48th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition
4 - 7 January 2010, Orlando, Florida

AIAA 2010-177

Behavior of Magnetorheological Fluid Composites
Employing Carrier Fluids Certified for Landing Gear UseLouise A. Ahuré1 and Norman M. Wereley.2
Smart Structures Laboratory, Alfred Gessow Rotorcraft Center, University of Maryland, College Park, MD, 20742
Magnetorheological (MR) fluid composites were formulated in order to investigate theirperformance for potential use in landing gear hydraulic systems, such as shock struts. MR
fluids prepared in this study utilized three hydraulic oils certified for use in landing gear as
carrier fluids, two different average diameters of spherical magnetic particles, and a
Lecithin surfactant. The fluids were characterized in order to measure and analyze theirrheological behavior. Hence, different characteristics were considered, such as 1)
magnetorheology as a function of magnetic field, 2) cycling of a small-scale damper
undergoing sinusoidal excitations at frequencies of 2.5 and 5 Hz, and 3) impact testing for arange of magnetic field strengths and velocities using a free-flight drop tower facility. The
goal of this research is to analyze the performance of these particular MR fluids, to compare
their behavior to standard commercial MR fluids, and to determine their feasibility for use
in helicopter landing gear.

Nomenclature
•

γ

Cpo
Cpr
f(t)
Fy
τ
τy
µ
v
vy

M

=
=
=
=
=
=
=
=
=
=

shear rate
postyield damping
preyield damping
nonlinear biviscous modelyield force
shear stressyield stresspost-yield viscosity
velocity
preyield velocity

I. Introduction

agnetorheological (MR) fluids are suspensions of micron-sized magnetic particles, such as iron or cobalt, in asilicone or hydraulic oil carrier fluid.1 Moreover, MR fluids have the ability to change properties when amagnetic field is applied and are used in an increasing range of applications, such as primary vehicle suspensions
and semi-active vibration absorbing systems.2 Yet, from published research, studies of MR fluids targeting
applications including landing gear systems have not been widely investigated.3-5 During landing, an aircraft issubjected to a short duration impulsive impact, which is a critical factor in structural fatigue damage, crew andpassenger discomfort, and dynamic stress. Consequently, several types of landing gear systems have been used in an
attempt to reduce the touchdown impact. One prospective method is to use smart fluids such as MR fluids, whose
advantage is to obtain continuously controllable rheological properties and fast response time to an applied magnetic
field.6,7 Hence, new types of MR fluids were developed in this study, employing three different carrier hydraulic oils
certified for landing gear use (MIL-H-5606, and MIL-PRF-83282 and MIL-PRF-87257). The fluid performance
depended on characteristics such as 1) rheological properties with magnetic field off and on, 2) dynamic stiffness
1

Graduate Research Assistant, Aerospace Engineering, 3181 Martin Hall College Park MD 20742, Student (SS)#287192.
2
Professor and Advisor, Department of Aerospace Engineering, 3181 Martin Hall College Park MD 20742,
Associate Fellow (MC).
1American Institute of Aeronautics and Astronautics
Copyright © 2010 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

measurements using a small scale MR damper, and 3) dynamic axial impact testing with varying velocities andmagnetic field strengths utilizing a drop tower facility. Consequently, this research investigates the effectiveness andfeasibility of magnetorheological (MR) fluids employing fluids certified for landing gear use and compares theirperformance to commercial MR fluids for prospective use in landing gear systems.

II. Experiments and Methods

Shear stress,τ y [kPa]Three certified landing gear hydraulic oils were utilized as carrier fluids to prepare MR fluid composites: amineral oil (MIL-H-5606) and two synthetic hydrocarbon oils (MIL-PRF-83282 and MIL-PRF-87257). The reason
for selecting these oils is topreserve
importantcharacteristics
neededforlanding gear systems, such as
wide operational temperature
ranges (-65 to 275ºF), excellent
anti-wear agents, and fire
resistance properties.8
Each hydraulic fluid wasthen used to prepare twocategories of MR fluid
Figure 1.
Fluid sample preparation and composites, depending on the
average diameter of sphericalnotation.
Figure 2.
Paariron particles present. The firstPhysica
MCR
300
set contained iron particles of larger diameter (6 to 10 µm denoted by I), and theparallel disk rheometer.
second set contained iron particles of smaller diameter (1 to 3 µm denoted by II). Also,
lecithin powder was added as a surfactant to reduce agglomeration and mediate
settling. In order to prepare stable MR fluids, a specific amount of hydraulic oil was mixed with 2 weight % (wt. %)of lecithin powder using a high speed shear mixer operating at 11,000 rpm. After consistently mixing for 30
minutes, iron particles were added in specific concentrations to each fluid and remixed for an additional hour.
Accordingly, samples varying from 60 to 80 wt. % (15 to 32
vol. %) in iron particle concentration were produced, and each40
0.60 T
sample notation includes three important parts: the hydraulic35
Experiment
oil used (e.g. 83282), iron particle size (I ≡ 6-10 µm or II ≡ 1Bingham-Plastic
303 µm), and iron particle concentration (e.g. d for 75 wt. %0.32 T
25iron), as presented in Fig. 1.
Magnetorheological characterization was performed on all20
0.20 Tof the MR fluid composites using a Paar Physica MCR 300
15parallel disk rheometer shown in Fig. 2. First, neat hydraulic
10
oils and the prepared MR fluids were tested singly in the
0.12 T5
rheometer to approximate their dynamic viscosity (shear stress
0.04 T0
versus shear rate in the absence of magnetic field). Then, the
0
100
200
300
400
500
600
700
800
900
1000
-1
prepared samples were loaded onto the rheometer, which had
Shear rate [s ]a standard 1 mm gap separating the rotating disk from the Figure 3.
Typical MR fluid flow curves
platen. On-field testing was executed with increasing current modeled with the Bingham Plastic Methodfrom 0.2 to 2 A to obtain fluid flow curves and determine (Commercial fluid MRF 126CD).
magnetic saturation of each sample.
For this study, the fluid flow curves were characterized using the Bingham Plastic (BP) model, which is a•

generalized model for relating the shear stress, τ , to shear rate, γ , as a viscoplastic flow with a yield stress.1 The•

equation for γ > 0 is:•τ =τy + µγ

2American Institute of Aeronautics and Astronautics[1]

The Bingham Plastic model parameters are the yield stress, τy (in Pa) and the post-yield viscosity, µ (in Pa sec). τy is
the intercept of the high shear rate asymptote with the shear stress axis, and µ is the slope of that asymptote. Typicalflow curves fitted with the Bingham Plastic model are shown in Fig. 3. This model has also been used to model
Poiseuille flow in electrorheological andmagnetorheological dampers.6,9,10
Dynamic stiffness of the MR fluid
Hydrauliccomposites was determined by analyzing
cylinderthe behavior of a magnetorheologicaldamper, containing the fluids andundergoing sinusoidal loading. Only MR
fluidscontaining
iron
particle
concentration of 80 wt. % (or 32 vol. %)were tested in the damper, whose behavior(b)depended on amplitude and frequency of (a)Figure4.
(a)CrosssectionofaRheonetics
SD-1000-2 MR damper
motion. In order to obtain a magnetic field
from
Lord
Corporation;
(b)
Setup
of
a
modified
Rheonetics MR
inside the damper, electric current wasdamperon
MTSmachine.
used to trigger the magnetic circuit. The
MR damper utilized was a modified
Rheonetics SD-1000-2, manufactured by Lord Corporation and tested on an 810 Material Test System (MTS). A
cross section of the damper and the test setup are shown in Fig. 4a and 4b. The damper hydraulic cylinder shown in
Fig. 4b is technically 102 mm in length and 45 mm in diameter, and it
encloses the damper piston, in which a magnetic circuit is mounted.
Inside and at the base of the hydraulic cylinder, a nitrogen accumulator
is used to pressurize the approximate 50 ml of MR fluid contained. The
accumulator also helps prevent cavitation in the fluid at the lowpressure side of the piston while in motion, discussed by Dyke et al. in
Ref. 11.
Advantages can result from applying MR fluids in dampers forlanding gear systems, such as crashworthiness enhancement.12,13 Also,a significant factor in the design of dampers is the evaluation of system
dynamics using simulation, for which the nonlinear behavior of the MR
damper need to be correctly represented.2 Therefore, to satisfy thisFigure 5. Typical MR damper test data critical element and characterize MR fluid behavior, a characterization
modeled with the Nonlinear Biviscous method was employed, which was the Nonlinear Biviscous (NBV)(NBV) model.
model, developed by Stanway for squeeze film dampers, andsubsequently for linear stroke dampers.14 This method assumes that the
MR fluid is plastic in the preyield and postyield regions, and the preyield damping, C pr is greater than the postyielddamping, C po . The yield force, Fy in the damper model is represented by the high velocity asymptote intercept withthe force axis as shown in Fig. 5. This model has been used to analyze leakage in electrorheological dampers in
Refs. 15 and 16, and an advantage is that it accounts for the preyield damping part a of typical viscoplastic preyield
behavior of MR fluids. Three piecewise equations are used to characterizing the Nonlinear Biviscous (NBV) model:v ≥ vyC Po v + Fyf (t ) = CPr v
C v − Fy
 Po

[2]

− vy ≥ v ≤ vy

[3]v ≤ −v y[4]

The preyield velocity is:v y = Fy /(C Pr − C Po )[5]

Moreover, a free-flight drop test facility with a 59 kg drop carriage, as shown in Fig.7, was used at the
University of Maryland to conduct dynamic axial crash tests on a MR damper enclosing synthetic oil-based MR
fluid composite, containing 6 to 10 µm iron particles at a concentration of 80 wt. % (denoted mr83282-Ie). Recently,
3
American Institute of Aeronautics and Astronautics

MR dampers have been highlighted as a promising candidate for crashworthiness systems, and several impact tests
have been conducted.3,17,18,19 Therefore, the performance of one of the
prepared landing gear oil-based MR fluid composite in a MR damper
subjected to drop testing is importantto verify the tuning nature of the
Honeycomb
Drop carriage
blocksresponse of the device at different
stroking velocities representative ofhelicopter landing impacts. The
experiment instrumentation included
a load cell located on a base plate as
shown in Fig. 8, which was tightly
attached to the ground, a LVDT
fastened to the damper to monitor the
Load cell
displacement of the damper piston as
a function of time, and a power
supply to provide current necessary to
generate a magnetic field in thedamper. In addition, the raw signal
from the sensor was filtered through a Figure 8.
MRdamper2311 VISHAY signal conditioning mounted for drop testing.
Figure 7.
Drop
test amplifier and the data was transferredcarriage at the University of to a computer using a SCC-68 National Instruments data acquisition
Maryland College Park.
interface (which changed analog data to digital). Then, the data wasexported to MATLAB compatible formats for analysis.
Testing of the MR fluid composite enclosed in the damper was achieved by positioning the damper vertically on
the base plate. Then, a block of aluminum honeycomb measuring 90x60x60 mm was taped on a small plate, which
was mounted on the top end of the damper. The purpose of using the aluminum honeycomb block was to prevent theringing in the load cell due to metal-to-metal contact and reduce the inertial spike associated with the impact
acceleration of the piston rod.20 Additional larger honeycomb blocks were placed on top of cement bricks and woodblocks (on each side of the damper) to stop the carriage platform after about 3.8 cm of stroke, which needed to beless than the LVDT stroke of 4.6 cm to prevent damaging it.
Before each test, the MR damper rod was extended, and the drop carriage was raised to a specific heightcorresponding to a selected impact velocity. Then, the drop carriage was released to freely fall under gravity until
impacting on the test assembly, and the load cell and LVDT data were recorded.

III. Results and Discussions
A. Rheology Results
Rheological tests were performed on all of the prepared MR fluids shown in table 1 at room temperature. A few
challenges were encountered in testing. MR fluid composites tended to be ejected from the test area when testing
low viscosity fluid at high shear rates, especially at low field strengths. Another difficulty was the inability for the
samples to stay within the test area without a small magnetic field being applied. For each test, 0.3 ml volume wasmeasured in the test area. Then, the fluid was sheared by the rotating top disk, as the bottom disk stayed stationary.
The off-state viscosity (zero magnetic field strength) measurements were performed with the shear rate ranging from10 to 1000 s-1. The shear rate of the flow curves (magnetic field on), shown in Fig. 3, ranged from 0.1 to 1000 s-1,
with current increasing from 0.2 to 2 A
in 0.2 A increments, and from 2.5 to 5
Table 1. MR fluid compositions.
A in 0.5 A increments. The maximum-1
shear rate was 1000 s to prevent fluid
from escaping the test area at higher
RPM, and the maximum current was 5
A (corresponding to a magnetic flux
density of about 1 Tesla). Flow curves
were measured using the supplied
US200 software compatible with therheometer.
4
American Institute of Aeronautics and Astronautics

(a)
(b)
Figure 9.
Zero-field viscosity of MR fluid composites as iron concentration increases. (a) 6-10 µm
particle diameter. (b) 1-3 µm particle diameter.
Rheology of neat hydraulic oils showed that the viscosity of the synthetic oil MIL-PRF-87257 is the lowest whencompared to the other oils. As iron particles (1 to 3 µm or 6 to 10 µm) were added to the oils, there was a
proportional increase, but the viscosity pattern was conserved as illustrated in Fig. 9a and 9b. Indeed, while the ironconcentration was increased, MR fluid composites containing the synthetic oil MIL-PRF-87257 had a 40% lower
zero-field viscosity compared to theother fluids. Moreover, the viscosityof the MR fluids depended
significantly on the choice of
hydraulic oil and the iron particle
concentration. For instance, eachsample viscosity increased by morethan 80% after adding 80 wt. % (or32 vol. %) iron particles to the carrier
fluids.
Flow curves of all samples were(a)(b)
Figure 10.
Selected and modeled (with Bingham Plastic) flow measured and modeled using the
curves for MR fluid composites: (a) synthetic oil-based mr83282-Ie (80 Bingham Plastic model, and Fig. 10aand 10b show selected flow curves
wt % iron) and (b) mineral oil-based mr5606-Ie.
modeled for MR fluid composites
containing one of the synthetic oils
(MIL-PRF-83282) and the mineral oil (MIL-H-5606). In
addition, the maximum yield stress saturation of all MR
fluid samples was determined and compared. As a result, it
was observed that the performance of the fluids did notstrongly depend on the choice of iron particle sizes or thetype of carrier hydraulic oil, but it was significantly
dependent on the iron particle concentration. As particle
concentration increased, the maximum yield stressincreased as well. The yield stress improved by more than
65% for all MR fluids containing smaller (1 to 3 µm)
particles and about 70% for fluids with larger (6 to 10 µm)
particles. The performance is illustrated in Fig. 11.
MR fluid rheology was compared with a commercial
MR fluid from Lord Corporation (MRF126CD ~ 26 vol. %Figure 11.
Maximum yield stress for all MR iron concentration). Synthetic oil-based fluid containing
fluid composites as a function of iron particle larger (6 to 10 µm) particles, at 75 wt. % (or 26 vol. %) ironconcentration (referred as mr83282-Id in table 1), had anconcentration.
off-state viscosity lower than the commercial MR fluid, as
illustrated in Fig. 12a. In fact, it is important to keep the offstate viscosity low to prevent adding excessive weight to MR devices21 and to maintain high frequency vibration
performance.22 Also, both fluid maximum yield stress performances followed similar trends, as Fig. 12b shows.
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(a)
(b)
Figure 12.
MR83282-Id performance compared to a commercial MR fluid. (a) Zero-field viscosity (b)yield stress (within 2%).
B. Damper Results
Hysteretic behavior of the linear stroke MR damper containing MR fluids was studied using high iron particle
concentration fluids (80 wt. % or 32 vol. %). The damper was tested using different sinusoidal shaft displacements:
1.27, 2.54, 5.08, and 7.62 mm, at two frequencies (2.5 and 5Hz), while the magnetic field was controlled using a
power supply. Current was applied from 0 to 4 A with 1 A increments to activate the damper electromagnet. The test
matrix presented in table 2 was programmed using thesoftware MTSTESWARE, which executed the inputs. The
MR damper test matrix. Tests
resulting force was measured by a load cell, mounted on the Table 2.
were executed at 2.5 and 5 Hz.
MTS-810 load frame.
Typical data shown in Fig. 13a and 13b are the force
versus displacement and the force versus velocity,respectively, for current values of 0 to 4 A at 2.5Hz. As theapplied current increased, so did the magnetic field;
therefore, the damping, which is represented by the area
enclosed by the force versus the displacement hysteresis
cycle in Fig. 13a, also increased. The force versus velocity
cycles in Fig. 13b show a behavior similar to a Bingham
Plastic.6,23 When the damper restoring force was less or greater than the yield force, Fy, the damper operated in thepreyield or the postyield region2. Hence, the NBV method was used to characterize the MR effect (preyield andpostyield regions). The model parameters (Cpr, Cpo, Fy, and vy) were identified from experimental data, and a
MATLAB® routine was used to program a constrained least- mean-squared error minimization. Parameters wereidentified for all MR fluid composites containing one of the synthetic oils. Mr83282 fluids followed similar patterns,
and their parameters are plotted versus thecurrent in Fig. 14a through 14d. As noted
earlier, the preyield damping, Cpr, isgreater than the postyield damping, Cpo, as
current increases at both frequencies,
shown in Fig. 14a and 14b. Furthermore,
the preyield velocity became larger as thefrequency increased. In addition, the yieldforce depended on the type of carrier fluid
used. For instance, the yield force of all
MR fluid composites containing the(a)(b)
synthetic oil MIL-PRF-87257 was 27%
Figure 13.
Hysteresis cycles of MR damper containing the lower than the yield force of MR fluids
commercial MR fluid. Sinusoidal excitation of 7.62 mm at 2.5 Hz. prepared with the other synthetic carrier(a) Force (N) versus displacement (mm); (b) Force versus velocity fluid (MIL-PRF-83282).
(mm/s).
MR fluids were tested in the damper
to verify the nonlinear behavior as a6
American Institute of Aeronautics and Astronautics

70

1400

60

1200
Yield Force, Fy (N)Damping, Cpr (N s/mm)

function of magnetic field. Figures 15a and 15b represent the force versus displacement and the force versus
velocity of the MR fluid composite containing the synthetic oil MIL-PRF-87257 for current values of 1 and 4 A at
2.5 Hz. The NBV model was used to characterize the preyield and postyield damping, as well as the yield force of

50
40
30
20

1000
800
600
larger particle fluid @ 2.5 Hzlarger particle fluid @ 5Hz
smaller particle fluid @ 2.5Hz
smaller particle fluid @ 5Hz

400
200

10

0

0
0

1

2

Current (A)

3

0

4

0.5

1

1.5

2

(a)

3

3.5

4

(b)

4.5

60
Yield Velocity, vy (mm/s)

Damping, Cpo (N s/mm)

2.5

Current (A)

4
3.5
3
2.5
2
1.5

larger particle fluid @ 2.5Hz
larger particle fluid @ 5Hz
smaller particle fluid @ 2.5Hz
smaller particle fluid @ 5Hz

1
0.5
0
0

1

2

Current (A)

3

50
40
30
20
10
0
0

4

1

2

3

4

Current (A)

(c)

(d)

Figure 14.
NBV model parameters (Cpr, Cpo, vy, and Fy) plotted vsersus appliedcurrent for mr83282 fluids. (a) preyield Damping, Ns/mm; (b) yield Force, N; (c) postyield
Damping, Ns/mm; (d) preyield velocity, mm/s.

(a)(b)
Figure 15.
NBV model of MR fluids with synthetic oil (MIL-PRF-87257) as a carrier fluid (6-10 µm
particles) at 2.5 Hz and currents of 1 and 4A. (a) Force versus displacement; (b) force versus velocity.
the MR damper. The NBV model accurately models the force versus displacement cycles, hence, the damping.
MR fluid composites (of 32 vol. % particle concentration) performance were compared with a commercial MR
fluid from Lord Corporation (MRF132 of the same particle concentration). Yield forces of fluids containing
synthetic oil MIL-PRF-83282 and mineral oil MIL-H-5606 with larger (6 to 10 µm) iron particles followed the same
pattern as the yield force of the commercial fluid, particularly at 5Hz. Figure 16 illustrates the results, and all three
fluid yield forces plotted versus the current show that the maximum yield forces are relatively close (within 5%).

7
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Figure 16.
Yield force (N) versus current (A) for the synthetic oil (mr83282-Ie)
and the mineral oil (mr5606-Ie) based MR fluid, and the commercial MR fluid(MRF132).
C. Drop Test Analysis
Synthetic hydraulic oil-based MR fluid composite with 6
to 10 µm iron particles at 80 wt. % concentration (denoted
Table 3. Peak stroking force (N) for applied
mr83282-Ie) was used inside the damper subjected to drop
current and impact velocity.
testing. This particular fluid was selected due to its favorable
performance based on the rheological and damper test results.
The software National Instruments executed the inputs andwas exported to MATLAB through a routine to filter and plot
the data. Hence, results of peak stroking force values for acorresponding range of impact velocities and applied magnetic

12000

12000
0A
0.5A
2A

10000

8000
Peak Force (N)8000
Peak Force (N)~1.1 m/s~2.8 m/s~4.1 m/s

10000

6000

6000

4000

4000

2000

2000

0

0
0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

0

0.5

Actual Impact Velocity (m/s)

1

1.5

2

2.5

Current (A)(a)(b)
Figure 17.
Peak stroke versus current and impact velocity. (a) Force versus velocity in m/s. (b) Forceversus current in ampere.

Stroking Force (KN)

field strengths were recorded and are shown in table 3. The
8Filtered data 0A
peak force increased as the magnetic field and the impact
Filtered data 0.5A
7
Filtered data 2A
velocity increased. In Fig. 17a, while the velocity increased,
Actual data 0A
6
an increase in the peak stroking force was observed and was
Actual data 0.5A
Actual data 2A5
due to the viscosity related component of the shear stress.
4
Also, in Fig. 17b, as the current, hence the magnetic field was
3augmented, there was an increase in the peak stroking force,
which was attributed to the increase in the yield stress related
2
component of the shear stress.22 Moreover, the ability to1significantly tune the level of the stroking force was higher at
0
0
0.005
0.01
0.015
0.02
0.025
lower impact velocity but slightly reduced as the velocity
Time (s)increased. The synthetic oil-based MR fluid composite
Stroking force versus time at(mr83282-Ie) used for the drop test showed tunable behavior Figure 18.
impactvelocityof
2.8 m/s.
as the magnetic field was changed. In fact, the peak stroking
force changed as a function of time, and the energy generated
by the MR damper altered as well, as observed in Fig. 18 for one specific impact velocity (of 2.8 m/s).
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IV. Conclusion
An analysis of the behavior of magnetorheological (MR) fluid composites, prepared using three different carrier
hydraulic oils certified for landing gear use, was conducted, and the feasibility and effectiveness for prospective use
in landing gear systems were assessed. First, magnetorheology was tested as a function of applied field, and theexperimental data were characterized using the Bingham Plastic model. Using flow curve data, the yield stress andviscosity of the MR fluid composites were identified. The MR landing gear fluid composite compared favorablywith a commercial MR fluid (both containing 26 vol. % magnetic particles). Then, the performance of a linear stroke
MR damper, containing the MR fluids, was characterized using a Nonlinear Biviscous (NBV) model. The NBVmodel was used to successfully reconstruct the force versus displacement diagram and to identify the pre-and-postyield damping and yield force using the force versus velocity plots.
MR damper behavior was compared to a second commercial MR fluid (of 32 vol. % particle concentration).
The yield forces of the MR fluid composites containing the larger (6 to 10 µm) iron particles (with 32 vol. %particles) compared favorably with the commercial fluid force, and their maximum yield forces remained within 5%of each other.
Last, synthetic oil-based MR fluid (containing 32 vol. % iron particles) was utilized in an MR damper andsubjected to high shear rate drop testing to experimentally verify the tuning nature of the MR device at different the
impact velocities and magnetic field strengths. Consequently, the peak stroking force and the energy dissipated by
the MR damper strongly depended on the changes in the magnetic field strengths.
Different tests done to characterize these particular MR fluid rheological behaviors showed that typical landing
gear hydraulic oils can be utilized to make suitable MR fluids.

Acknowledgments
Research support provided by the Center for Rotorcraft Innovation and the National Rotorcraft Technology
Center, Aviation and Missile Research, Development and Engineering Center under Technology Investment
Agreement with W911W6-06-2-0002. Additional research support provided under a contract from the U.S. Navy
(Form Fit and Function Engineering as prime contractor).

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American Institute of Aeronautics and Astronautics

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American Institute of Aeronautics and Astronautics