl iron particle filled silicone with 65% W metallic content, and a carbonyl iron particle filled silicone gel with 65% W particle content were exposed to identical force measurement tests. Fig. 6 shows the attractive force versus distance for each of these three materials.
5. Conclusions The compression tests on cylindrical specimens from particle iron-filled silicone showed a marked increase in stiffness and strength as compared with pure silicone samples. At a strain of 30%, the compressed pure silicon
experienced a stress of 13.8 kPa, while for the particle filled gel a stress of 75.2 kPa, i.e. more than five times higher was obtained. In the case of the magnetostrictive gel compressed under a BZ0.32 T magnetic field a compressive stress of 114 kPa, i.e. a 52% increase relative to the case BZ0 T and for BZ0.44 T magnetic field a stress of 148 kPa that is a 97% increase relative to the case BZ0 T was observed. At 30% strain, the tangent compressive modulus of pure silicon was 60 kPa, while for the particle-filled gel the compressive modulus amounted to 427 kPa that is 7 times the modulus of unfilled gel. Under a magnetic field of 0.23 T, the calculated modulus was 616 kPa, that is a 44% increase from the case BZ0 T. Under a magnetic field induction of 0.44 T the compression modulus reached the value 861 kPa, which marked a 100% increase from the case BZ0 T. In all cases, it was found that an increase in the magnetic induction had stiffening and strengthening effect on the magnetostrictive gels. Magnetostrictive very soft elastomers offer potential applications including variable stiffness components, large strain actuators, and electro-magnetically active damping elements. Polymer-based gels may also have potential applications as artificial muscles and damping components.
References [1] M. Farshad, A.N. Benine, Magnetoactive elastomer composites, Polymer Testing 23 (2004) 347–353. [2] T. Shiga, A. Okada, T. Kurauchi, Magnetoviscoelastic behavior of composite gels, Journal of Applied Polymer Science 58 (1995) 787–792. [3] Y. An, B. Liu, Soft gels with ordered iron particles: fabrication and electrorheological response, International Journal of Modern Physics B 16 (17 and 18) (2002) 2440–2446.
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M. Farshad, M. Le Roux / Polymer Testing 24 (2005) 163–168
[4] Y. An, T. Montgomery, Actuating properties of soft gels with ordered iron particles: basis for a shear actuator, Smart Material and Structures 12 (2003) 157–163. [5] M. Zrinyi, Generation of smart elastomers, 4th Annual UNESCO School and IUPAC Conference, April 7–11 (2001). [6] M. Zrinyi, Szabo, Muscular contraction mimiced by magnetic gels, International Journal of Modern Physics 15 (6–7) (2001) 557–563.
[7] M. Zrinyi, D. Szabo, J. Feher, Comparative studies of electricand magnetic field sensitive polymer gels, Proceedings of SPIE 3667 (1999) 406–413. [8] T. Mitsumata, et al., Compressive Modulus of Ferrite Containing Polymer Gels, International Journal of Modern Physics 16 (17 and 18) (2002) 2419–2425. [9] G.Y. Zhou, J.R. Li, Dynamic behavior of a magnetorheological elastomer under uniaxial deformation: I. Experiment, Smart Materials and Structures 12 (2003) 859–872.