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Dot-patterned hybrid magnetorheological elastomer developed by 3D printing
Journal of Magnetism and Magnetic Materials 494 (2020) 165825
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Research articles
Dot-patterned hybrid magnetorheological elastomer developed by 3D printing A.K. Bastolaa,b, M. Paudela,b, L. Lia, a b
T
⁎
School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore Institute for Sports Research, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore
A R T I C LE I N FO
A B S T R A C T
Keywords: 3D printing Magnetorheological elastomers MR effect Stiffness
This article presents the development of dot-patterned magnetorheological (MR) elastomers (MREs) via 3D printing technology and their magnetorheological characterization. The 3D printed MR elastomer consists of three different materials; magnetic particles, magnetic particles carrier medium, and an elastomer. In such 3D printing, a controlled volume of MR fluid is encapsulated layer-by-layer within the elastomer matrix. The capability of 3D printing technology has been successfully demonstrated by developing the various dot patterns MR elastomers namely isotropic, anisotropic and configurations inspired from basic crystal structures such as BCC and FCC. The magneto-mechanical properties of such 3D printed MR elastomers (3DP-MREs) are studied using a cyclic compression and through a forced vibration testing. In the presence of a magnetic field, a clear change in stiffness of 3DP-MREs has been achieved. Moreover, the anisotropic behavior of 3DP-MREs has also been demonstrated. The experimental results suggested that the 3D printing method makes it possible to develop various structured MREs even without applying a magnetic field during the fabrication process.
1. Introduction In recent decades, additive manufacturing or three-dimensional (3D) printing has been the hot cake of research in several fields including but not limited to construction, footwear, automotive, aerospace, dental and medical industries. 3D printing technology is a fabrication method where the structures are constructed in the layer by layer fashion. The fast and precise manufacturing process, suitable to produce highly customizable products, has also opened the door for additive manufacturing of the smart materials and structures [1–3]. The smart materials are such materials that can change their shapes or properties under the influence of external stimuli such as temperature, electric or magnetic field, and pH, etc. The additively fabricated smart materials or structures can alter their shape or properties over time. Therefore, the fabrication of smart materials or structure via 3D printing is often regarded as 4D printing [2–4]. In recent years, one of the smart materials, magnetorheological (MR) elastomers have gained considerable attention because of the various advantages over the MR fluids such as leakage, the requirement of storage and environmental contaminations [5–10]. A plethora of research works have been performed to explore the various properties of MR elastomers in order to seek the possibility of implementing them in the real engineering system. Some of the potential applications of MR ⁎
elastomers include active vibration absorbers, vibration isolators, sandwich beams, soft actuators or sensors [7,8,11–14]. The MR elastomers are composed of magnetically polarizable micron-sized particles embedded within the polymeric elastomer [15–18]. Typically, to fabricate such MR elastomers, the mixture of the magnetic particles and elastomer resin is allowed to cure in a mold. A magnetic field is often applied during the crosslinking process in order to align the magnetic particles along the direction of the magnetic flux and are locked on the final curing. If the magnetic field is applied during the crosslinking process the MR elastomer is regarded as anisotropic MREs otherwise isotropic MREs are produced. In recent decades, these two types of MREs have been extensively studied [12,13]. It has been also proven that the anisotropic MREs exhibit higher MR effect when the direction of the applied magnetic field is parallel to the chains of magnetic particles. But the need of magnetic field is unavoidable to develop such anisotropic MREs. A few techniques to develop MREs include injection molding [19], compression molding [20] cast molding [20,21], and vacuum-assisted resin transfer molding [21,22]. On the other hand, to avoid the need of a magnetic field for fabricating anisotropic MR elastomer researchers have also developed patterned magnetorheological elastomer using conventional molding or manual patterning [23]. However, all methods used in the literature to fabricate MR elastomers are unable to exactly control the dispersion and
Corresponding author. E-mail address: [email protected] (L. Li).
https://doi.org/10.1016/j.jmmm.2019.165825 Received 28 May 2019; Received in revised form 23 August 2019; Accepted 9 September 2019 Available online 10 September 2019 0304-8853/ © 2019 Published by Elsevier B.V.
Journal of Magnetism and Magnetic Materials 494 (2020) 165825
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Four different amounts of current (0, 1, 2, and 3 A) to electromagnet was supplied to generate the various amount of magnetic flux density. Tests were conducted at 0.1 Hz cyclic frequency and 10% strain level. The raw data were collected using the Instron Bluehill 3 software then analyzed and plotted in Microsoft Excel 2019. The detail of the forced vibration experimental setup can be found in the previous article [24]. Forced vibration testing was conducted at four different amounts of the magnetic flux densities (0, 110, 300 and 500 mT). The various magnetic flux densities could easily be generated by changing the distance between two permanent magnets. The MRE samples were tested using the frequency sweep (from 50 Hz to 200 Hz) at a constant acceleration amplitude of 0.5 g. The MRE sample is in a squeeze mode of the operation and the magnetic field was applied parallel to the printed layers. The MRE sample was clamped on the base of the shaker, which forms a single degree of freedom system. Two accelerometers were used, one records the excitation signal and the other records the response signal. Transmissibility curve was obtained with the help of the excitation and response signals. Thereafter, the transmissibility curves were utilized to obtain elastic properties and then the MR effect of 3D printed MREs. Fig. 1(c) provides the direction of the applied magnetic field during the experiments. The application of the magnetic field for 3D printed MREs can be understood with respect to the plane of the printed MR layers. In cyclic compression, the magnetic field is applied in the normal direction to the printed MR layers. In forced vibration testing, the magnetic field is applied parallelly to the printed MR layers.
distribution of magnetizable particles. Hence, an innovative fabrication technique has to be looked for, which must be able to easily control the configuration of magnetic particles in MR elastomer without applying a magnetic field. One of the possible ways to configure magnetic particles without applying a magnetic field is to use 3D printing method. The biggest advantage of 3D printing is its ability to exactly control where and what to be deposited on a substrate from a printing head. Thus, 3D printing or additive manufacturing could be the only and best way to add magnetizable particles exactly to the pre-desired locations in an MR elastomer to be fabricated without applying a magnetic field. Recently, in 2017, our group has introduced a 3D printing method to fabricate hybrid MR elastomers [24,25]. In such a 3D printing method, MR fluid filaments are configured layer-by-layer inside the elastomer matrix. Various patterns of MR fluid such as line, circle and dots could be printed without applying a magnetic field. 3D printing of MR elastomer was achieved by means of multi-material printing. Two nozzles were used. One of the nozzles is used to dispense the elastomer resin and the other nozzle prints various patterns of MR fluids on top of the elastomer layer. The details of the 3D printing process used to develop hybrid MR elastomers can be found in the previous work [24]. In conventional anisotropic MR elastomers, magnetic particles line up in chains in the direction of an external magnetic field, thus, anisotropy has been observed in both mechanical and magnetic properties [12,13]. However, as presented in our previous study [24], 3D printing of MRE could only provide the anisotropy of magnetic field dependent mechanical properties such as stiffness by 3D printing various spatial structures of magnetic particles within the elastomer matrix. The anisotropic MREs developed in this study are not exactly same as that of conventional anisotropic MREs, however, are an alternative to conventional anisotropic MREs. In this work, we report the fabrication, and characterization of a new type of the configuration of the MR fluid within the elastomer matrix via 3D printing method, namely dot patterns. A small amount of the MR fluid is dispensed from the nozzle at a distinct locus, thus, patterns do not have a continuous structure rather they are disjoint to each other. Firstly, the development of various dot patterns including isotropic, anisotropic, BCC and FCC configurations using 3D printing are presented. Thereafter, magneto-mechanical characterizations of such dot patterns 3D printed MR elastomers are presented. The magneto-mechanical characterization was studied under a cyclic compression loading and through a force vibration technique without and with the application of a magnetic field.
4. Results and discussion Firstly, the details of 3D printing of various dot-patterns and development of BCC and FCC structures are discussed. Thereafter, MR effects of 3D printed MR elastomers are presented as achieved from a cyclic compression and a forced vibration testing. 4.1. 3D printing The hybrid MR elastomer samples were obtained by multi-material printing as illustrated in Fig. 2. A controlled volume of the MR fluid was precisely encapsulated within the elastomeric layer. Again, the detail of the printing process can be found in our previous work [24]. The printing of distinct dots is very similar to ink-jet printing. So, the printing can also be referred as the discontinuous printing process. The work explores the capability of the 3D printing to control the size, location, and distribution of MR fluid dots to produce various hybrid MR elastomers such as isotropic, anisotropic, BCC and FCC structures. Firstly, a fixed volume of the MR fluid was divided into 5 different amounts, as 1, 1/4, 1/8, 1/16 and 1/32, henceforth, the 3D printed MR elastomers are named as Dot_1, Dots_4, Dots_8, Dots_16, and Dots_32 respectively. All five samples have the same amount of the MR fluid printed however configured in a different way with disjoint dots as shown in Fig. 3. The Dot_1 sample was produced by combining the four dots. The size and the location of MR fluid dots were controlled by controlling the printing pressure, initial height, feed rate and time. The crucial printing parameter for controlling the size of dots is the printing time. All other parameters (extrusion pressure, initial height, and feed rate) were kept constant and only dispensing time was controlled to control the size of the dot at the given location. Printing condition of all five different samples is given in Table 1. As can be seen from Table 1, the total time to print the MR fluid layer is the same, while the dispensing time per dot can be seen in the geometric series for different samples (i.e. printing time per dot = a*r^n − 1, where a = 3, r = 2 and n = 1, 2, 3, 4 and 5 for Dots_32, Dots_16, Dots_8, Dots_4 and Dot_1 respectively). The SEM image of the cross-section of the 3D printed samples is presented in Fig. 4. The MR fluid dots are sandwich between the elastomer layers. As given in Fig. 5, the base of the printed dots is very close to a
2. Materials A UV curable silicone is used as an elastomer matrix which serves as a solid matrix of 3D printed hybrid MR elastomer samples. The homogenous mixture of the magnetic particles and a highly viscous silicone is used as a printing material. Carbonyl iron powders (CIP) of size 3–5 µm were used as magnetic particles. As the materials used are the same as in the previous work [24], the details of the materials used can be found in the same article. 3. Magneto-mechanical testing The 3D printed MR elastomer samples were characterized using a cyclic compression loading and through a forced vibration testing to study the magneto-mechanical properties in both the absence and presence of a magnetic field. Schematic illustration of the application of a magnetic field and the photograph of experimental equipment are given in Fig. 1. The detail of the cyclic compression setup can be found in the previous article [26]. Cyclic compression was conducted with a universal testing Instron 5569 equipped with a 500 N load cell with an electromagnet to apply a magnetic field during the testing. The electromagnet comes with an air gap where the MRE sample can be placed. 2
Journal of Magnetism and Magnetic Materials 494 (2020) 165825
A.K. Bastola, et al.
Fig. 1. Experimental equipment (a) Cyclic compression: schematic of the magnetic circuit and a photograph of the experimental setup as connected to a load cell. (b) Forced vibration testing: illustration of the magnetic field direction and a photograph of experimental setup connected to the control box and data acquisition system. (c) Illustration of the direction of the magnetic field with respect to the orientation of the printed MR layers.
printing process is summarized in Table 3. The number of dots is higher in FCC structure and size of the dots are bigger for BCC structure.
perfect circle and the size of dots is increased with increasing printing time. The size of the printed dots is analyzed based on two dimensions: diameter and peak height of the dots. The average diameter and average peak height of the dots are presented in Table 2. Both diameter and peak height are increased with increasing printing time. The curve fitting shows that both dimensions (diameter and height) increase linearly with increasing time. In the second phase, to demonstrate the potential of 3D printing, the MR fluid dots were printed in such a way that the dots form basic crystal structures of the elements such as body-centered cubic (BCC) and face-centered cubic (FCC). BCC and FCC structures were selected in this study due to the ease of configuration of MR dots. The atoms of BCC and FCC unit cell are replaced by MR dots. Printing of the unit cell was performed by printing three MR layers. The morphology of 3D printed hybrid MREs with BCC and FCC structure is shown in Fig. 6. As can be seen in Fig. 6, for BCC structure, 1st and 3rd layers are identical and have 4 MR dots while the 2nd layer only has 1 MR dot per unit cell. Similarly, for FCC structure, 1st and 3rd layers are also identical and have 5 MR dots while the 2nd layer has 4 MR dot per unit cell. Total of 9-unit cells were combined and printed within the elastomer matrix. The total volume of the printed MR fluid has been kept the same for both BCC and FCC structures. The MR fluid volume was again controlled by controlling the dispensing time, the information of the
4.2. Magneto-mechanical characterization As in our previous work [24], there are two types of hybrid MR elastomer developed by 3D printing. The detail of 3D printed samples can be found in the same article. The first type of 3D printed MR elastomer (3DP-MRE) must completely solidify after being printed, whereas in the second type of 3DP-MRE the encapsulated MR fluid dots should remain fluid even after printing. Hereafter, the 3D printed MR elastomer with solid MR structures inside the elastomer matrix is regarded as 3DP-MRE1 and the 3D printed MR elastomer with MR fluid structures inside the elastomer matrix is regarded as 3DP-MRE2. 4.2.1. Cyclic compression Cyclic compression was performed to study the effect of patterning the MR dots within the elastomer matrix in both cases: without and with the application of a magnetic field. A typical stress-strain loop under cyclic compression is given in Fig. 7. Firstly, the effect of the magnetic field can be visualized as the slope of the stress-strain curve is increased with increasing current to the electromagnet. Similarly, the second way to visualize the magnetic field effect is to study the change 3
Journal of Magnetism and Magnetic Materials 494 (2020) 165825
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Fig. 2. Schematic illustration of dot patterned MRE fabrication via 3D printing.
divided by strain at any given value of stress or strain. It also is called the stress-strain ratio. Small non-linearity in stress-strain response can provide a bigger variation in tangent modulus as it is a slope at a point, so tangent modulus becomes highly biased. The unbiased modulus is secant modulus as it is simply a ratio of stress and strain. Thus, in this study, in order to investigate the MR effects, the stress-strain curves are analyzed by obtaining secant modulus. Absolute and relative MR effect for all five types of samples (Dot_1, Dots_4, Dots_8, Dots_16, and Dots_32) of both categories (3DP-MRE1 & 3DP-MRE2) are given in Fig. 8. 3DP-MRE2 samples showed higher MR effect than that of 3DP-MRE1. The MR effect is increased with increasing current to the electromagnet, with Dot_1 sample achieving the highest MR effects. This is attributed to the bulk volume of the MR fluid. MR effect curves start from almost zero or minus in the smallstrain region that increase and remains constant within 10% strain. The minus MR effects are most visible in low strain region, this is because of remnant deformation of magnetic particle chains or even the elastomeric case upon the cyclic compression, a similar type of finding has been reported by Schubert and Harrison [32]. Hence, causing some amount of plastic strain. Nevertheless, the MR effect increased to a positive value and became stable within 10% strain. The maximal MR effects from Fig. 8 are recorded and plotted against the different type of sample as shown in Fig. 9. As shown in Fig. 9, for 3DP-MRE1 samples, the relative MR effect is smaller than that of 3DP-MRE2 samples and is the highest for Dot_1 sample and is decreased with an increasing number of dots. Similarly,
in the area under the loading and unloading curves. In the absence of a magnetic field, 3DP-MRE1 has a higher modulus than the 3DP-MRE2. For five different samples (Dots_32, Dots_16, Dots_8, and Dots_4 and Dot_1), they almost have a similar modulus for 3DP-MRE1, however, for 3DP-MRE2 samples, it was also noted that the Dot_1 sample has a slightly lower modulus than others. Upon the application of a magnetic field, 3DP-MRE1 samples showed very small changes, while the 3DP-MRE2 sample showed noticeable changes. This is attributed to the free magnetic particles within the printed fluid dots of 3DP-MRE2 samples, the free magnetic particles can freely move and make chains along the magnetic field. The effect of the magnetic field can be studied by obtaining the absolute and relative MR effect. The absolute MR effect is defined as the change in the modulus by the application of a magnetic field.
MRabs = EB − E B0
(1)
where EB and E B0 are the moduli with and without the application of a magnetic field, respectively. Secondly, relative MR effect is defined as the relative change between properties
MRrel = (EB / E B0 − 1)
(2)
Relative MR effect can also be expressed as (EB / E B0 − 1) × 100% when defined as a percentage value. The moduli such as secant and tangent moduli are obtained to study the MR effect of current MREs [27–32]. The tangent modulus is the slope of the stress-strain response at any point. Secant modulus is stress
Fig. 3. 3D printed MR elastomers with different sized MR fluid dots, Dots_32, Dots_16, Dots_8, Dots_4, Dot_1 and pure elastomer from right to left respectively. 4
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Table 1 Summary of the printing parameters for Dots 32, Dots 16, Dots 8, Dots 4 and Dot 1 samples. Sample
Extrusion pressure (Bars)
Initial height (mm)
Feed rate (mm/min)
Dispensing time per dot (s)
Total time per MR fluid layer (s)
Dot 1 Dots 4 Dots 8 Dots 16 Dots 32
2.1 2.1 2.1 2.1 2.1
1.2 1.2 1.2 1.2 1.2
150 150 150 150 150
96 24 12 6 3
96 96 96 96 96
anisotropic sample and the sample obtained by offsetting the MR layer is regarded as isotropic. The secant moduli obtained from the cyclic compressive testing for the spatially structured samples are given in Fig. 10(b). For 3DP-MRE2, in the absence of the magnetic field, moduli of isotropic and anisotropic samples are similar, however, in the presence of a magnetic field, isotropic sample significantly lags the increase in modulus compared to the anisotropic sample. This result demonstrates that the 3D printing method can also be used to develop isotropic and anisotropic MR elastomers. The maximum MR effect for the isotropic sample was found to be 0.23, where anisotropic sample showed the MR effect of 0.75, these values are for 3DP-MRE2 samples. Similarly, moduli for BCC and FCC structured samples of both categories in the absence and presence of a magnetic field are given in Fig. 11. The main aim of developing BCC and FCC structure is to study the MR effect of the same amount of the MR materials but configured differently inspired by basic crystal structures. Both BCC and FCC structure have the same amount of MR materials. Cyclic compression result revealed that BCC structure showed slightly higher MR effect than that of FCC structured 3DP-MREs. This is believed that the effect of size of the MR dots, the BCC patterns have bigger dots and, therefore, must have shown higher MR effect.
for the 3DP-MRE2 category, the MR effect is also decreased as the size of the MR dots decreases and the MR effect becomes almost similar for Dots_8, Dots_16, and Dots_32 samples. While the Dots_4 sample showed almost similar MR effect as that of Dot_1 sample, while the MR effect is considerably decreased (almost by 50%) for Dots_8, Dots_16, and Dots_32 samples. The experimental results suggested that the division of the given amount of MR fluid into smaller dots could uphold a reasonable MR effect. Such division is also beneficial when leakage and particle settling are concerned. The finding suggested that dividing the given volume of MR fluid into 4 dots can maintain a similar MR effect as that of a single MR dot. On the other hand, increasing the number of dots beyond 4 is not recommended if the MR effect is the main concern, however, can be considered if the leakage and sedimentation are the main concern. Thereafter, two different types of spatially structured dot-patterned samples are developed, by off-setting the specified printing location in the successive MR layers, picture and illustration are presented in Fig. 10(a). Hereafter, for easiness of evoking the samples, the spatially structured samples are named as anisotropic and isotropic, the names are given by only considering the orientation of the printed dots. It should be noted that the isotropic and anisotropic samples named in this study are different than that of conventional anisotropic MREs. The sample printed without offsetting the MR layers is regarded as the
Fig. 4. Crossectional micrograph of 3D printed MR elastomer under SEM. 5
Journal of Magnetism and Magnetic Materials 494 (2020) 165825
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Fig. 5. Variation of the size of the dots. (a) diameter and (b) height Dots_32, Dots_16, Dots_8, and Dots_4.
4.2.2. Forced vibration test MR effects of 3DP-MREs were also studied through forced vibration testing. Four different magnetic flux densities (0, 110, 300 and 500 mT) were applied during the experiment. The direction of the magnetic flux is parallel to the printed layers as shown in Fig. 12(a). Typical results of the forced vibration testing of dot-patterned 3DPMRE are presented in Fig. 12(b). The magnetic field was applied as illustrated in Fig. 12(a). Here, the magnetic field is parallel to the plane of the printed layers. A number of features can be noted in Fig. 12(b). Firstly, when the magnetic field is not applied, the transmissibility
Table 2 Summary of the variation of the size of the printing dots: diameter and height. Sample
Dots Dots Dots Dots
4 8 16 32
Size of the dots Diameter (mm)
Height (mm)
5.1 4.2 3.6 2.6
1.8 1.6 1.4 1.2
± ± ± ±
0.2 0.15 0.1 0.1
± ± ± ±
0.1 0.1 0.1 0.1
Fig. 6. (a) Unit cell of BCC and FCC structures and (b) Morphology of the BCC and FCC structured obtained via 3D printing. 6
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Table 3 Summary of the printing condition for BCC and BCC MR elastomers. Sample
Extrusion pressure (Bars)
Initial height (mm)
Feed rate (mm/min)
Dispensing time per dot (s)
Total number of dots
Total time (s)
BCC FCC
2.1 2.1
1.2 1.2
150 150
6 4
41 62
246 248
Fig. 7. Stress-strain loops for Dots_4 samples of two categories 3DP-MRE1 (left) and 3DP-MRE2 (right) at 4 different amounts of current to the electromagnet.
Fig. 8. Absolute and relative MR effect, calculated with secant modulus, for all five different patterns of 3DP-MRE1 (top figures) and 3DP-MRE2 (bottom figures), achieved with 3 different amounts of current to the electromagnet versus compressive strain.
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Journal of Magnetism and Magnetic Materials 494 (2020) 165825
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Fig. 9. Relative MR effect achieved with 1A, 2A and 3A current and obtained using Eq. (2) are listed for all five types of samples (Dot_1–Dots_32) of two categories. 3DP_MRE1 (left) and 3DP-MRE2 (right) samples. These MR effect values were obtained from the relative MR effect curves that are given in Fig. 8.
Fig. 10. (a) Illustration of spatially structured samples and the application of a magnetic field. (b) Secant moduli for spatially structured samples of both categories.
Fig. 11. Results of the BCC and FCC structure 3D printed MR elastomer of both categories, 3DP-MRE1 (left) and 3DP-MRE2 (right).
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Journal of Magnetism and Magnetic Materials 494 (2020) 165825
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Fig. 12. Magnitude transmissibility versus excitation frequency of the dot-patterned 3DP-MREs at various magnetic flux densities, 3DP-MRE1 (left) and 3DP-MRE2 (right).
Fig. 13. Stiffness and damping ratio versus magnetic flux density five types of dot sample of both 3DP-MRE1 and 3DP-MRE2.
the damping ratio of the samples are almost constant or slightly decreased with the increasing magnetic field as the peak of the transmissibility curve is almost same or increased upon the application of a magnetic field. The effect of changing the size of the dots are discussed by obtaining the stiffness and damping ratio as given in Fig. 13. When the magnetic field is not applied, as obtained in cyclic compression, it was found that the 3DP-MRE1 samples are stiffer than that of 3DP-MRE2 samples. Similarly, the 3DP-MRE1 samples have a low damping ratio than that of 3DP-MRE2 samples. Dots_1 sample was found to be least stiff compared to other samples for 3DP-MRE2 samples and the stiffness was found to be increased as the size of MR dots decreased. This can be attributed to
curves of the two types of samples (3DP-MRE1 and 3DP-MRE2) are easily distinguishable. The 3DP-MRE1 samples have higher natural frequency and higher peaks at the natural frequency. Higher natural frequency signifies that the 3DP-MRE1 samples are stiffer than that of 3DP-MRE2 samples and higher peak of transmissibility curve indicates that the damping ratio of the 3DP-MRE1 is smaller than that of the 3DPMRE2. Upon the application of a magnetic field, the transmissibility curves are moving toward the right. Shifting of the curves toward right indicate that stiffness of the 3DP-MRE is increased [24]. Again, the shifting of the curve is more pronounced for 3DP-MRE2 samples. Yet again, this must be attributed to the free magnetic particles that are suspended within the dots of 3DP-MRE2 samples. On the other hand, 9
Journal of Magnetism and Magnetic Materials 494 (2020) 165825
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Fig. 14. (a) Illustration of the direction of the applied magnetic field. (b) Stiffness versus magnetic flux density isotropic and anisotropic samples of both 3DP-MRE1 (left) and 3DP-MRE2 (right).
Fig. 15. Stiffness versus magnetic flux density FCC and BCC samples of both 3DP-MRE1 (left) and 3DP-MRE2 (right).
should be noted that if the same amount of volume of MR fluid is divided into the different amount of volume the MR effect gets affected. Nonetheless, it is worthy that if dots are divided into four parts, the MR effect would not significantly decrease. This is also one of the reasons that the printing of four dots are considered to develop isotropic and anisotropic samples. In the cyclic compression, the magnetic field was applied normal to the printed layer as given in Fig. 10(a). Whereas, in the forced vibration testing, the magnetic field is applied as given in Fig. 14(a), which is to observe the difference of isotropic and anisotropic configuration in MR effect when magnetic field is applied parallel to the printed layers. The stiffness achieved via frequency sweep with isotropic and anisotropic samples of both 3DP-MRE1 and 3DP-MRE2 is given in Fig. 14(b). The
the size of the dots; as the smaller fluid dots could behave like viscoelastic solid dots. On the other hand, for 3DP-MRE1 samples, the stiffness was found to increase up to 8 dots and start to decrease with an increasing number of dots. This can be attributed to the soft elastomeric casing where the presence of smaller dots could not significantly alter the stiffness of the elastomer matrix. Upon the application of a magnetic field, MR effect exhibited by 3DP-MRE1 samples was found to be much smaller compared to 3DP-MRE2 samples and the effect of size of the dots are not significant. Whereas for 3DP-MRE2, MR effect was found to be decreased with an increasing number of dots. The Dot_1 sample has relative MR effect of 77% while Dots_32 sample only has 28% at 500 mT. However, the difference between Dots_4 and Dot_1 sample is not very significant, Dots_4 has 70% MR effect at 500 mT. Here, it 10
Journal of Magnetism and Magnetic Materials 494 (2020) 165825
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MR effect exhibited by the isotropic and anisotropic 3DP-MREs is found to be similar when the magnetic field was applied parallel to the printed layers. Yet again, the MR effect is higher for 3DP-MRE2 samples. This result provides the evidence that the anisotropic nature chiefly depends upon the direction of the applied magnetic field and 3D printing has the capability to configure the MR fluid or magnetic particles in a unique fashion that MRE exhibits anisotropic nature. Similarly, the results of BCC and FCC structured 3DP-MREs samples are given in Fig. 15. As achieved in cyclic compression result, the significant difference of BCC and FCC structure was also not observed through the vibration testing even when the magnetic field was applied parallel to the printed layers. However, BCC structures seemingly showed slightly higher MR effect than that of FCC, which is again attributed to the higher size of MR dots as described previously. Even though the difference in MR effect was not observed by printing the basic crystal structures such developments provide the capability of a 3D printing method, which are difficult to be performed by other fabrication technique even with the in-situ application of a magnetic field.
[6]
[7]
[8]
[9]
[10]
[11]
[12] [13]
5. Conclusions
[14] [15]
The capability of 3D printing technology for the fabrication of various structured dot-patterns are presented. Like the current MREs, the stiffness of the 3DP-MREs was found to be increased with an increasing magnetic field. The change in the properties was more pronounced for 3DP-MRE2 than that of 3DP-MRE1, which is because of the presence of the MR fluid filaments. Experimental results suggested that the division of bulk volume of MR fluid into smaller dots could be beneficial as smaller dots are less susceptible to sedimentation and leakage, however, MR effect can be maintained to a certain level. Furthermore, 3DP-MREs also exhibited an anisotropic behavior when the direction of the magnetic field was varied with respect to the orientation of the printed dots. It is verified that the multi-materials 3D printing can provide the exact and precise control of various configurations of MR fluid dots or magnetic particles within the elastomeric matrix without applying a magnetic field.
[16]
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[22]
Acknowledgments [23]
This work was supported by the Academic Research Funds (RG189/ 14) from the Ministry of Education, Singapore.
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Appendix A. Supplementary data [26]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jmmm.2019.165825.
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Research articles
Dot-patterned hybrid magnetorheological elastomer developed by 3D printing A.K. Bastolaa,b, M. Paudela,b, L. Lia, a b
T
⁎
School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore Institute for Sports Research, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore
A R T I C LE I N FO
A B S T R A C T
Keywords: 3D printing Magnetorheological elastomers MR effect Stiffness
This article presents the development of dot-patterned magnetorheological (MR) elastomers (MREs) via 3D printing technology and their magnetorheological characterization. The 3D printed MR elastomer consists of three different materials; magnetic particles, magnetic particles carrier medium, and an elastomer. In such 3D printing, a controlled volume of MR fluid is encapsulated layer-by-layer within the elastomer matrix. The capability of 3D printing technology has been successfully demonstrated by developing the various dot patterns MR elastomers namely isotropic, anisotropic and configurations inspired from basic crystal structures such as BCC and FCC. The magneto-mechanical properties of such 3D printed MR elastomers (3DP-MREs) are studied using a cyclic compression and through a forced vibration testing. In the presence of a magnetic field, a clear change in stiffness of 3DP-MREs has been achieved. Moreover, the anisotropic behavior of 3DP-MREs has also been demonstrated. The experimental results suggested that the 3D printing method makes it possible to develop various structured MREs even without applying a magnetic field during the fabrication process.
1. Introduction In recent decades, additive manufacturing or three-dimensional (3D) printing has been the hot cake of research in several fields including but not limited to construction, footwear, automotive, aerospace, dental and medical industries. 3D printing technology is a fabrication method where the structures are constructed in the layer by layer fashion. The fast and precise manufacturing process, suitable to produce highly customizable products, has also opened the door for additive manufacturing of the smart materials and structures [1–3]. The smart materials are such materials that can change their shapes or properties under the influence of external stimuli such as temperature, electric or magnetic field, and pH, etc. The additively fabricated smart materials or structures can alter their shape or properties over time. Therefore, the fabrication of smart materials or structure via 3D printing is often regarded as 4D printing [2–4]. In recent years, one of the smart materials, magnetorheological (MR) elastomers have gained considerable attention because of the various advantages over the MR fluids such as leakage, the requirement of storage and environmental contaminations [5–10]. A plethora of research works have been performed to explore the various properties of MR elastomers in order to seek the possibility of implementing them in the real engineering system. Some of the potential applications of MR ⁎
elastomers include active vibration absorbers, vibration isolators, sandwich beams, soft actuators or sensors [7,8,11–14]. The MR elastomers are composed of magnetically polarizable micron-sized particles embedded within the polymeric elastomer [15–18]. Typically, to fabricate such MR elastomers, the mixture of the magnetic particles and elastomer resin is allowed to cure in a mold. A magnetic field is often applied during the crosslinking process in order to align the magnetic particles along the direction of the magnetic flux and are locked on the final curing. If the magnetic field is applied during the crosslinking process the MR elastomer is regarded as anisotropic MREs otherwise isotropic MREs are produced. In recent decades, these two types of MREs have been extensively studied [12,13]. It has been also proven that the anisotropic MREs exhibit higher MR effect when the direction of the applied magnetic field is parallel to the chains of magnetic particles. But the need of magnetic field is unavoidable to develop such anisotropic MREs. A few techniques to develop MREs include injection molding [19], compression molding [20] cast molding [20,21], and vacuum-assisted resin transfer molding [21,22]. On the other hand, to avoid the need of a magnetic field for fabricating anisotropic MR elastomer researchers have also developed patterned magnetorheological elastomer using conventional molding or manual patterning [23]. However, all methods used in the literature to fabricate MR elastomers are unable to exactly control the dispersion and
Corresponding author. E-mail address: [email protected] (L. Li).
https://doi.org/10.1016/j.jmmm.2019.165825 Received 28 May 2019; Received in revised form 23 August 2019; Accepted 9 September 2019 Available online 10 September 2019 0304-8853/ © 2019 Published by Elsevier B.V.
Journal of Magnetism and Magnetic Materials 494 (2020) 165825
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Four different amounts of current (0, 1, 2, and 3 A) to electromagnet was supplied to generate the various amount of magnetic flux density. Tests were conducted at 0.1 Hz cyclic frequency and 10% strain level. The raw data were collected using the Instron Bluehill 3 software then analyzed and plotted in Microsoft Excel 2019. The detail of the forced vibration experimental setup can be found in the previous article [24]. Forced vibration testing was conducted at four different amounts of the magnetic flux densities (0, 110, 300 and 500 mT). The various magnetic flux densities could easily be generated by changing the distance between two permanent magnets. The MRE samples were tested using the frequency sweep (from 50 Hz to 200 Hz) at a constant acceleration amplitude of 0.5 g. The MRE sample is in a squeeze mode of the operation and the magnetic field was applied parallel to the printed layers. The MRE sample was clamped on the base of the shaker, which forms a single degree of freedom system. Two accelerometers were used, one records the excitation signal and the other records the response signal. Transmissibility curve was obtained with the help of the excitation and response signals. Thereafter, the transmissibility curves were utilized to obtain elastic properties and then the MR effect of 3D printed MREs. Fig. 1(c) provides the direction of the applied magnetic field during the experiments. The application of the magnetic field for 3D printed MREs can be understood with respect to the plane of the printed MR layers. In cyclic compression, the magnetic field is applied in the normal direction to the printed MR layers. In forced vibration testing, the magnetic field is applied parallelly to the printed MR layers.
distribution of magnetizable particles. Hence, an innovative fabrication technique has to be looked for, which must be able to easily control the configuration of magnetic particles in MR elastomer without applying a magnetic field. One of the possible ways to configure magnetic particles without applying a magnetic field is to use 3D printing method. The biggest advantage of 3D printing is its ability to exactly control where and what to be deposited on a substrate from a printing head. Thus, 3D printing or additive manufacturing could be the only and best way to add magnetizable particles exactly to the pre-desired locations in an MR elastomer to be fabricated without applying a magnetic field. Recently, in 2017, our group has introduced a 3D printing method to fabricate hybrid MR elastomers [24,25]. In such a 3D printing method, MR fluid filaments are configured layer-by-layer inside the elastomer matrix. Various patterns of MR fluid such as line, circle and dots could be printed without applying a magnetic field. 3D printing of MR elastomer was achieved by means of multi-material printing. Two nozzles were used. One of the nozzles is used to dispense the elastomer resin and the other nozzle prints various patterns of MR fluids on top of the elastomer layer. The details of the 3D printing process used to develop hybrid MR elastomers can be found in the previous work [24]. In conventional anisotropic MR elastomers, magnetic particles line up in chains in the direction of an external magnetic field, thus, anisotropy has been observed in both mechanical and magnetic properties [12,13]. However, as presented in our previous study [24], 3D printing of MRE could only provide the anisotropy of magnetic field dependent mechanical properties such as stiffness by 3D printing various spatial structures of magnetic particles within the elastomer matrix. The anisotropic MREs developed in this study are not exactly same as that of conventional anisotropic MREs, however, are an alternative to conventional anisotropic MREs. In this work, we report the fabrication, and characterization of a new type of the configuration of the MR fluid within the elastomer matrix via 3D printing method, namely dot patterns. A small amount of the MR fluid is dispensed from the nozzle at a distinct locus, thus, patterns do not have a continuous structure rather they are disjoint to each other. Firstly, the development of various dot patterns including isotropic, anisotropic, BCC and FCC configurations using 3D printing are presented. Thereafter, magneto-mechanical characterizations of such dot patterns 3D printed MR elastomers are presented. The magneto-mechanical characterization was studied under a cyclic compression loading and through a force vibration technique without and with the application of a magnetic field.
4. Results and discussion Firstly, the details of 3D printing of various dot-patterns and development of BCC and FCC structures are discussed. Thereafter, MR effects of 3D printed MR elastomers are presented as achieved from a cyclic compression and a forced vibration testing. 4.1. 3D printing The hybrid MR elastomer samples were obtained by multi-material printing as illustrated in Fig. 2. A controlled volume of the MR fluid was precisely encapsulated within the elastomeric layer. Again, the detail of the printing process can be found in our previous work [24]. The printing of distinct dots is very similar to ink-jet printing. So, the printing can also be referred as the discontinuous printing process. The work explores the capability of the 3D printing to control the size, location, and distribution of MR fluid dots to produce various hybrid MR elastomers such as isotropic, anisotropic, BCC and FCC structures. Firstly, a fixed volume of the MR fluid was divided into 5 different amounts, as 1, 1/4, 1/8, 1/16 and 1/32, henceforth, the 3D printed MR elastomers are named as Dot_1, Dots_4, Dots_8, Dots_16, and Dots_32 respectively. All five samples have the same amount of the MR fluid printed however configured in a different way with disjoint dots as shown in Fig. 3. The Dot_1 sample was produced by combining the four dots. The size and the location of MR fluid dots were controlled by controlling the printing pressure, initial height, feed rate and time. The crucial printing parameter for controlling the size of dots is the printing time. All other parameters (extrusion pressure, initial height, and feed rate) were kept constant and only dispensing time was controlled to control the size of the dot at the given location. Printing condition of all five different samples is given in Table 1. As can be seen from Table 1, the total time to print the MR fluid layer is the same, while the dispensing time per dot can be seen in the geometric series for different samples (i.e. printing time per dot = a*r^n − 1, where a = 3, r = 2 and n = 1, 2, 3, 4 and 5 for Dots_32, Dots_16, Dots_8, Dots_4 and Dot_1 respectively). The SEM image of the cross-section of the 3D printed samples is presented in Fig. 4. The MR fluid dots are sandwich between the elastomer layers. As given in Fig. 5, the base of the printed dots is very close to a
2. Materials A UV curable silicone is used as an elastomer matrix which serves as a solid matrix of 3D printed hybrid MR elastomer samples. The homogenous mixture of the magnetic particles and a highly viscous silicone is used as a printing material. Carbonyl iron powders (CIP) of size 3–5 µm were used as magnetic particles. As the materials used are the same as in the previous work [24], the details of the materials used can be found in the same article. 3. Magneto-mechanical testing The 3D printed MR elastomer samples were characterized using a cyclic compression loading and through a forced vibration testing to study the magneto-mechanical properties in both the absence and presence of a magnetic field. Schematic illustration of the application of a magnetic field and the photograph of experimental equipment are given in Fig. 1. The detail of the cyclic compression setup can be found in the previous article [26]. Cyclic compression was conducted with a universal testing Instron 5569 equipped with a 500 N load cell with an electromagnet to apply a magnetic field during the testing. The electromagnet comes with an air gap where the MRE sample can be placed. 2
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Fig. 1. Experimental equipment (a) Cyclic compression: schematic of the magnetic circuit and a photograph of the experimental setup as connected to a load cell. (b) Forced vibration testing: illustration of the magnetic field direction and a photograph of experimental setup connected to the control box and data acquisition system. (c) Illustration of the direction of the magnetic field with respect to the orientation of the printed MR layers.
printing process is summarized in Table 3. The number of dots is higher in FCC structure and size of the dots are bigger for BCC structure.
perfect circle and the size of dots is increased with increasing printing time. The size of the printed dots is analyzed based on two dimensions: diameter and peak height of the dots. The average diameter and average peak height of the dots are presented in Table 2. Both diameter and peak height are increased with increasing printing time. The curve fitting shows that both dimensions (diameter and height) increase linearly with increasing time. In the second phase, to demonstrate the potential of 3D printing, the MR fluid dots were printed in such a way that the dots form basic crystal structures of the elements such as body-centered cubic (BCC) and face-centered cubic (FCC). BCC and FCC structures were selected in this study due to the ease of configuration of MR dots. The atoms of BCC and FCC unit cell are replaced by MR dots. Printing of the unit cell was performed by printing three MR layers. The morphology of 3D printed hybrid MREs with BCC and FCC structure is shown in Fig. 6. As can be seen in Fig. 6, for BCC structure, 1st and 3rd layers are identical and have 4 MR dots while the 2nd layer only has 1 MR dot per unit cell. Similarly, for FCC structure, 1st and 3rd layers are also identical and have 5 MR dots while the 2nd layer has 4 MR dot per unit cell. Total of 9-unit cells were combined and printed within the elastomer matrix. The total volume of the printed MR fluid has been kept the same for both BCC and FCC structures. The MR fluid volume was again controlled by controlling the dispensing time, the information of the
4.2. Magneto-mechanical characterization As in our previous work [24], there are two types of hybrid MR elastomer developed by 3D printing. The detail of 3D printed samples can be found in the same article. The first type of 3D printed MR elastomer (3DP-MRE) must completely solidify after being printed, whereas in the second type of 3DP-MRE the encapsulated MR fluid dots should remain fluid even after printing. Hereafter, the 3D printed MR elastomer with solid MR structures inside the elastomer matrix is regarded as 3DP-MRE1 and the 3D printed MR elastomer with MR fluid structures inside the elastomer matrix is regarded as 3DP-MRE2. 4.2.1. Cyclic compression Cyclic compression was performed to study the effect of patterning the MR dots within the elastomer matrix in both cases: without and with the application of a magnetic field. A typical stress-strain loop under cyclic compression is given in Fig. 7. Firstly, the effect of the magnetic field can be visualized as the slope of the stress-strain curve is increased with increasing current to the electromagnet. Similarly, the second way to visualize the magnetic field effect is to study the change 3
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Fig. 2. Schematic illustration of dot patterned MRE fabrication via 3D printing.
divided by strain at any given value of stress or strain. It also is called the stress-strain ratio. Small non-linearity in stress-strain response can provide a bigger variation in tangent modulus as it is a slope at a point, so tangent modulus becomes highly biased. The unbiased modulus is secant modulus as it is simply a ratio of stress and strain. Thus, in this study, in order to investigate the MR effects, the stress-strain curves are analyzed by obtaining secant modulus. Absolute and relative MR effect for all five types of samples (Dot_1, Dots_4, Dots_8, Dots_16, and Dots_32) of both categories (3DP-MRE1 & 3DP-MRE2) are given in Fig. 8. 3DP-MRE2 samples showed higher MR effect than that of 3DP-MRE1. The MR effect is increased with increasing current to the electromagnet, with Dot_1 sample achieving the highest MR effects. This is attributed to the bulk volume of the MR fluid. MR effect curves start from almost zero or minus in the smallstrain region that increase and remains constant within 10% strain. The minus MR effects are most visible in low strain region, this is because of remnant deformation of magnetic particle chains or even the elastomeric case upon the cyclic compression, a similar type of finding has been reported by Schubert and Harrison [32]. Hence, causing some amount of plastic strain. Nevertheless, the MR effect increased to a positive value and became stable within 10% strain. The maximal MR effects from Fig. 8 are recorded and plotted against the different type of sample as shown in Fig. 9. As shown in Fig. 9, for 3DP-MRE1 samples, the relative MR effect is smaller than that of 3DP-MRE2 samples and is the highest for Dot_1 sample and is decreased with an increasing number of dots. Similarly,
in the area under the loading and unloading curves. In the absence of a magnetic field, 3DP-MRE1 has a higher modulus than the 3DP-MRE2. For five different samples (Dots_32, Dots_16, Dots_8, and Dots_4 and Dot_1), they almost have a similar modulus for 3DP-MRE1, however, for 3DP-MRE2 samples, it was also noted that the Dot_1 sample has a slightly lower modulus than others. Upon the application of a magnetic field, 3DP-MRE1 samples showed very small changes, while the 3DP-MRE2 sample showed noticeable changes. This is attributed to the free magnetic particles within the printed fluid dots of 3DP-MRE2 samples, the free magnetic particles can freely move and make chains along the magnetic field. The effect of the magnetic field can be studied by obtaining the absolute and relative MR effect. The absolute MR effect is defined as the change in the modulus by the application of a magnetic field.
MRabs = EB − E B0
(1)
where EB and E B0 are the moduli with and without the application of a magnetic field, respectively. Secondly, relative MR effect is defined as the relative change between properties
MRrel = (EB / E B0 − 1)
(2)
Relative MR effect can also be expressed as (EB / E B0 − 1) × 100% when defined as a percentage value. The moduli such as secant and tangent moduli are obtained to study the MR effect of current MREs [27–32]. The tangent modulus is the slope of the stress-strain response at any point. Secant modulus is stress
Fig. 3. 3D printed MR elastomers with different sized MR fluid dots, Dots_32, Dots_16, Dots_8, Dots_4, Dot_1 and pure elastomer from right to left respectively. 4
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Table 1 Summary of the printing parameters for Dots 32, Dots 16, Dots 8, Dots 4 and Dot 1 samples. Sample
Extrusion pressure (Bars)
Initial height (mm)
Feed rate (mm/min)
Dispensing time per dot (s)
Total time per MR fluid layer (s)
Dot 1 Dots 4 Dots 8 Dots 16 Dots 32
2.1 2.1 2.1 2.1 2.1
1.2 1.2 1.2 1.2 1.2
150 150 150 150 150
96 24 12 6 3
96 96 96 96 96
anisotropic sample and the sample obtained by offsetting the MR layer is regarded as isotropic. The secant moduli obtained from the cyclic compressive testing for the spatially structured samples are given in Fig. 10(b). For 3DP-MRE2, in the absence of the magnetic field, moduli of isotropic and anisotropic samples are similar, however, in the presence of a magnetic field, isotropic sample significantly lags the increase in modulus compared to the anisotropic sample. This result demonstrates that the 3D printing method can also be used to develop isotropic and anisotropic MR elastomers. The maximum MR effect for the isotropic sample was found to be 0.23, where anisotropic sample showed the MR effect of 0.75, these values are for 3DP-MRE2 samples. Similarly, moduli for BCC and FCC structured samples of both categories in the absence and presence of a magnetic field are given in Fig. 11. The main aim of developing BCC and FCC structure is to study the MR effect of the same amount of the MR materials but configured differently inspired by basic crystal structures. Both BCC and FCC structure have the same amount of MR materials. Cyclic compression result revealed that BCC structure showed slightly higher MR effect than that of FCC structured 3DP-MREs. This is believed that the effect of size of the MR dots, the BCC patterns have bigger dots and, therefore, must have shown higher MR effect.
for the 3DP-MRE2 category, the MR effect is also decreased as the size of the MR dots decreases and the MR effect becomes almost similar for Dots_8, Dots_16, and Dots_32 samples. While the Dots_4 sample showed almost similar MR effect as that of Dot_1 sample, while the MR effect is considerably decreased (almost by 50%) for Dots_8, Dots_16, and Dots_32 samples. The experimental results suggested that the division of the given amount of MR fluid into smaller dots could uphold a reasonable MR effect. Such division is also beneficial when leakage and particle settling are concerned. The finding suggested that dividing the given volume of MR fluid into 4 dots can maintain a similar MR effect as that of a single MR dot. On the other hand, increasing the number of dots beyond 4 is not recommended if the MR effect is the main concern, however, can be considered if the leakage and sedimentation are the main concern. Thereafter, two different types of spatially structured dot-patterned samples are developed, by off-setting the specified printing location in the successive MR layers, picture and illustration are presented in Fig. 10(a). Hereafter, for easiness of evoking the samples, the spatially structured samples are named as anisotropic and isotropic, the names are given by only considering the orientation of the printed dots. It should be noted that the isotropic and anisotropic samples named in this study are different than that of conventional anisotropic MREs. The sample printed without offsetting the MR layers is regarded as the
Fig. 4. Crossectional micrograph of 3D printed MR elastomer under SEM. 5
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Fig. 5. Variation of the size of the dots. (a) diameter and (b) height Dots_32, Dots_16, Dots_8, and Dots_4.
4.2.2. Forced vibration test MR effects of 3DP-MREs were also studied through forced vibration testing. Four different magnetic flux densities (0, 110, 300 and 500 mT) were applied during the experiment. The direction of the magnetic flux is parallel to the printed layers as shown in Fig. 12(a). Typical results of the forced vibration testing of dot-patterned 3DPMRE are presented in Fig. 12(b). The magnetic field was applied as illustrated in Fig. 12(a). Here, the magnetic field is parallel to the plane of the printed layers. A number of features can be noted in Fig. 12(b). Firstly, when the magnetic field is not applied, the transmissibility
Table 2 Summary of the variation of the size of the printing dots: diameter and height. Sample
Dots Dots Dots Dots
4 8 16 32
Size of the dots Diameter (mm)
Height (mm)
5.1 4.2 3.6 2.6
1.8 1.6 1.4 1.2
± ± ± ±
0.2 0.15 0.1 0.1
± ± ± ±
0.1 0.1 0.1 0.1
Fig. 6. (a) Unit cell of BCC and FCC structures and (b) Morphology of the BCC and FCC structured obtained via 3D printing. 6
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Table 3 Summary of the printing condition for BCC and BCC MR elastomers. Sample
Extrusion pressure (Bars)
Initial height (mm)
Feed rate (mm/min)
Dispensing time per dot (s)
Total number of dots
Total time (s)
BCC FCC
2.1 2.1
1.2 1.2
150 150
6 4
41 62
246 248
Fig. 7. Stress-strain loops for Dots_4 samples of two categories 3DP-MRE1 (left) and 3DP-MRE2 (right) at 4 different amounts of current to the electromagnet.
Fig. 8. Absolute and relative MR effect, calculated with secant modulus, for all five different patterns of 3DP-MRE1 (top figures) and 3DP-MRE2 (bottom figures), achieved with 3 different amounts of current to the electromagnet versus compressive strain.
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Fig. 9. Relative MR effect achieved with 1A, 2A and 3A current and obtained using Eq. (2) are listed for all five types of samples (Dot_1–Dots_32) of two categories. 3DP_MRE1 (left) and 3DP-MRE2 (right) samples. These MR effect values were obtained from the relative MR effect curves that are given in Fig. 8.
Fig. 10. (a) Illustration of spatially structured samples and the application of a magnetic field. (b) Secant moduli for spatially structured samples of both categories.
Fig. 11. Results of the BCC and FCC structure 3D printed MR elastomer of both categories, 3DP-MRE1 (left) and 3DP-MRE2 (right).
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Fig. 12. Magnitude transmissibility versus excitation frequency of the dot-patterned 3DP-MREs at various magnetic flux densities, 3DP-MRE1 (left) and 3DP-MRE2 (right).
Fig. 13. Stiffness and damping ratio versus magnetic flux density five types of dot sample of both 3DP-MRE1 and 3DP-MRE2.
the damping ratio of the samples are almost constant or slightly decreased with the increasing magnetic field as the peak of the transmissibility curve is almost same or increased upon the application of a magnetic field. The effect of changing the size of the dots are discussed by obtaining the stiffness and damping ratio as given in Fig. 13. When the magnetic field is not applied, as obtained in cyclic compression, it was found that the 3DP-MRE1 samples are stiffer than that of 3DP-MRE2 samples. Similarly, the 3DP-MRE1 samples have a low damping ratio than that of 3DP-MRE2 samples. Dots_1 sample was found to be least stiff compared to other samples for 3DP-MRE2 samples and the stiffness was found to be increased as the size of MR dots decreased. This can be attributed to
curves of the two types of samples (3DP-MRE1 and 3DP-MRE2) are easily distinguishable. The 3DP-MRE1 samples have higher natural frequency and higher peaks at the natural frequency. Higher natural frequency signifies that the 3DP-MRE1 samples are stiffer than that of 3DP-MRE2 samples and higher peak of transmissibility curve indicates that the damping ratio of the 3DP-MRE1 is smaller than that of the 3DPMRE2. Upon the application of a magnetic field, the transmissibility curves are moving toward the right. Shifting of the curves toward right indicate that stiffness of the 3DP-MRE is increased [24]. Again, the shifting of the curve is more pronounced for 3DP-MRE2 samples. Yet again, this must be attributed to the free magnetic particles that are suspended within the dots of 3DP-MRE2 samples. On the other hand, 9
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Fig. 14. (a) Illustration of the direction of the applied magnetic field. (b) Stiffness versus magnetic flux density isotropic and anisotropic samples of both 3DP-MRE1 (left) and 3DP-MRE2 (right).
Fig. 15. Stiffness versus magnetic flux density FCC and BCC samples of both 3DP-MRE1 (left) and 3DP-MRE2 (right).
should be noted that if the same amount of volume of MR fluid is divided into the different amount of volume the MR effect gets affected. Nonetheless, it is worthy that if dots are divided into four parts, the MR effect would not significantly decrease. This is also one of the reasons that the printing of four dots are considered to develop isotropic and anisotropic samples. In the cyclic compression, the magnetic field was applied normal to the printed layer as given in Fig. 10(a). Whereas, in the forced vibration testing, the magnetic field is applied as given in Fig. 14(a), which is to observe the difference of isotropic and anisotropic configuration in MR effect when magnetic field is applied parallel to the printed layers. The stiffness achieved via frequency sweep with isotropic and anisotropic samples of both 3DP-MRE1 and 3DP-MRE2 is given in Fig. 14(b). The
the size of the dots; as the smaller fluid dots could behave like viscoelastic solid dots. On the other hand, for 3DP-MRE1 samples, the stiffness was found to increase up to 8 dots and start to decrease with an increasing number of dots. This can be attributed to the soft elastomeric casing where the presence of smaller dots could not significantly alter the stiffness of the elastomer matrix. Upon the application of a magnetic field, MR effect exhibited by 3DP-MRE1 samples was found to be much smaller compared to 3DP-MRE2 samples and the effect of size of the dots are not significant. Whereas for 3DP-MRE2, MR effect was found to be decreased with an increasing number of dots. The Dot_1 sample has relative MR effect of 77% while Dots_32 sample only has 28% at 500 mT. However, the difference between Dots_4 and Dot_1 sample is not very significant, Dots_4 has 70% MR effect at 500 mT. Here, it 10
Journal of Magnetism and Magnetic Materials 494 (2020) 165825
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MR effect exhibited by the isotropic and anisotropic 3DP-MREs is found to be similar when the magnetic field was applied parallel to the printed layers. Yet again, the MR effect is higher for 3DP-MRE2 samples. This result provides the evidence that the anisotropic nature chiefly depends upon the direction of the applied magnetic field and 3D printing has the capability to configure the MR fluid or magnetic particles in a unique fashion that MRE exhibits anisotropic nature. Similarly, the results of BCC and FCC structured 3DP-MREs samples are given in Fig. 15. As achieved in cyclic compression result, the significant difference of BCC and FCC structure was also not observed through the vibration testing even when the magnetic field was applied parallel to the printed layers. However, BCC structures seemingly showed slightly higher MR effect than that of FCC, which is again attributed to the higher size of MR dots as described previously. Even though the difference in MR effect was not observed by printing the basic crystal structures such developments provide the capability of a 3D printing method, which are difficult to be performed by other fabrication technique even with the in-situ application of a magnetic field.
[6]
[7]
[8]
[9]
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[11]
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5. Conclusions
[14] [15]
The capability of 3D printing technology for the fabrication of various structured dot-patterns are presented. Like the current MREs, the stiffness of the 3DP-MREs was found to be increased with an increasing magnetic field. The change in the properties was more pronounced for 3DP-MRE2 than that of 3DP-MRE1, which is because of the presence of the MR fluid filaments. Experimental results suggested that the division of bulk volume of MR fluid into smaller dots could be beneficial as smaller dots are less susceptible to sedimentation and leakage, however, MR effect can be maintained to a certain level. Furthermore, 3DP-MREs also exhibited an anisotropic behavior when the direction of the magnetic field was varied with respect to the orientation of the printed dots. It is verified that the multi-materials 3D printing can provide the exact and precise control of various configurations of MR fluid dots or magnetic particles within the elastomeric matrix without applying a magnetic field.
[16]
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[20] [21]
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Acknowledgments [23]
This work was supported by the Academic Research Funds (RG189/ 14) from the Ministry of Education, Singapore.
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Appendix A. Supplementary data [26]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jmmm.2019.165825.
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