Material Characterization of a Magnetorheological Fluid Subjected to Long-Term Operation in Damper

materials

Article Material Characterization of a Magnetorheological Fluid Subjected to Long-Term Operation in Damper

Dewi Utami 1, Ubaidillah 2,3,* , Saiful A. Mazlan 1, Fitrian Imaduddin 2 , Nur A. Nordin 1, Irfan Bahiuddin 1,4 , Siti Aishah Abdul Aziz 1, Norzilawati Mohamad 1 and Seung-Bok Choi 5,* 1 Malaysia-Japan International Institute of Technology, Universiti Teknologi Malaysia, Jalan Sultan Yahya Petra, 54100 Kuala Lumpur, Malaysia; [email protected] (D.U.); [email protected] (S.A.M.); [email protected] (N.A.N.); [email protected] (I.B.); [email protected] (S.A.A.A.); [email protected] (N.M.) 2 Mechanical Engineering Department, Faculty of Engineering, Universitas Sebelas Maret (UNS), Jl. Ir. Sutami 36A, Kentingan, Surakarta, 57126 Central Java, Indonesia; ﬁ[email protected] 3 National Center for Sustainable Transportation Technology (NCSTT), 40132 Bandung, Indonesia 4 Department of Mechanical Engineering, Vocational College, Universitas Gadjah Mada (UGM), Jl. Yacaranda Sekip Unit IV, 55281 Yogyakarta, Indonesia 5 Department of Mechanical Engineering, Inha University, 253, Yonghyun-dong, Namgu, 22212 Incheon, Korea * Correspondence: [email protected] (U.); [email protected] (S.-B.C.); Tel.: +62-857-9952-7552 (U.); +82-32-860-7319 (S.-B.C.) Received: 8 October 2018; Accepted: 1 November 2018; Published: 6 November 2018

Abstract: This paper investigates the field-dependent rheological properties of magnetorheological (MR) fluid used to fill in MR dampers after long-term cyclic operation. For testing purposes, a meandering MR valve was customized to create a double-ended MR damper in which MR fluid flowed inside the valve due to the magnetic flux density. The test was conducted for 170,000 cycles using a fatigue dynamic testing machine which has 20 mm of stroke length and 0.4 Hz of frequency. Firstly, the damping force was investigated as the number of operating cycles increased. Secondly, the change in viscosity of the MR fluid was identified as in-use thickening (IUT). Finally, the morphological observation of MR particles was undertaken before and after the long-term operation. From these tests, it was demonstrated that the damping force increased as the number of operating cycles increases, both when the damper is turn on (on-state) and off (off-state). It is also observed that the particle size and shape changed due to the long operation, showing irregular particles.

Keywords: MR fluid; long-term operation; double-ended damper; magnetic field; rheological properties; morphology observation

1. Introduction It is known that magnetorheological (MR) fluid belongs to one of the smart materials featuring the magnetic field-dependent rheological properties and, hence, control performance of various application systems in which MR fluid is used as carrier fluid medium can be achieved by simply controlling the magnitude of the field intensity [1–4]. More specifically, MR fluid behaves like a Newtonian fluid (fluid-like phase) in the absence of the magnetic field, but its behavior rapidly changes to the non-Newtonian fluid (solid-like phase) by applying the magnetic field [5–9]. This special characteristic of MR fluid can provide a quiet, simple, and rapid-response interface between electronic controls and mechanical systems [10]. So far, numerous application devices and systems, such as automotive

Materials 2018, 11, 2195; doi:10.3390/ma11112195 www.mdpi.com/journal/materials Materials 2018, 11, 2195 2 of 17 damper, have been investigated, and some of the systems are used in the practical environment. Some of the specific applications include truck seat damper [11], a prosthetic knee joint [12], and fan clutches [13]. Although these devices or systems have shown great performances, some problems were not evident in the initial phase, and need to be addressed at a later time. One of the major problems is the phenomenon called in-use-thickening (IUT), where the fluid is permanently damaged after long-term operation [11]. The IUT phenomenon frequently happens when the MR fluid is subjected to high shear rate and high shear stress over time. The original low viscosity MR fluid will thicken and change into an unmanageable paste-like material. Therefore, under these circumstances, MR fluid is no longer suitable for controlling the performances of the applied devices and systems [6,11,14,15]. The early IUT problem was observed in the truck seat damper, type Motion Master™ RD-1005 made by Lord Corporation, Cary, NC, USA [11]. The damper was continuously operated under 1200 N force with a frequency of 1 Hz, 25.4 mm stroke length, and 1 A of applied current to generate the magnetic field, for 600,000 cycles. The damper’s operation was periodically varied between on-state and off-state conditions, by applying a certain current during the observation. The result revealed that the initial force (off-state) increased more than 100% after the operation: from 200 to 500 N. The initial force increase caused by the viscosity increment of MR fluid is not acceptable for the performance of the damper, since it reduces rider’s comfort. For the case of the prosthetic knee joint MR device, it is generally known that the proper performance should last for 3,000,000 cycles [12]. However, the IUT effect has made MR fluid unusable once the device reaches 1,000,000 cycles. The prosthetic knee joint is one of the applications of MR fluid operated under direct shear mode [15]. This work identified that the knee joint stiffness increased with increasing of operation time and number of cycles. Such problems occurred as the off-state torque force of the knee joint increased in long-term operation until, eventually, the knee joint needed to be replaced due to a decrease in effectiveness. The degradation of the knee joint effectiveness is likely related to the degradation of the MR fluid caused by the IUT phenomenon. Other studies also reported similar IUT problems with most MR devices and systems after long-term operation applications [16–18]. Of interest is a study conducted on long-term operation of an MR fan clutch, which operated for 540 h to observe the change in MR fluid characteristics [13]. The study focused on analysis of the iron particles within the MR fluid. Three samples were observed during the durability test: one before the test, one after 108 h, and the last one after 540 h. The result showed that the torque of the fan decreased by 15% for both 3000 and 5000 rpm at the end of 540 h. Through the observation, it was identified that the magnetic particles (irons) in the MR fluid were oxidized over the time. Further observations indicated that the surface of the magnetic particles showed porous layers and, appeared rough as the result of the oxidation process. Thus, oxidation of the magnetic particles can be considered another issue that needs to be resolved in MR device application and systems subjected to long-term operation. The above studies showed the significance of material characteristics of MR fluid when it is utilized for a long time as the main function for controlling force or torque in diverse applications. In other words, the durability or reliability of MR fluid itself plays a critical role in the lifetime operation of MR applications. For example, an MR device could not be used when the problems of IUT and particles oxidation of MR fluid occurred, even though the devices might still in good condition. More importantly, the performances of MR devices are critically affected by MR fluid condition. As commercialization of MR applications expands, the endurance of MR products becomes a more important issue to be resolved. However, research on material characterization of MR fluid used in long-term operation is considerably rare. Consequently, the main technical contribution of this work is to experimentally investigate material properties of MR fluid used as the main carrier fluid to generate field-dependent force or torque for long-term operation. In order to accomplish this goal, MR damper operated as a flow mode is adopted and operated for a long time. After completing the operation, the MR fluid is collected, and its field-dependent viscosity tested. In addition, the particle shapes are observed via scanning electron microscope (SEM) and the surface roughness of the MR damper’s spacer bar is investigated to determine the wear effect of the iron particles. It is remarked, Materials 2018, 11, 2195 3 of 17 here, that the material characterization of MR fluid, which is operated for a long time in a flow mode MR damper, has not been reported, so far, to the best of the authors’ knowledge.

2. Experimental Setup

2.1. MR Fluid The MR ﬂuid used in this study is a commercial one, model MRC-C1L, supplied by CK Material Laboratory (http://www.ckmaterialslab.com), Seoul, Korea, for general applications such as dampers, brakes and shock absorbers. The ﬂuid contains magnetic particles with the weight ratio of approximately 78%, and the principal properties of MRC-C1L are listed in Table1.

Table 1. Principal properties of MRC-C1L.

Appearance Dark Grey Liquid Solid Weight Percentage (wt %) 77–80 Density (g/m3) 2.75–2.95 Shear Stress (kPa @ 1500/s 570 mT) 68.0 ± 7 Viscosity (Pa s) 0.106 + 0.020 Flash Point (◦C) >140 Operating Temperature (◦C) −40 to +140 Sedimentation Stability (vol %/30 days) 4.00

2.2. MR Damper and Testing Machine An experimental setup configuration for the MR valve measurement is established, as shown in Figure1. The details of MR damper are shown in Figure2. It was customized to accommodate the long-term operation experiment with detailed geometrical dimensions as follows: bore size = 50 mm, rod size = 30 mm, stroke length = 60 mm, and cylinder length = 500 mm. The rod and bore differ greatly in size, to emphasize volume of flowing MR fluid inside the MR damper for each stroke motion. The meandering MR valve structure of the damper is divided into three main components: casing, valve, and core. The casing, made from magnetic material, acting not only as an outer shell of the whole structure of the valve but, also, as the connector of the valve to other devices through its embedded fittings. The other functions of the casing are to guide the magnetic flux from the coil in order to minimize the loss of flux to the air, which reduces efficiency. The coil was wound around the coil bobbin, made from non-magnetic material, to generate the magnetic field in the MR valve structure. The core was used as a platform to provide an annular and radial flow channel for the path of MR fluid flow. The detailed configuration of the MR valve, including finite element magnetic simulation using FEMM, as well as magnetic flux distribution in all effective areas in the MR valve, is shown in Figure3. The MR valve consists of 5 effective areas, namely, annular 1, radial 1 to 4, and annular 2. It can be seen that the amount of magnetic flux density between annular and radial areas are different at the same applied current. Therefore, it cannot be mentioned, definitively, the conversion of a certain current flow to the value of magnetic fields. Therefore, the magnetic flux density during an on-state experiment, Table2 provides the amount of magnetic flux density in each region, at 1 and 2 A. This configuration allows the provision of high magnetic field strength at the fluid gaps through a combination of casing and core embedded with the bobbin, as fully described in [19]. Materials 2018, 10, x FOR PEER REVIEW 4 of 17 MaterialsMaterials2018 2018, 11,, 10 2195, x FOR PEER REVIEW 4 of4 17of 17

Table 2. Magnetic fields in magnetorheological (MR) valve. Table 2. Magnetic fields in magnetorheological (MR) valve. Table 2. MagneticFlux ﬁelds Density in magnetorheological in MR Valve Areas (MR) (T) valve. Current (A) Flux Density in MR Valve Areas (T) Current (A)Annular 1 Radial 1 Radial 2 Radial 3 Radial 4 Annular 2 Annular 1 RadialFlux Density 1 Radial in MR 2 ValveRadial Areas 3 (T) Radial 4 Annular 2 Current1 (A) 0.63 0.84 0.84 0.84 0.84 0.63 1 Annular 0.63 1 Radial 0.84 1 Radial0.84 2 Radial0.84 3 Radial0.84 4 Annular 0.63 2 0.8 0.52 0.72 0.72 0.72 0.72 0.52 10.8 0.630.52 0.84 0.72 0.840.72 0.840.72 0.840.72 0.63 0.52 0.6 0.38 0.58 0.58 0.58 0.58 0.38 0.80.6 0.52 0.38 0.72 0.58 0.720.58 0.720.58 0.720.58 0.52 0.38 0.5 0.31 0.50 0.50 0.50 0.50 0.31 0.60.5 0.38 0.31 0.58 0.50 0.580.50 0.580.50 0.580.50 0.38 0.31 0.40.5 0.24 0.31 0.43 0.50 0.43 0.50 0.43 0.50 0.43 0.50 0.24 0.31 0.4 0.24 0.43 0.43 0.43 0.43 0.24 0.20.4 0.13 0.24 0.24 0.43 0.24 0.43 0.24 0.43 0.24 0.43 0.14 0.24 0.20.2 0.13 0.13 0.24 0.24 0.240.24 0.240.24 0.240.24 0.14 0.14

Figure 1. Schematic configuration of MR valve testing. FigureFigure 1. Schematic1. Schematic conﬁguration configuration of MRof MR valve valve testing. testing.

Figure 2. Sketch of MR damper conﬁguration.

Figure 2. Sketch of MR damper configuration. Figure 2. Sketch of MR damper configuration.

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Figure 3. Schematic diagram for the meandering MR valve of the damper. Figure 3. Schematic diagram for the meandering MR valve of the damper. 2.3. Sample Characterization 2.3. Sample Characterization The field-dependent rheological properties, such as viscosity of MR fluid (MRC-C1L), are investigatedThe field-dependent before filling rheological the MR damper. properties, The rheologicalsuch as viscosity properties of ofMR MR fluid fluid (MRC-C1L), are investigated are investigatedvia a rotational before shear filling test the of MR the parallel-platedamper. The rheo rheometerlogical properties (Anton Paar, of MR Physica, fluid MCRare investigated 302, Graz, viaAustria), a rotational at room shear temperature. test of the The parallel-plate viscosity and rheometer shear rate (Anton of MR Paar, fluid Physica, is tested MCR at on-state 302, Graz, and Austria),off-state condition,at room temperature. using a 20 mmThe viscosity diameter and plate shea (PP20/MRD/TI).r rate of MR fluid The is on-statetested at conditionon-state and for off- the stateviscosity condition, and shear using rate a 20 are mm conducted diameter plate under (PP20/MRD/TI). the following conditions The on-state of condition the magnetic for the field: viscosity range andfrom shear 1 A and rate 2 are A, andconducted sweep shearunder rate the from following 0.01 to co 1000nditions s−1. From of the the magnetic experiment, field: the range magnetic from field1 A −1 andof 1 2 A A, is approximatelyand sweep shear 0.19 rate tesla, from and 0.01 2 A to approximately 1000 s . From 0.40the experiment, tesla. In addition, the magnetic the magnetic field of particles 1 A is approximatelyare extracted from 0.19 the tesla, MR and fluid 2 usingA approximately a solvent, acetone, 0.40 tesla. to characterize In addition, the the morphological magnetic particles properties are extractedbefore the from operation. the MR The fluid morphological using a solvent, characteristics acetone, to characterize related to size the and morphological shape of the properties magnetic beforeparticles the in operation. the MR fluid The were morphological observed using characteristics low vacuum related scanning to size electron and shape microscopy of the (LV-SEM),magnetic particlesmodel T300 in the LV MR from fluid Jeol, were Japan, observed with a magnificationusing low vacuum of 3000 scanning× and 5000 electron× at themicroscopy accelerating (LV-SEM), voltage modelof 5 kV. T300 Meanwhile, LV from Jeol, the corresponding Japan, with a magnification composition ofof magnetic3000× and particles 5000× at wasthe accelerating analyzed using voltage the ofenergy 5 kV. dispersive Meanwhile, X-ray the spectroscopycorresponding (EDS) composition equipped of inmagnetic the LV-SEM. particles The was same analyzed tests as theusing above the energymentioned dispersive were undertaken X-ray spectroscopy by collecting (EDS) MR equipp fluid fromed in the the damper LV-SEM. after The the same long-term tests as operation the above of mentioned170,000 cycles. were undertaken by collecting MR fluid from the damper after the long-term operation of 170,000 cycles. 2.4. Experimental Procedures 2.4. Experimental Procedures The experimental works are conducted using the fatigue dynamic test machine, made by ShimadzuThe experimental Company (Japan). works The are machine conducted shown using in Figure the fatigue4 is capable dynamic of generating test machine, compressed made and by Shimadzuextended forcesCompany of MR (Japan). damper. The MR machine damper shown is filled in with Figure MR 4 fluid, is capable where of the generating damper is compressed actuated by andthe dynamicextended test forces machine of MR and damper. the changed MR damper behavior is fi oflled the with fluid MR can fluid, be analyzed where the through damper the is measured actuated bystrokes the dynamic of the damper. test machine The experiment and the ischanged started onbeha thevior off-state of the condition fluid can by be setting analyzed the strokethrough length the measuredat 20 mm withstrokes a frequency of the damper. of 0.4 The Hz. Then,experiment for the is on-statestarted on condition, the off-state the magnetic condition field by setting of 0.5 Athe is strokecontinuously length at applied 20 mm for with the a longfrequency operation of 0.4 of Hz. 170,000 Then, cycles. for the Theon-state results condition, in terms the of dampingmagnetic forcefield ofversus 0.5 A number is continuously of cycles areapplied plotted for to the identify long oper the changeation of in 170,000 the damping cycles. forceThe generatedresults in terms from theof dampingyield stress force of the versus MR fluid.number Eventually, of cycles theare applied plotted currentto identify is cut the off change after the in last the cycle. damping After force that, generated from the yield stress of the MR fluid. Eventually, the applied current is cut off after the last cycle. After that, the experiment is conducted under off-state condition with a similar setup: the

Materials 2018, 11, 2195 6 of 17

Materials 2018, 10, x FOR PEER REVIEW 6 of 17 the experiment is conducted under off-state condition with a similar setup: the stroke length at 20 mm stroke length at 20 mm with a frequency of 0.4 Hz. The off-state damping forces before and after the with a frequency of 0.4 Hz. The off-state damping forces before and after the long-term operation are long-term operation are then compared. then compared.

Figure 4. Experimental setup using fatigue dynamic test machine, including MR valve and MR damper

that would be operating for 170,000 cycles. Figure 4. Experimental setup using fatigue dynamic test machine, including MR valve and MR 3. Resultsdamper and that Discussion would be operating for 170,000 cycles.

3.1.3. Results Long-Term and Exposure Discussion Figure5 shows the force versus displacement graph for the on-state condition of 0.5 A as a function of3.1. the Long-Term operating Exposure cycle under the fixed oscillation frequency of 0.4 Hz, and the excitation displacement amplitudeFigure of 5 20 shows mm. Thethe influenceforce versus of the displacement number of cycles graph on for the the MR on-state damper dampingcondition force of 0.5 is clearlyA as a observedfunction fromof the the operating results. Incycle particular, under thethe maximumfixed oscillation damping frequency forces are of identified 0.4 Hz, and by3.96, the excitation 4.38, 4.68, 5.07,displacement 5.04, 5.08, amplitude 5.15, 5.08, 5.16, of 20 and mm. 5.70 The kN influence for 1000; of 25,000; the number 50,000; of 75,000; cycles 100,000; on the MR 125,500; damper 150,000; damping and 170,000force iscycles, clearly respectively. observed from It shows the results. that the In lowest particular, peak forcethe maximum appears with damping the least forces number are identified of cycles, whileby 3.96, the 4.38, highest 4.68, force 5.07, value 5.04, is5.08 associated, 5.15, 5.08, with 5.16, the and largest 5.70number kN for 1000; of cycles. 25,000; In general,50,000; 75,000; the maximum 100,000; damping125,500; 150,000; forces proportionally and 170,000 cycles, increased respectively. with the increasingIt shows that operating the lowest cycles. peak The force phenomenon appears with can bethe further least number observed of incycles, Figure while6. As the shown highest in the force figure, value the is maximum associated values with ofthe the largest damping number force of increasecycles. In with general, the increasing the maximum number ofdamping cycles. Theforces peak proportionally damping force increased increment, with from the the beginningincreasing ofoperating the operation cycles. to The 170,000 phenomenon cycles, is can approximately be further ob 1.74served kN, in or Figure 44%, with 6. As respect shown to in the the initial figure, peak the force.maximum This resultvalues indicatesof the damping that the force utilized increase MR fluid with is the affected increasing by the number long-term of cycles. operation The of peak the MRdamping damper. force In fact,increment, from the from beginning the beginning of the operation of the operation cycles to to 170,000 170,000 cycles, cycles, the is forceapproximately increases, although,1.74 kN, or after 44%, 100,000 with respect cycles, to the the force initial slightly peak decreases.force. This Meanwhile,result indicates from that 75,000 the cyclesutilized to MR 150,000, fluid thereis affected is no considerableby the long-term difference operation in the of forcethe MR increment. damper. The In fact, result from also the shows beginning that, after of the 8000 operation cycles, thecycles damping to 170,000 forces cycles, remain the force almost increases, unchanged, although which, after has 100,000 revealed cycles, the damping the force forces’slightly saturation decreases. points.Meanwhile, This phenomenonfrom 75,000 cycles occurs to when 150,000, the there MR fluid is no is considerable continuously difference activated in by the the force magnetic increment. field forThe a result long duration. also shows Even that, though after 8000 the experiment cycles, the hasdamping already forces finished, remain it wasalmost assumed unchanged, that the which MR fluidhas revealed still under the thedamping influence forces’ of magnetic saturation field. points This. This may phenomenon be caused by occurs the increased when theviscosity MR fluid of is thecontinuously MR fluid. activated Figure7 shows by the the magnetic force-displacement field for a long results duration. for the Even off-state though condition, the experiment before and has afteralready the finished, long-term it operationwas assumed of the that MR the damper. MR fluid The still result under indicates the influence that the of peak magnetic force field. increases This may be caused by the increased viscosity of the MR fluid. Figure 7 shows the force-displacement results for the off-state condition, before and after the long-term operation of the MR damper. The result indicates that the peak force increases after the long-term operation, particularly after 170,000

Materials 2018, 11, 2195 7 of 17

Materials 2018, 10, x FOR PEER REVIEW 7 of 17 afterMaterials the 2018 long-term, 10, x FOR operation, PEER REVIEW particularly after 170,000 cycles. The initial force of MR damper7 of 17 is approximately 2 kN, and increases to 3.8 kN after the long operation. This increment of the initial cycles. The initial force of MR damper is approximately 2 kN, and increases to 3.8 kN after the long force phenomenon indicates that the properties of the MR ﬂuid are affected by the continuous load in operation. This increment of the initial force phenomenon indicates that the properties of the MR a long-term operation, even for the off-state condition. This might be due to the increased viscosity of fluid are affected by the continuous load in a long-term operation, even for the off-state condition. theThis MR might ﬂuid be after due the to the long increased operation. viscosity of the MR fluid after the long operation.

FigureFigure 5.5. Force-displacementForce-displacement results at the on-state condition. condition.

FigureFigure 6.6. TheThe incrementincrement ofof dampingdamping force with respect to the number number of of cycles. cycles.

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FigureFigure 7. The 7. The force-displacement force-displacement resultresult in in the the off-state off-state condition. condition.

3.2. In Use Thickening (IUT) 3.2. In Use Thickening (IUT) Theoretically, as explained by Carlson [11], the viscosity of MR fluid increases during the long-term useTheoretically, of application as thatexplained caused theby Carlson IUT phenomenon. [11], the viscosity The study of revealed MR fluid that, increases with increasing during usage the long- termperiod, use of theapplication off-state viscosity that caused also increased, the IUT phen resultingomenon. in an increase The study in the revealed force and/or that, thewith torque increasing of usagethe period, MR application the off-state devices viscosity and systems. also increased, In this work, resulting the same in phenomenon an increase of in increasing the force peak and/or MR the torquedamper of the force MR isapplication observed in devic the experimentales and systems. results, In as this shown work, in Figuresthe same5 and phenomenon7. In another possibleof increasing peakscenario, MR damper although force MR is observed fluid is free in to the flow experimental during the operation, results, as some shown of the in magnetic Figures 5 particles and 7. In might another possiblebe trapped scenario, and although deposited atMR the fluid MR valve. is free This to phenomenonflow during is the also operation, referred to assome IUT. of The the trapped magnetic particlesmagnetic might particles be trapped may cause and a deposited clot at the MR at the valve, MR interrupting valve. This the phenomenon flow of MR fluid is andalso resulting referred in to as IUT. aThe higher trapped force duringmagnetic the longerparticles operation may cause process. a clot The at physical the MR observation valve, interrupting of the MRvalve the flow female of MR fluidcasing and resulting is shown in in a Figure higher8. Theforce figure during exhibits the long theer condition operation of the process. MR valve The female physical casing observation before of the operation, while Figure8b shows the same casing after 170,000 cycles and the deposited magnetic the MR valve female casing is shown in Figure 8. The figure exhibits the condition of the MR valve particles surrounding the valve, marked with a red arrow. Figure8c shows the casing with the wear female casing before the operation, while Figure 8b shows the same casing after 170,000 cycles and effect inside the tube after the long-term operation. After cleaning the casing, the wear effect can the depositedstill be observed magnetic at the particles channel surrounding surface, along the with va thelve, remaining marked clotwith that a red still arrow. surrounds Figure the valve.8c shows the casingThis clot with affects the thewear MR effect fluid inside flow inside the tube the valveafter bythe narrowing long-term the operation. flow channel; After this cleaning is particularly the casing, the wearevident effect at the can tube. still Therefore, be observed the restriction at the channel of MR fluidsurface, flow along causes with thedamping the remaining force to clot increase, that still surroundsas discussed the valve. in Section This 3.1 clot. Besides, affects the the clot MR becomes fluid thicker flow inside over time the because valve moreby narrowing particles will the be flow channel;dumped this onis particularly the channel. evident at the tube. Therefore, the restriction of MR fluid flow causes the damping force to increase, as discussed in Section 3.1. Besides, the clot becomes thicker over time because more particles will be dumped on the channel.

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Clot

(a) (b)

Wear effect

(c)

FigureFigure 8. The female casing of the MR valve ( a) before before the operation, operation, ( b) after after the the long-term long-term operation, operation, andand ((cc)) thethe wearwear insideinside thethe tube.tube.

TheThe steadysteady state state model model of theof meanderingthe meandering MR valve MR typevalve can type be utilized can be to utilized analyze theto analyze relationship the betweenrelationship the depositedbetween the magnetic deposited particles magnetic and theparticle risings and peak the of rising the force. peak The of the MR force. valve The model MR can valve be expressedmodel can bybe [expressed20] by [20] ∆P = ∆Pviscous(Q) + ∆Pyield(B), (1) ∆ = ∆() + ∆(), (1) where ∆P, ∆Pviscous(Q), and ∆Pyield(B) are the total pressure drop, the viscous pressure drop as a functionwhere ∆ of, ﬂow∆ rate() (Q),, and and the∆ yield() pressure are the droptotal aspressure a function drop, of magneticthe viscous ﬁeld pressure (B), respectively. drop as a Meanwhile,function of flow in general, rate (),∆ Pandviscous theand yield∆ Ppressureyield(B) can drop be as expressed a function as of follows. magnetic field (), respectively. Meanwhile, in general, ∆ and ∆() can be expressed as follows. 1 ∆P∆viscous = = K1Q (2) d3 (2)

1 ∆P∆yield= =K2τ(B()) (3)(3) d wherewhere dis is the the channel channel gap gapwithin within thethe MRMR valve, K1 andand K2are are constants constants for for pressure pressure dropdrop viscousviscous andand yield within the MR valve, respectively, and τ((B)) is is the the yield yield stress stress as as aa functionfunction ofof thethe magneticmagnetic ﬁeldfield B.. WhileWhile magneticmagnetic particlesparticles are depositeddeposited in the gap, the gap ((d)) valuesvalues decrease,decrease, indirectlyindirectly increasingincreasing thethe totaltotal pressurepressure dropdrop inin thethe ﬂowflow channel.channel. If the force producedproduced byby MRMR damperdamper hashas aa correlationcorrelation with the pressure drop thatthat occurs betweenbetween the before and after states of the MR valve, thethe increaseincrease ofof pressurepressure dropsdrops meansmeans thethe increaseincrease ofof dampingdamping force.force. InIn addition,addition, thethe differencedifference inin thethe peakpeak forceforce beforebefore andand afterafter thethe long-termlong-term operationoperation in thethe off-stateoff-state conditioncondition (refer to FigureFigure7 7)) can can bebe considerablyconsiderably higher than in thethe on-stateon-state conditioncondition (refer toto FigureFigure5 5),), and and can can be be explained explained in in the the

Materials 2018, 11, 2195 10 of 17 Materials 2018, 10, x FOR PEER REVIEW 10 of 17 steady state model. The peak force in the off-state condition is mainly affected by ∆P , where the steady state model. The peak force in the off-state condition is mainly affected by viscous∆, where channelthe channel gap, gap,d, is raised, is raised to the to power the power of three. of three.

3.3. Surface Roughness 3.3. Surface Roughness In this work, the wear effect of the device due to the impingement of iron particles is also In this work, the wear effect of the device due to the impingement of iron particles is also investigated by visual observation on the surface of some MR valve components. Figure9 shows the investigated by visual observation on the surface of some MR valve components. Figure 9 shows the spacers after 170,000 cycles of operation. Some dented and uneven surfaces were observed in each spacers after 170,000 cycles of operation. Some dented and uneven surfaces were observed in each spacer. The main reason for this problem is the clotting of the magnetic particles at the component spacer. The main reason for this problem is the clotting of the magnetic particles at the component surfaces, which are directly in contact with the fluid as a flow path. This eventually creates obstacles surfaces, which are directly in contact with the fluid as a flow path. This eventually creates obstacles for the smooth flow motion of MR fluid. In this case, a higher force is required for the MR damper for the smooth flow motion of MR fluid. In this case, a higher force is required for the MR damper piston to move inward and outward [21]. piston to move inward and outward [21].

FigureFigure 9.9. SpacerSpacer afterafter long-termlong-term operation.operation.

3.4.3.4. Rheological PropertiesProperties InIn thisthis work,work, thethe rheologyrheology teststests areare conductedconducted usingusing MCRMCR 302,302, AntonAnton PaarPaar Companies,Companies, Graz,Graz, Austria,Austria, underunder two two different different conditions: conditions: on- on- and and off-states. off-states. The The results results from from the the rheological rheological tests tests for thefor off-statethe off-state (without (without applied applied current) current) and for and the for on-state the on-state (at 1 A and(at 1 2 AA ofand applied 2 A of current) applied conditions current) areconditions shown inare Figure shown 10 in. Figure 10.10a Figure shows 10a the show graphs the of viscositygraph of versusviscosity shear versus rate shear for the rate off-state for the condition.off-state condition. The results The show results that show the viscosity that the decreasesviscosity decreases with increasing with increasing shear rate shear before rate and before after theand 170,000 after the cycles, 170,000 for cycles, both the for off both and the on off cases. and Inon particular, cases. In particular, for the off-state for the experiment, off-state experiment, the initial −1 −1 viscositythe initial for viscosity the fluid for before the fluid use before is 1 kPa use s is with 1 kPa 0.01 s with s shear 0.01 s rate−1 shear and, rate after and, 1000 after s 1000shear s−1 rates,shear therates, viscosity the viscosity decreased decreased to 1 Pa to s.1 Pa For s. theFor same the same off-state off-state experiment, experiment, the the after-use after-use MR MR fluid, fluid, that that is, −1 theis, the MR MR fluid fluid that that has has been been subjected subjected to to 170,000 170,000 cycles, cycles, has has an an initial initial viscosity viscosity of of 1 1 Pa Pa s s for for 0.01 0.01 s s−1 −1 shearshear rate,rate, andand thisthis valuevalue isis furtherfurther decreaseddecreased toto 0.10.1 PaPa s s for for 1000 1000 s s−1 shearshear rates.rates. There is aa largelarge − differencedifference inin viscosityviscosity betweenbetween newnew andand usedused MRMR fluid,fluid, especially especially at at the the initial initial shear shear rates rates of of 0.01 0.01 s s−11. Indeed,Indeed, afterafter beingbeing subjectedsubjected toto thethe 170,000 cycles, the MR fluidfluid exhibits an initial viscosity of 1 Pa s, − whichwhich isis 10001000 timestimes lowerlower thanthan whatwhat itit exhibitsexhibits beforebefore thethe cycles:cycles: 11 kPakPa ss atat aa shearshear raterate ofof 0.01 0.01 s s−11. − However,However, aa smallersmaller differencedifference in in the the viscosity viscosity is is observed observed for for 1000 1000 s s−11 shear rates; the viscosity ofof thethe usedused MRMR fluidfluid (0.1(0.1 PaPa s),s), thethe fluidfluid subjectedsubjected toto thethe 170,000170,000 cycles,cycles, isis 1010 timestimes lowerlower thanthan itsits viscosityviscosity beforebefore thethe longlong cycliccyclic operationoperation (1(1 PaPa s).s). Furthermore, Figure 10b,c show the viscosity of the MR fluid before and after the long operation in the on-state condition. A similar pattern to the results shown in Figure 10a is observed. From the results shown in Figure 10b, the initial viscosity of the MR fluid is identified as 1000 kPa s for 0.01 s−1 shear rate. It then decreases to 100 Pa s for 1000 s−1 shear rates. In this on-state condition of 1 A, the viscosity of the before-use MR fluid is about 10 times higher than the viscosity of the after-use MR fluid either for 0.01 or 1000 s−1 shear rates. A similar pattern is observed for 2 A magnetic field, as shown in Figure 10c, with higher values of viscosity compared to 1 A magnetic field. Figure 10c shows that the initial viscosity of the MR fluid is 10,000 kPa s, with 0.01 s−1 shear rate, and that it decreases to 100 Pa s for 1000 s−1 shear rates. In brief, the viscosity decreases along with the increase of shear rate for both cases, for before- and after-use MR fluid. However, the viscosity for the on-state

Materials 2018, 11, 2195 11 of 17 conditions decrease in a manner that is inversely proportional to the shear rate, while this is not the case for the off-state condition. For the on-state condition, the viscosity of the MR ﬂuid increases, with increasingMaterials 2018 applied, 10, x FOR currents. PEER REVIEW 11 of 17

(a)

(b)

Figure 10. Cont.

Materials 2018, 11, 2195 12 of 17 Materials 2018, 10, x FOR PEER REVIEW 12 of 17

(c)

FigureFigure 10. Shear10. Shear rate-viscosity rate-viscosity of of MR MR ﬂuid fluid before before and and after after the the operationoperation (a) without applied applied current, current, (b) at(b 1) at A 1 and A and (c) at(c) 2 at A 2 of A appliedof applied current. current.

FigureFurthermore, 11a compares Figure the 10b,c viscosity show vs the shear viscosity rate forof the MR MR fluid fluid before before the and long-endurance after the long operation test for off andin on the conditions, on-state condition. while Figure A similar 11b compares pattern to these the results same parameters shown in Figure for MR 10a fluid is observed. that has experienceFrom the theresults long-endurance shown in Figure test. As10b, a the general initial trend, viscosity the of viscosity the MR fluid of MR is identified fluid decreases as 1000 askPa the s for shear 0.01 rates−1 −1 increasesshear rate. for both It then off- decreases and on-state to 100 conditions. Pa s for 1000 For s instance, shear rates. the In relationship this on-state between condition the of viscosity1 A, the andviscosity shear rates of the in thebefore-use presence MR of fluid current is about is inversely 10 times linear,higher whilethan the the viscosity relationship of the under after-use off-state MR −1 conditionfluid either (0 current) for 0.01 shows or 1000 an s inverse shear exponentialrates. A similar pattern. pattern Indeed, is observed the difference for 2 A magnetic in viscosity field, of MRas shown in Figure 10c, with higher values of viscosity compared to 1 A magnetic field. Figure 10c shows fluid with 1 A is approximately 1000 times higher than the viscosity at 0 A. Meanwhile, the viscosity of that the initial viscosity of the MR fluid is 10,000 kPa s, with 0.01 s−1 shear rate, and that it decreases the after-use MR fluid with 1 A is 1,000,000 times higher than the viscosity at 0 A. In brief, the results to 100 Pa s for 1000 s−1 shear rates. In brief, the viscosity decreases along with the increase of shear of viscosity of MR fluid after the long-term operation are considerably lower compared to the viscosity rate for both cases, for before- and after-use MR fluid. However, the viscosity for the on-state of the initial MR fluid for both on- and off-state conditions. The decrease in viscosity especially after conditions decrease in a manner that is inversely proportional to the shear rate, while this is not the long-endurancecase for the off-state use of thecondition. MR fluid For is the due on-state to the condition, reduction the in theviscosity number of the of magneticMR fluid increases, particles inwith the MRincreasing fluid as time applied goes currents. on. After the long-term operation with the continuous load of the magnetic field, theFigure magnetic 11a particlescompares are the deposited viscosity vs on shear the MR rate valve for MR (refer fluid to before Figure the8), confirminglong-endurance that thetest MRfor fluidoff after and the on longconditions, operation while has Figures a lower 11b number compares of magnetic these same particles, parameters compared for MR to fluid the conditionthat has beforeexperience the operation. the long-endurance This lower content test. As of a magnetic general tr particlesend, the inviscosity the MR of fluid MR resultsfluid decreases in a decrease as the in viscosityshear atrate both increases the off- for and both on-state off- and conditions. on-state conditions. For instance, the relationship between the viscosityIt is known and thatshear the rates rheological in the presence properties of curren of MRt is fluid inversely are affected linear, while by particle the relationship size and volumeunder fractionoff-state [22], condition while the (0 MR current) fluid shows viscosity an isinverse greatly exponential affected by pattern. the magnetic Indeed, field the anddifference magnetic in particlesviscosity [23]. of In MR fact, fluid the viscositywith 1 A ofis theapproximately material, before 1000 andtimes after higher the longthan operation,the viscosity significantly at 0 A. changed.Meanwhile, The magnetic the viscosity particles of the are after-use evenly MR mixed fluid in wi theth carrier 1 A is 1,000,000 fluid of the times MR higher fluid insidethan the the viscosity damper for theat 0 first A. In few brief, strokes, the results before sedimentationof viscosity of occursMR fluid inside after the the damper long-term [15], operation affecting theare contentconsiderably of MR fluidlower particles. compared With furtherto the viscosity operation of of the the initial system, MR sedimentation fluid for both may on- lead and to off-state a smaller conditions. volume of The MR particles,decrease and in a resultingviscosity lowerespecially viscosity after oflong-endurance the MR fluid.There use of are the several MR fluid other is possibilitiesdue to the reduction for the lower in viscosity.the number In this of research, magnetic for particles example, in the the MR MR fluid fluid was as time deposited goes on. at After the valve the long-term when the fluidoperation was forcedwith the continuous load of the magnetic field, the magnetic particles are deposited on the MR valve (refer by high pressure. The deposit of MR particles would agglomerate into a paste-like phase that remained to Figure 8), confirming that the MR fluid after the long operation has a lower number of magnetic in contact with the valve. Therefore, a smaller number of magnetic particles remained in suspension, contributing to the smaller composition ratio of particles to carrier fluid. In other words, the sample of

Materials 2018, 11, 2195 13 of 17

MR fluid after the operation contained a smaller number of magnetic particles compared to the number of magnetic particles in the initial or before-use MR fluid. Consequently, the viscosity of MR fluid decreased at the end of the long-term operation. The study by Cvek et al. [24] revealed that the number of magnetic particles in MR fluid affects its viscosity. It showed that the greater concentration of magnetic particles causeMaterials higher 2018 shear, 10, x stresses FOR PEER and, REVIEW eventually, this higher shear stress causes higher viscosity [24]. However,13 of 17 this result is different with the previous studies conducted by Carlson et al. [11], who claimed that the viscosityparticles, of the compared MR fluid to increased the condition as the before system’s the operation. operating This time lower extended. content of magnetic particles in the MR fluid results in a decrease in viscosity at both the off- and on-state conditions.

(a)

(b)

FigureFigure 11. 11.Shear Shear rate-viscosity rate-viscosity of of ( a(a)) before-usebefore-use MR MR fluid ﬂuid at at 0 0A, A, 1 A, 1 A,and and 2 A 2 of A applied of applied currents, currents, (b) (b) after-useafter-use MR fluid ﬂuid at at 0 0 A, A, 1 1 A, A, and and 2 2A A of of the the applied applied currents. currents.

It is known that the rheological properties of MR fluid are affected by particle size and volume fraction [22], while the MR fluid viscosity is greatly affected by the magnetic field and magnetic particles [23]. In fact, the viscosity of the material, before and after the long operation, significantly changed. The magnetic particles are evenly mixed in the carrier fluid of the MR fluid inside the

Materials 2018, 11, 2195 14 of 17

The meandering MR valve geometry, which is the combination of multiple annular and radial gaps, increases the path length of MR fluid flow [21]. This special geometry configuration consequently makes the magnetic particles and the carrier fluid easy to separate. Furthermore, the material of the valve itself, is magnetic, and is assumed to have remnants of a magnetic field, even though the operation has already finished. Another possibility is the applied mode operation. The principal operation of the damper commonly uses flow mode operation. Hence, in this particular MR valve, studies focused on flow mode operation. The magnetic particles produced an oxide layer at the outer surface of the particles as a result of the oxidation process between the iron particles and the surrounding environment. In addition, the magnetic particles of the MR fluid have a special characteristic, which is the capability to segregate among the particles themselves, and easily sediment in a certain area. There are multiple reasons behind the segregation of magnetic particles, such as shear-induced migration, gravity force, and centrifugal forces [25]. The segregation of the particles can noticeably change the volume fraction of the MR fluid from the initial condition.

3.5. Morphological Observation SEM micrographs of the magnetic particles extracted from MR ﬂuid, before and after the long-term operation, are shown in Figure 12. The initial morphology of the magnetic particles before the operation is quite similar for all particles and present a spherical shape (Figure 12a), with the mean size of 1 to 5 µm. However, after the long-term operation, the magnetic particles deteriorated, both in shape and size, as presented in Figure 12b. The magnetic particles’ shape changed to irregular, and they also grew bigger in size. This might be due to the surface interaction between magnetic particles and the MR valve during the dynamic testing. In particular, during the dynamic testing, the MR ﬂuid experienced an environment with high temperatures and elevated pressures. This high temperature leads to the welding of particles, increasing their size and changing their shapes. This similar condition has been reported by previous researchers [26]. Carlson et al. [11], stated that the oxidation process occurred during the operation, and resulted in the formation of an oxide layer surrounding the magnetic particles. This oxide layer may come from the composition of native oxides, carbides, and nitrides,

Materialsthat might 2018 result, 10, x FOR from PEER the REVIEW reaction of the material with the environment. 15 of 17

(a) (b)

Figure 12.12. SEMSEM images images of of magnetic magnetic particle particle of MR of fluidsMR fluids (a) before (a) andbefore (b) afterand ( theb) long-termafter the long-term operation. operation. 3.6. Oxidation 3.6. OxidationOxidation of magnetic particles in the MR fluid becomes an obstacle in long-term operation of MR dampers,Oxidation dueof magnetic to its impact particles on thein the behavior MR fluid of MRbecomes fluid. an Normally, obstacle in the long-term addition operation of additives of MR dampers, due to its impact on the behavior of MR fluid. Normally, the addition of additives to MR fluid should be considered to reduce the effect of oxidation [27–30]. The energy dispersive X-ray spectroscopy (EDX) measurement of the magnetic particles, before and after the operation, was investigated. It is observed that oxygen increased from 0.75% to 1.75%, indicating that the oxidation affected the MR fluid iron particles during the long operation. This finding confirms the study conducted by Desrosiers et al. [14], who worked on slippage clutch systems. Another study conducted by Plachy et al. [31] investigated the impact of thermal and chemical oxidative processes of carbonyl iron particles on MR performances, and found that the oxidized particles showed lower values of the yield stress that then affect the viscosity of MR fluid. Although the rheological and morphological properties of MR fluid have been shown to impact long-term operation, it is, however, still not clear which factor affects MR fluid performances, magnetic particles, or the carrier fluid more. Thus, further investigation in these areas are important, especially on the significant characteristics of carrier fluid and magnetic particles to the MR fluid performances in long-term operation.

4. Conclusions In this study, the rheological characteristics of the commercial MR fluid used in MR dampers has been investigated after long-term operation. The performance of MR fluid was confirmed to change after long cyclic loading applied by a fatigue dynamic machine. Firstly, it was observed that the damping force increases after long-term operation under both on- and off-states. It was observed that the size and shapes of magnetic particles in the MR fluid had changed after long-term operation. The damping force of the MR damper increased by about 44%, compared to the initial force for the on-state condition and 90% for the off-state condition after 170,000 operating cycles. Besides, the viscosity of the MR fluid was reduced for both on- and off-states, because of IUT and oxidation phenomenon in MR fluid during the long operation. The magnetic particles of the MR fluid also changed considerably in both shape and size after the operation. Regarding the rheology of the MR fluid, before it is subjected to long-term operation, it has a viscosity of 1 kPa s with 0.01 s−1, while after experimental work, the MR fluid’s viscosity was 1 Pa s with 0.01 s−1 shear rate. The used MR fluid had 1 Pa s viscosity, which is 1000 times lower than the respective initial viscosity of non-used MR fluid, which is approximately 1 kPa s. This is because a smaller number of magnetic particles are retained after the long-term operation, compared to the amount of the magnetic particles present on the initial MR fluid. The authors believe that these preliminary results would be very useful for

Materials 2018, 11, 2195 15 of 17 to MR fluid should be considered to reduce the effect of oxidation [27–30]. The energy dispersive X-ray spectroscopy (EDX) measurement of the magnetic particles, before and after the operation, was investigated. It is observed that oxygen increased from 0.75% to 1.75%, indicating that the oxidation affected the MR fluid iron particles during the long operation. This finding confirms the study conducted by Desrosiers et al. [14], who worked on slippage clutch systems. Another study conducted by Plachy et al. [31] investigated the impact of thermal and chemical oxidative processes of carbonyl iron particles on MR performances, and found that the oxidized particles showed lower values of the yield stress that then affect the viscosity of MR fluid. Although the rheological and morphological properties of MR fluid have been shown to impact long-term operation, it is, however, still not clear which factor affects MR fluid performances, magnetic particles, or the carrier fluid more. Thus, further investigation in these areas are important, especially on the significant characteristics of carrier fluid and magnetic particles to the MR fluid performances in long-term operation.

4. Conclusions In this study, the rheological characteristics of the commercial MR fluid used in MR dampers has been investigated after long-term operation. The performance of MR fluid was confirmed to change after long cyclic loading applied by a fatigue dynamic machine. Firstly, it was observed that the damping force increases after long-term operation under both on- and off-states. It was observed that the size and shapes of magnetic particles in the MR fluid had changed after long-term operation. The damping force of the MR damper increased by about 44%, compared to the initial force for the on-state condition and 90% for the off-state condition after 170,000 operating cycles. Besides, the viscosity of the MR fluid was reduced for both on- and off-states, because of IUT and oxidation phenomenon in MR fluid during the long operation. The magnetic particles of the MR fluid also changed considerably in both shape and size after the operation. Regarding the rheology of the MR fluid, before it is subjected to long-term operation, it has a viscosity of 1 kPa s with 0.01 s−1, while after experimental work, the MR fluid’s viscosity was 1 Pa s with 0.01 s−1 shear rate. The used MR fluid had 1 Pa s viscosity, which is 1000 times lower than the respective initial viscosity of non-used MR fluid, which is approximately 1 kPa s. This is because a smaller number of magnetic particles are retained after the long-term operation, compared to the amount of the magnetic particles present on the initial MR fluid. The authors believe that these preliminary results would be very useful for further consideration in MR-based devices’ design. This is because almost all operating conditions of the devices will be cyclic loading. Therefore, to achieve better reliability for MR-based devices, the prediction of MR fluids’ degradation must be investigated.

Author Contributions: Conceptualization, D.U., U., S.A.M., F.I., I.B., N.A.N., S.A.A.A., N.M.; Data Curation, D.U., U., S.A.M., F.I., I.B., N.A.N., S.A.A.A., N.M.; Formal Analysis D.U., U., S.A.M., F.I., I.B., N.A.N., S.A.A.A., N.M.; Funding Acquisition, U., S.A.M., F.I.; Investigation, D.U., I.B.; Methodology, D.U., U., S.A.M., F.I., I.B., N.A.N.; Project Administration, D.U., I.B.; Resources, D.U., U.; Supervision, D.U., U., S.A.M., F.I., I.B., N.A.N., S.A.A.A., N.M., S.-B.C.; Visualization, D.U., U., S.A.M., F.I., I.B., N.A.N., S.A.A.A., N.M., S.-B.C.; Validation, D.U., U., S.A.M., F.I., I.B., N.A.N., S.A.A.A., N.M., S.-B.C.; Writing—Original Draft Preparation, D.U.; Writing—Review & Editing, D.U., U., S.A.M., F.I., I.B., N.A.N., S.A.A.A., N.M., S.-B.C. Funding: “This research was funded by Universiti Teknologi Malaysia under GUP Grant (Vot No: 13H55) and USAID through Sustainable Higher Education Research Alliances (SHERA) Program—Centre for Collaborative (CCR) National Center for Sustainable Transportation Technology (NCSTT) with Contract No. IIE00000078-ITB-1, as well as Universitas Sebelas Maret (UNS) through Hibah Kolaborasi Internasional 2019” and “The APC was funded by Universitas Sebelas Maret through Hibah Kolaborasi Nasional”. Multimedia University also support the research through Mini Fund Grant (MMUI/180172). Acknowledgments: Authors are grateful to Muhammad Hafiz and Norhiwani Hapipi for their assistance during the experiment. Conflicts of Interest: The authors declare no conflict of interest. Materials 2018, 11, 2195 16 of 17

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