A comparison of stack load-line (including blocked force and free displacement) as well as dynamic response of two
single crystal PMN-32%PT stacks is provided in this study. The first stack is a 7mm diameter by 0.5mm thickness 60
layer single crystal stack while the second stack is a 6mm diameter by 0.3mm thickness 100 layer single crystal stack.
Blocked force and free displacement measurements were both performed under DC driving conditions. Free
displacement measurements showed that under 500V driving conditions displacements approaching 87&mgr;m (~2500ppm)
and 48&mgr;m (~1450ppm) were obtained for the 6mm and 7mm diameter stacks, respectively. Experimental blocked force
measurements correlated well with theoretical predictions with experimental values approaching 709N and 685N for the
6mm and 7mm diameter stacks, respectively. The error between the theoretical predictions and experimental values was
attributed to the linear load line assumption in the theoretical model whereas the stack stiffness is dependent upon the
applied force. Dynamic measurements performed under a pre-stress of 4MPa indicated an increase in the strain at
frequencies above 500Hz for driving frequencies up to 1000Hz. This was unexpected as the PMN stack resonance was
calculated to be on the order of several kHz.
The primary objective of this study is to estimate the parameters of constitutive models characterizing the rheological properties of ferrous and cobalt nanoparticle-based magnetorheological fluids. Constant shear rate rheometer measurements were carried out using suspensions of nanometer sized particles in hydraulic oil. These measurements yielded shear stress vs. shear rate as a function of applied magnetic field. The MR fluid was characterized using both Bingham-Plastic and Herschel-Bulkley constitutive models. Both these models have two regimes: a rigid pre-yield behavior for shear stress less than a field-dependant yield stress, and viscous behavior for higher shear rates. While the Bingham-Plastic model assumes linear post-yield behavior, the Herschel-Bulkley model uses a power law dependent on the dynamic yield shear stress, a consistency parameter and a flow behavior index. Determination of the model parameters is a complex problem due to the non-linearity of the model and the large amount of scatter in the experimentally observed data. Usual gradient-based numerical methods are not sufficient to determine the characteristic values. In order to estimate the rheological parameters, we have used a genetic algorithm and carried out global optimization. The obtained results provide a good fit to the experimental data.
Magnetorheological (MR) fluids can be used in a variety of smart semi-active systems. MR dampers have especially great potential to mitigate environmentally induced vibration and shocks. MR fluid can be used effectively in valve networks to control the flow from a hydraulic source to enable a fully active actuator. These devices are simple, have few moving parts and can be easily miniaturized to provide a compact, high energy density pressure source. The present study describes a prototype MR-piezo hybrid actuator that combines the piezo-pump and MR valve actuator concepts, resulting in a self-contained hydraulic actuation device without active electro-mechanical valves. Durability and miniaturization of the hybrid device are major advantages due to its low part count and few moving parts. An additional advantage is the ability to use the MR valve network in the actuator to achieve controllable damping. The design, construction and testing of a prototype MR-piezo hybrid actuator is described. The performance and efficiency of the device is derived using ideal, biviscous and Bingham-plastic representations of MR fluid behavior. Unidirectional performance, or constant velocity actuator shaft motion, is assessed analytically and compared to experimental data. A experimental assessment of the magnetorheological birectional flow control capability is also provided.
The primary objective of this study is to estimate the parameters of a constitutive model characterizing the rheological properties of a ferrous nanoparticle-based magnetorheological fluid. Constant shear rate rheometer measurements were carried out using suspensions of nanometer sized iron particles in hydraulic oil. These measurements provided shear stress vs. shear rate as a function of applied magnetic field. The MR fluid was characterized using both a Bingham-Plastic constitutive model and a Herschel-Bulkley constitutive model. Both these models have two regimes: a rigid pre-yield behavior for shear stress less than a field-dependant yield stress, and viscous behavior for higher shear rates. While the Bingham-Plastic model assumes linear post-yield behavior, the Herschel-Bulkley model uses a power law dependent on the dynamic yield shear stress, a consistency parameter and a flow behavior index. Determination of the model parameters is a complex problem due to the non-linearity of the model and the large amount of scatter in the experimentally observed data. Usual gradient-based numerical methods are not sufficient to determine the characteristic values. In order to estimate the rheological parameters, we have used a genetic algorithm and carried out global optimization. The obtained results provide a good fit to the data and support the choice of the Herschel-Bulkley fluid model.
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