Abstract
Magnetorheological fluid composites were formulated in this study to investigate their performance for potential use in landing gear hydraulic systems, such as shock struts. The magnetorheological fluids synthesized here utilized three hydraulic oils certified for use in landing gear, two average diameters of spherical magnetic particles, and a lecithin surfactant. The magnetorheology of these fluids was characterized, including (a) magnetorheology (yield stress and viscosity) as a function of magnetic field, (b) sedimentation analysis using an inductance-based sensor, (c) cycling of a small-scale magnetorheological damper undergoing sinusoidal excitations at frequencies of 2.5 and 5 Hz, and (d) impact testing of an magnetorheological damper for a range 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 magnetorheological fluid composites, compare their behavior to standard commercial magnetorheological fluid, and determine their feasibility for use in helicopter landing gear.
Introduction
Magnetorheological fluids (MRFs) are suspensions of micron-sized magnetic particles, such as iron (Fe) or cobalt, in a silicone or hydraulic oil carrier fluid (Carlson and Jolly, 2000; Wereley et al., 2006; Park et al., 2009). Furthermore, MRFs have the ability to change rheological properties, such as yield stress and viscosity, when a magnetic field is applied. MRFs are used in an increasing range of applications, such as primary vehicle suspensions and semi-active vibration absorbing systems (Choi et al., 2005; Snyder et al., 2001). However, the use of MRFs targeting such applications as landing gear systems has not been widely investigated (Batterbee et al., 2007; Choi et al., 2012; Choi and Wereley, 2003).
During landing, an aircraft is subjected to a short-duration impulsive impact, which is a contributing factor to structural fatigue damage, crew and passenger discomfort, and dynamic stress. One possibility to reduce these impact loads is to incorporate MRFs into the shock struts of landing gear. By doing so, a landing gear can exploit the field adjustable yield stress to control the stroking load of a landing gear oleo to minimize impact loads transmitted to the aircraft fuselage. Magnetorheology depends on several factors including particle shape (Bell et al., 2007), and coatings (Fang and Choi, 2008) or passive particles (Powell et al., 2012). However, in this study, the focus will be on the feasibility of synthesizing MRFs employing three different carrier fluids, which are hydraulic oils licensed for use in landing gear (MIL-H-5606, MIL-PRF-83282, and MIL-PRF-87257). MRF performance was characterized through a series of measurements in order to assess the suitability of these MRFs: (1) rheological properties (yield stress and viscosity) were measured using a parallel disk rheometer as a function of magnetic field, (2) sedimentation rate was measured using an inductance-based sensor to measure particle settling rates, (3) dynamic behavior was measured using a small-scale MR damper, and (4) dynamic impact loads were measured as a function of varying magnetic field strengths utilizing a drop tower facility. The MRFs synthesized in this study are also compared to commercially available MRFs with similar solids loading (wt% of Fe powder). The key goal of this study is to assess the effectiveness and feasibility of using these newly synthesized MRFs in landing gear stroking elements such as shock struts or oleos.
Experimentation
Sample preparation
Three certified landing gear hydraulic oils were utilized as carrier fluids to prepare MRFs: a mineral oil (MIL-H-5606) and two synthetic hydrocarbon oils (MIL-PRF-83282 and MIL-PRF-87257). These particular oils were selected to preserve important characteristics needed for landing gear systems, such as wide operational temperature ranges (e.g. −65°F to 275°F), excellent antiwear agents, and fire resistance properties (Aviation Maintenance and Training Manual, 2003). Each hydraulic fluid was then used to prepare two categories of MRF composites, depending on the average diameter of spherical Fe particles present. The first sample set contained Fe particles of larger diameter (6–10 µm denoted by I), and the second set contained Fe particles of smaller diameter (1–3 µm denoted by II). Also, lecithin powder (2 wt%) was added as a surfactant to reduce agglomeration and to minimize settling. Accordingly, samples varying from 60 to 80 wt% (15–32 vol%) in Fe particle concentrations were synthesized, and each sample notation includes three important parts: the hydraulic oil number used (e.g. 83282 or 5606), Fe particle sizes (e.g. I ≡ 6–10 µm or II ≡ 1–3 µm), and Fe particle concentration (e.g. “d” for 75 wt% Fe and “e” for 80 wt% Fe).
Magnetorheological testing
MRF characterization was performed on all the MRF samples using a Paar Physica MCR 300 parallel disk rheometer. This instrument was used to measure the flow curve (i.e. shear stress vs shear rate) as a function of applied field. The prepared samples (0.3 mL) were loaded onto the rheometer, which had a standard 1 mm gap separating the rotating disk from the platen. The current was increased from 0.2 to 5 A to measure flow curves as a function of the applied field and to determine magnetic saturation of each sample.
The flow curves were characterized using the Bingham plastic (BP) model (Wereley et al., 2006), whose constitutive law is
where τy is the yield stress, µ is the postyield viscosity, and
Sedimentation testing
The effectiveness of the MRF composite to maintain the suspension was analyzed by quantifying the settling rates of Fe particles using a 1/4-in inductance-based sensing coil. In fact, after settling, the Fe particles may form a hard cake due to remnant particle magnetization, which is difficult to redisperse, rendering the MRF ineffective. Therefore, in order to prepare stable MRF, adding a surfactant, such as lecithin, improves mixability by reducing this tendency to form a hard cake. The sedimentation setup is described in Powell et al. (2012) and Ngatu and Wereley (2007).
As settling progresses, the sensing coil tracks a distinct boundary, or “mudline,” between the clarified carrier fluid above and the MRF below, and measures the inductance, which depends on the permeability of MRFs (also dependent on the volume fraction of dispersed particles) enclosed by the sensing coil. As the mudline travels downward through the sensing coil, the permeability of the fluid volume contained therein decreases. The slope of this curve yields the sedimentation rate (Ngatu and Wereley, 2007; Powell et al., 2012). The samples were tested twice: a sedimentation test was performed immediately after synthesizing the fluid and a second test after being stored quiescent for 1 month in order to assess remixability.
Damper testing
Performance of the MRFs was determined by the response of a modified Rheonetics SD-1000-2 magnetorheological (MR) damper from Lord Corporation to sinusoidal loads. Only MRFs containing Fe particle concentration of 80 wt% or 32 vol% were tested. The damper was subjected to sinusoidal loading on an 810 Material Test System (MTS) machine. In order to apply a magnetic field inside the damper, electric current (from 0 to 4 A) was used to power the magnetic circuit. A cross section of the damper, the test setup, and further discussion of this MR damper can be found in Snyder et al. (2001) and Powell et al. (2012).
Force versus displacement data were analyzed using the nonlinear biviscous (NBV) model (Wereley et al., 2004). The NBV model was employed to represent force versus velocity damper response and provided a means to estimate yield force (Fy) and postyield damping (Cpo). The model is piecewise continuous in velocity and assumes that the MR fluid is plastic in both the preyield, Cpr, and the postyield, Cpo, conditions with the preyield damping being much greater than the postyield damping (Wereley et al., 2004).
The piecewise continuous equations describing the model are shown in equations (2) and (3)
The preyield velocity is
Impact testing
A free-flight drop test facility with a drop carriage of 59 kg, as shown in Figure 1, was used to conduct impact tests on a magnetorheological energy absorber (MREA) filled with synthetic oil-based MRF containing 6–10 µm Fe particles at a concentration of 80 wt% (denoted mr83282-Ie). Recently, MREAs have been highlighted as a promising candidate for crashworthiness systems, and several impact tests have been conducted (Facey et al., 2005; Mao et al., 2007). Therefore, the performance of mr83282-Ie in a MREA subjected to drop testing is important to verify the adjustability of MREA response at impact velocities representative of sink rates that occur during a helicopter landing.
MREA drop testing setup at the University of Maryland.
Before each test, the MREA rod was extended, and the drop carriage was raised to a specific height corresponding to a selected impact velocity; then, the drop carriage was released to freely fall under gravity until impacting the test assembly (Figure 1).
Results
Magnetorheology
Rheological tests were performed on all of the prepared MR fluids at room temperature. The fluid was sheared by the rotating top disk of the rheometer as the bottom disk stayed stationary. Figure 2 shows selected flow curves with the BP model of mr83282-Ie with synthetic oil (MIL-PRF-83282). Synthetic oil-based MRF containing larger (6–10 µm) Fe particles with 75 wt% (or 26 vol%) Fe concentration (referred as mr83282-Id) rheology was compared to a commercial MRF from Lord Corporation (MRF126CD) of the same Fe concentration. The MRF sample mr83282-Id had an off-state (dynamic) viscosity lower than that of the commercial MRF, as illustrated in Figure 3. It is important to keep the off-state viscosity as low as possible in order to maintain high-frequency transmissibility in a base-excited isolation mount as low as possible (Choi et al., 2005). Also, both fluids had field-dependent yield stress curves that followed similar trends, as shown in Figure 4.
Sample flow curve or shear stress versus shear rate for oil-based mr83282-Ie (80 wt% Fe).
Off-state viscosity (Pa s) of mr83282-Id and MRF126CD (both 75 wt% and 26 vol% particle concentration).
Yield stress (kPa) as a function of applied current (A).
Sedimentation
Particle sedimentation rates of the tested MRFs were measured several times to verify redispersion consistency. For example, the inductance of the MRF containing synthetic oil and larger particles at 80 wt% (i.e. mr83282-Ie) was measured while the fluid mudline descended through the sensor. After performing the first set of tests, the fluid was not remixed for 1 month until the second set of tests was conducted. Testing was repeated several times on the same sample, and Figure 5 shows that the results are similar. This demonstrates that the particles were easily remixed in the MRF after being left alone for at least 1 month.
Settling rates of mr83282-Ie (containing 6-10 µm particles at 80 wt% concentration). Testing was performed 1 month apart.
Damping behavior
The hysteretic behavior of a linear stroke MR damper containing MRF was studied using high Fe particle concentration fluids of 80 wt% or 32 vol% (specifically, MRFs denoted mr83282-Ie and mr5606-Ie). The damper was tested using different sinusoidal shaft displacements: 1.27, 2.54, 5.08, and 7.62 mm, at two different frequencies (2.5 and 5 Hz), while the magnetic field was controlled using a power supply. The NBV model was used to characterize the yield force of the MR damper. The performances of both MRF samples, that is, mr83282-Ie and mr5606-Ie, were compared to a commercial MRF from Lord Corporation (MRF132) of the same particle concentration. The yield force of all three fluids followed the same trend as a function of applied field, particularly at 5 Hz. Figure 6 illustrates these results, and all three fluid yield forces plotted versus current show that the maximum yield forces are within 5% of each other.
Yield force (N) versus current (A) for synthetic oil-based (mr83282-Ie) and mineral oil-based (mr5606-Ie) MRFs and commercial MRF (Lord MRF132) at 5 Hz.
Impact loads
An MREA was filled with a synthetic hydraulic oil-based MRF with 6–10 µm Fe particles at 80 wt% concentration (denoted mr83282-Ie) and subjected to drop tests. This particular fluid was selected due to its favorable performance based on the rheological and damper test results. Peak stroking loads measured for a range of impact velocities and applied magnetic field strengths are in Table 1. The peak force increased as magnetic field and impact velocities increased. In Table 1, for a given field strength, the peak stroking load increased as velocity increased as a result of the velocity squared viscous force component. As the current (or magnetic field) increased, the peak stroking load also increased, which resulted in the increase in yield stress or MREA force. Tunability of the stroking load was greater at lower impact velocity and reduced slightly as the velocity increased. The MRF (mr83282-Ie) enabled tunable behavior as the magnetic field varied. The peak stroking load, as well as the energy absorbed by the MREA, could be adjusted by increasing magnetic field for an impact velocity of 2.8 m/s (Figure 7).
Peak stroking load (N) for applied current and impact velocity.
Stroking load time history for three different applied field strengths for an impact velocity of 2.8 m/s.
Conclusion
MRFs were synthesized using three different carrier hydraulic oils certified for landing gear use, and the feasibility of these MRFs for potential use in landing gear systems was assessed. A series of sample MRFs were synthesized for different solid loadings (vol% of carbonyl Fe) using landing gear fluids as the carrier fluids: MIL-H-5606, MIL-PRF-83282, and MIL-PRF-87257. A lecithin surfactant was used (2 wt%) to maintain the suspension and to prevent particle agglomeration. These fluids were also compared to commercially available fluids from Lord Corporation with comparable solid loading.
First, magnetorheological properties were tested as a function of applied field, and the experimental data were characterized using the BP model. Using flow curve data, the yield stress and viscosity of the MR fluid composites were identified. The MR landing gear fluid composite compared favorably with a commercial MR fluid (both containing 26 vol% magnetic particles).
Second, a particle sedimentation study was performed on the fluids using an inductance coil-based sedimentation rate monitoring system. Consequently, particle dispersion stability was effective, and redispersion showed similar results, even though fluids were left quiescent for over a month.
Third, the performance of a linear stroke MR damper, filled with MRFs, was characterized using a NBV model. The NBV model was used to successfully identify the yield force. MR damper behavior was compared to the damper behavior using a commercial MRF (of 32 vol% particle concentration). The yield forces of the MRFs containing the larger (6–10 µm) Fe particles (32 vol%) compared favorably with that of the commercial MRFs, and measured yield forces of the MR damper with either MRF were within 5% of each other.
Finally, synthetic oil-based MR fluid (of 32 vol% Fe particles) was utilized in an MR damper and subjected to high shear rate drop testing to experimentally verify the tuning nature of the MR device at different 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.
Based on this range of tests used to characterize MRFs synthesized, using certified landing gear fluids, it has been shown that it is feasible to utilize such hydraulic oils as the carrier fluids in suitable MR fluids. Additional testing is warranted to ensure that the addition of particle solids and surfactants does not affect key properties of the hydraulic carrier fluids such that operating temperature range and resistance to flammability are preserved in landing gear applications.
Footnotes
Funding
This study was supported by the National Rotorcraft Technology Center under Technology Investment Agreement W911W6-06-2-0002.