IEEE TRANSACTIONS ON MAGNETICS, VOL. 49, NO. 8, AUGUST 2013 4721
Processing and Properties of Magnetorheological Fluids for ProspectiveApplication in a Passive Armour
Joanna Kozłowska and Marcin Leonowicz
Faculty of Materials Science and Engineering, Warsaw University of Technology, Warsaw 02-507, Poland
Magnetorheological fluids (MRFs) on a basis of synthetic oils and carbonyl iron were synthesized and studied. It was found that in-troduction of the appropriate stabilizer plays an important role in formation of usable magnetorheological properties. For the MRFs,synthesized in this study, the shear modules, obtained in a magnetic field of 159 kA/m, were as follows: complex shear modulusMPa, storage modulus MPa and loss modulus G” MPa. The MRFs were applied as a component of the composite passivearmour. The application of the magnetic field for such structures results in improvement of the protective properties of the armour.
Index Terms—Magnetorheological fluids, rheological and viscoelastic properties, smart armour, smart materials.
I. INTRODUCTION
I N recent decades, a great deal of attention has been devotedto the field of smart or multifunctional materials (SM). One
group of these materials constitutes smart magnetic materials(SMMs) with characteristics of reacting to the magnetic field innoticeable and reversible changes in their properties [1]. SMMscomprise such materials as magnetorheological, magnetostric-tive, magnetoresistive, giant magnetocaloric and ferromagneticshape memory alloys [2].MRFs are suspensions of micron-sized, magnetizable parti-
cles, dispersed in a nonmagnetic carrier liquid. Under applica-tion of the magnetic field, MRFs demonstrate significant andreversible change in their rheological properties as a conse-quence of a dramatic transition in the microstructure. The re-sulting phenomenon, called the magnetorheological effect, cor-responds to the significant increase in the shear stress with amagnetically variable yield stress [3]. MRFs can exhibit yieldstress of 10–100 kPa at a magnetic field of 150–280 kA/m [3],[4].Characteristic properties of MRFs attract attention in a wide
range of applications such as shock absorbers, damping devices,clutches, brakes, artificial joints and in magnetorheological pol-ishing systems [5], [6].The concept of MRF application in smart protective armours
arises from their characteristic ability to absorb and dissipateenergy in a wide spectrum, by varying the magnetic field inten-sity. For now, there are only few literature studies (see [7]–[9])on the subject of potential MRF application as a smart armourcomponent.In the present studies the influence of stabilizing agent type
on the rheological properties of the MRFs was analyzed. Thesecond objective was to identify the performance of the MRF-Kevlar® composites in light of their use in a passive armour.
Manuscript received February 07, 2013; accepted March 13, 2013. Date ofcurrent version July 23, 2013. Corresponding author: J. Kozłowska (e-mail: [email protected]).Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/TMAG.2013.2254473
II. EXPERIMENTAL PROCEDURE
A. Materials
All synthesized fluids had the same 75% w/w of magneticpowder particles, 24% w/w of carrier oil, and 1% w/w of stabi-lizing additives.The carbonyl iron powder was composed of spherical shape
particles with average diameters of 1.8 m. Synthetic oil wasapplied as the carrier fluids with density of 0.9 g/cm . The kine-matic viscosity of oil was 100 mm /s (viscosity class of 100) at40 C.As stabilizer agent, two types of fumed silica: hydrophilic (di-
ameter 12 nm, BET ) and hydrophobic (diameter 16 nm,BET ), and oleic acid were chosen. The MRFs wereprepared by mixing appropriate amounts of constituent compo-nents by a stirrer until a uniform mixture was formed.Synthesized MRFs were used in the structures of passive
armour samples with Kevlar® fabric in two methods. Oneway of implementation of the MR fluid to the structure was tosoak the polyurethane (PU) foam with the MRF. The secondmethod involved filling the MRF into a polyethylene bag. Suchmagnetorheological materials were subsequently combinedwith layers of Kevlar® Style 660 ( g/m , DuPont) orKevlar® XP ( g/m , DuPont) fabrics. All compositesample targets were prepared in a size of 100 100 mm.
B. Methods
1) Characterization of Rheological Properties: The rheolog-ical characterization of the MRFs was conducted using rationalAres TA Instruments Rheometer, equipped in a magnetic coil. Aplate-plate geometry with diameter of 20 mm and 1mm gap wasused. The rheological characterization of the MRFs was carriedout in the steady shear and dynamic oscillatory modes. Steadyshear tests were conducted in a shear rate range of 0.1–630 swithout magnetic field and in 159 kA/m field. In the oscillatorymode, frequency of angular deformation was 6.28 rad/s (1 Hz)and the angular deformation was %. The measurementswere carried out under linear magnetic field increment from 0to 230 kA/m.2) Stab and Ballistic Resistance Testing: Vertical structures
were selected for stab and ballistic resistance testing. Ballistic
4722 IEEE TRANSACTIONS ON MAGNETICS, VOL. 49, NO. 8, AUGUST 2013
Fig. 1. Drop mass with titanium edge for stab resistance testing.
Fig. 2. (a) Puncture resistance testing and (b) deformation in a clay backingmaterial.
Fig. 3. Dynamic viscosity versus shear rate forMRFs with different stabilizers,without magnetic field and in 159 kA/m.
clay was used as backing material in stab and bullet-proof resis-tance testing.Magnetic fields of 478 kA/m and 159 kA were applied in
ballistic and stab resistance testing, respectively. Lines of themagnetic field were perpendicular to the direction of knife/pro-jectile motion axes. In the stab resistance testing, titanium edgewas used as penetrator and equipped within drop mass (Fig. 1).This system was dropped down from 1 m altitude, which cor-responded to the hit energy of 24 J. After dropping down theknife, the depth of base deformation and total depth of penetra-tion into base in a case of sample’s full puncture, were measured(Fig. 2). In the ballistic test, Parabellum 9 mm projectiles withvelocities of 350–380 m/s were used. The depth of deformationof ballistic clay and number of damaged layers of Kevlar® fab-rics were measured after each test.
III. RESULTS AND DISCUSSION
1) Rheological Properties of MRFs: The dynamic viscosityversus shear rate, for the MRFs with different types of stabi-lizing agents, measured without and in 159 kA/m external mag-netic field, is shown in Fig. 3.
Fig. 4. Complex, storage and loss modules versus magnetic field 0–230 kA/mfor MRFs with different stabilizers.
As it can be noticed, the lowest off-state (no field applied)viscosity values exhibits the MRF with oleic acid addition. TheMRFs with fumed silica, both hydrophilic and hydrophobic, arecharacterized by higher off-state viscosities. This is obviouslycaused by higher proportion of solid component in the systemswith fumed silica addition in comparison to the MRF with oleicacid. On the one hand, low viscosity is a convenient parameterfor the enhanced MR effect. On the other, lack of a stabilizermay create unstable composition with the unavoidable tendencyof sedimentation, leading to radical deterioration of the func-tional properties of the MR suspension.All the investigated MRFs display viscosity decrease in a
function of shear rate evidencing non-Newtonian behavior, de-fined as shear thinning.The application of the 159 kA/m magnetic field results in
an abrupt jump of the viscosity, by up to 1000 times, for allexamined MRFs. Apparently, the highest on-state viscosity of698.9 kPa s, at shear rate of 0.1 s , shows the MRF with hy-drophilic silica fumed. The on-state viscosity of the examinedMRFs again decreases drastically with growing shear rate, char-acterizing a typical shear thinning behavior. These results areconsistent with those in [10]–[12].The viscoelastic parameters such as the complex shear mod-
ulus, storage modulus and loss modulus, for various stabilizers,are shown, versus magnetic field of 0–230 kA/m, in Fig. 4.The storage modulus G’ is a measure of the energy stored
elastically during deformation, while the loss modulus G” quan-tifies the energy dissipated into heat. The complex modulusrefers to overall viscoelastic properties and quantifies the totalresistance to the oscillatory flow. These modules can be appli-cable in predicting field-induced aggregation of the MRF underthe external magnetic field.In current experiments, all the three modules exhibit similar
behavior for studied fluids. As observed, application of the mag-netic field results in a substantial increase (three orders of mag-nitude) of the modules with different intensity, depending on thetype of the stabilizing agents. For instance, the MRF containinghydrophilic silica exhibits complex and storage modules growthfrom the initial values of 389 and Pa to the 1.2 and
MPa, respectively, with increasing magnetic fieldfrom 0 up to 230 kA/m.
KOZŁOWSKA AND LEONOWICZ: PROCESSING AND PROPERTIES OF MRFS 4723
TABLE IRESULTS OF KNIFE-RESISTANCE TESTS, 24 J
Noticeably, the lowest complex shear modulus MPain a field of 230 kA/m, shows the fluid containing addition ofthe hydrophobic silica. The fluid with addition of the oleic acidis located in the middle part with the parameter value around0.57 MPa.The highest MPa in 230 kA/m, and thus the best rhe-
ological properties, exhibit the magnetorheological fluid, con-taining addition of the hydrophilic silica. Similar behavior andproportional properties show the other two modules and .However, the complex shear and storage modules showthe most intense growth for the fluid with hydrophilic silica, asa function of external magnetic field, whereas the loss modulus
exhibits a somewhat narrower spread between the proper-ties. Obviously, in the entire magnetic field range, values of thestorage modulus of the MRFs, with different stabilizers types,exceed the loss modulus. This result clearly evidences dom-inance of the elastic properties over viscous behavior for theMRFs and directly indicates an enhancing chain-like structureformed under higher external magnetic field.These results allow us to draw some conclusions. The ad-
dition of a proper stabilizer enables achievement of high andpractically useful rheological properties. The stabilizers pro-mote formation of individual chains of ferromagnetic particlesparallel to the external magnetic field giving rise to substan-tial change of the rheological properties. The hydrophobic silicadoes not wet the iron particles contributing only to the increaseof the viscosity. On the other hand, the hydrophilic silica formsmore effective gel-network in the carrier fluid, which enablesseparation of the particles and prevents aggregation.2) Stab and Bullet Resistance Testing: The results of stab
resistance testing with titanium edge for the two structures ofcomposite materials based onKevlar® fabrics, without and withMRF, are presented in Table I.As one can see, application of the MRF component to the
structure of armour targets caused a reduction in a total depthof ballistic clay penetration, both for the Kevlar® Style andKevlar® XP.The addition of the polyethylene bag filled with MRF to the
Kevlar Style fabrics (26 layers) results in a decrease of the base
TABLE IIRESULTS OF BULLET RESISTANCE TESTS, PARABELLUM 9 MM
clay’s deformation depth by 8%, under external field of 478kA/m. At the same time, the areal density of MRF-Kevlar®Style sample twice exceeds the value for the neat Kevlar® Style.Comparing Kevlar® XP (12 layers) with a neat polyurethane
foam with the same target containing magnetorheological foam,noticeable differences cannot be observed when no magneticfield is applied. However, the sample with MRF filled-foam re-veals significant reduction in deformation depth (50%) in com-parison withmeasurement obtained withoutmagnetic field. Thisclearly is evidence that application of the magnetic field to thetargets with MRF foam can improve the stab resistance perfor-mance. However, there is a dichotomy in the stab test results.On a one side, the magnetorheological fluids can improve thestab performance of neat Kevlar fabrics. On the other side, theareal density of targets with MRFs grows, which is an undesir-able effect in the need of the ballistic materials.The ballistic performance of Kevlar® targets with and
without addition of the MRF under projectile Parabellum 9 mmare shown in Table II.The results obtained for the MRF- Kevlar® Style composite
target shows reduced depth of deformation by 20% in compar-ison with the neat Kevlar® fabrics. The areal density of MRF-Kevlar® composite is twice higher than neat Kevlar sample.The implementation of theMRF to the composite Kevlar® Stylestructure does not change the number of damaged layers.Comparing ballistic performance of Kevlar® XP targets
without and with the implementation of MRF (in a polyeth-ylene bag) at the same areal density, the 29 layers of a neatKevlar® XP can ensure slightly smaller deformation in ballisticclay than 13 layers with the MRF component. Moreover, itcan be demonstrated that MRF’s implementation can enhanceballistic performance of Kevlar XP fabric. At the same numberof Kevlar® XP layers, the deformation of the target with theMRF in a polyethylene bag is 20% smaller (corresponding tothe 13 layers of neat Kevlar® XP). The number of damagedlayers was significantly reduced for the sample with MRFunder magnetic field of 154 kA/m.
IV. CONCLUSION
A series of magnetorheological fluids was synthesized on abasis of synthetic oil and carbonyl iron with various stabilizers.
4724 IEEE TRANSACTIONS ON MAGNETICS, VOL. 49, NO. 8, AUGUST 2013
It was found that application of an appropriate stabilizer playsa crucial role in formation of usable magnetorheological prop-erties. In our studies the best properties presented the MRFsprocessed with application of hydrophilic nanosilica. The re-spective shear modules for this MRF, obtained in a magneticfield of 230 kA/m, were as follows: complex shear modulus
MPa, storage modulus MPa and loss modulusMPa. The stabilizers promote formation of individual
chains of ferromagnetic particles, parallel to the external mag-netic field, giving rise to substantial change of the rheologicalproperties. The hydrophobic silica does not wet the iron parti-cles contributing only to the increase of the viscosity.The stab and ballistic tests results indicate that application the
MRFs with Kevlar® targets may cause some improvement inballistic performance and the overall energy dissipating processunder projectiles or edge impacts. From one side, compositesMRF-Kevlar are characterized by high areal density of targets.From the other side, implementation of the MRF components tothe structure of the high-strength ballistic materials can enhancethe energy absorbing process and can be applicable in reducingthe trauma blunt effect.
ACKNOWLEDGMENT
This project was co-financed by the E.U. within the Euro-pean Regional Development Fund and by the Polish Govern-ment within the Innovative Economy Programme, priority 1,activity 1.3, subactivity 1.3.1, agreement with MSHE nr UDA-POIG.01.03.01-00-060/08-00, 25.02.2009. The authors wouldlike to thank: Prof. A. Wiśniewski, D. Pacek (Military Instituteof Armament Technology Zielonka), Prof. M. Struszczyk, Dr.K. Olszewska, D. Zielińska (Institute of Security Technologies
MORATEX), and Ł. Wierzbicki (Warsaw University of Tech-nology) for assistance in research and support in preparing ma-terials for the stab and ballistic testing.
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