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offset about 30kPa (6psi) across the channel test section was observed. For the first ramp, the piston
pushes the MR fluid through the channel with a constant velocity with an externally applied magnetic
field. As a result of the MR effect, the pressure drop (Ap=Pi-P 2 ) across the magnetically activated
region increases significantly. As illustrated in Figure 3, we define this pressure drop as the dynamic
pressure drop, since its value depends on the combination of the applied magnetic flux density and the
input velocity of the piston. In this study, the dynamic pressure drop experimental data can establish
the shear stress and shear strain rate relationship needed to obtain the apparent viscosity and dynamic
yield stress of MR fluids.
For the second ramp, the piston returns to its original position. Both the pressures P! and P 2 decreased
while the accumulator pushes the MR fluid back. In addition, PI drops more than its initial pressure.
This means that a vacuum may have been formed in the inlet chamber. The material resistance due to
the MR effect may prevent the MR fluid returning back completely. A negative pressure drop is
obtained at this phase. We refer to this pressure drop as the static pressure drop, as shown in Figure 3.
The static pressure drop is highly dependent on the magnetic flux density and nearly is unaffected by
the piston movement.
When the piston is completely stopped, after returning to its initial position, the static pressure drop
can be kept in equilibrium until the magnetic field is switched off. After a short period following the
input electric current being turned off, the pressures PI and p2 recover to their original values, as
shown in Figure 3. This would suggest that the static pressure drop is the minimum pressure that can
induce MR fluid flow. Therefore, using Equation (1), the static shear yield stress of MR fluids could
be evaluated from the measurement data of the static pressure drop.
242
2000.00 0.40
-500.00 -1.60
I I I ~
0.00 5.00 10.00 15.00 20.00 25.00
Time (&\
Figure 3. A typical measured result of piston displacement and flow
pressures across the rectangular channel.