Theses and Dissertations at Montana State University (MSU)
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Item Finite element modeling of piezoelectric bimorphs with conductive polymer electrodes(Montana State University - Bozeman, College of Letters & Science, 2010) Lediaev, Laura Marie; Chairperson, Graduate Committee: V. Hugo SchmidtThe purpose of my research has been to find a good way to solve for the mechanical and electrical behavior of piezoelectric polymer bimorphs which are electroded with a low to medium conductivity material. Traditionally, metal with very high conductivity has been used as the electrode material. Any applied voltage to an electrode will be distributed nearly instantaneously and uniformly throughout the electrode. Because of this quality, the voltage was assumed to be known and uniform for any applied voltage signal, including high frequency signals. The disadvantage of metal is that it is stiffer than polymers, and thus impedes the bending of the bimorph to a greater extent than for comparable polymer electrodes. With the modern invention of conductive polymers with acceptably high conductivities, it is now possible to manufacture piezoelectric devices with finite conductivity electrodes. For all but the very lowest frequencies of applied voltage signals, the voltage distribution cannot be assumed to be uniform throughout the electrode, nor can it be assumed to be exactly in phase. With finite conductivity electrodes there will be a loss in voltage amplitude due to resistivity, and there will also be a phase lag. The piezoelectric problem involves solving a coupled set of differential equations which involve mechanical displacement and electric potential. The electrical behavior of the electrodes is also included in the formulation, so that the voltage distribution in the electrodes is solved for simultaneously with the mechanical displacement and electric potential in the piezoelectric sheets. In this dissertation the coupled set of differential equations was solved using the Finite Element Method with quadratic Lagrange finite elements. The piezoelectric polymer which was modeled was polyvinylidene fluoride (PVDF). The conductive polymer of interest was PEDOT-PSS, although the model is valid for any type of isotropic finite conductivity material. The results of the work show that for moderate conductivity, the mechanical response of the bimorph is very good. There will not be a large phase lag within the first frequency mode. The bimorph resonates at low frequencies, and so any large effect from finite conductivity would only occur at higher modes.Item Thermomechanical training and characterization of shape memory alloy axial actuators(Montana State University - Bozeman, College of Engineering, 2010) Becker, Marcus Patrick; Chairperson, Graduate Committee: David A. MillerAlthough considerable work has been performed to understand the key mechanisms of Shape Memory Alloy (SMA) behavior, little of this work follows a standard testing protocol, quantifies a conditioning methodology, or develops data appropriate for design of SMA actuators. One major issue that limits the ability of the material from being used directly as an actuator is the large, non-recoverable strains likely to accrue in the material during each training cycle, mechanical or thermal. When mechanical or thermal cycling is performed, a hysteresis curve develops and reaches a steady state strain recovery response. At the point where permanent plastic strain stops growing, or saturates, the SMA has been successfully trained. The focus of this work is oriented toward SMAs in general, but all testing and experimentation was carried out on Nickel-Titanium (NiTi) alloys. The experimentation and testing was performed on a combination of 4 different sizes and 3 different NiTi alloy compositions. Thermomechanical testing was performed to determine critical values to describe the stress-temperature phase space of the materials and parameters to model the applied stress and transformation strain relationship. All material size and alloy combinations were tested in the as-received, or as-machined, and fully annealed state. The results of the training and actuation strain characterization process developed in this work shows that the samples that experienced Transformation Induced Plasticity (TRIP), greater than 2% during the training process and exhibit Two-Way Shape Memory (TWSM) after being fully trained, share a very similar applied stress versus transformation strain curve. This curve is modeled by the Back Stress formulation derived from the Gibbs Free Energy constitutive model by Bo & Lagoudas. The design space created by the Back Stress formulation, recrystallization temperature, and training stress allows SMA materials to be characterized and implemented as stable 1-D actuators. This research formalized a thermomechanical training and characterization method for uniaxial SMA actuators by addressing the interaction between processing, recoverable and non-recoverable deformation. Using various sizes and NiTi alloy combinations, this research develops and evaluates a method to train and characterize a diverse range of SMAs through a set of thermomechanical and physical property measurements.Item Integration of actuators and sensors into composite structures(Montana State University - Bozeman, College of Engineering, 2009) Ehresman, Jonathan David; Chairperson, Graduate Committee: Douglas S. CairnsThe need for more efficient wind turbine blades is growing in our society. One step in accomplishing this task would be to make wind turbines blades into smart structures. A smart structure is one that incorporates sensors, complete control systems, and active control devices, in order to shed, or redistribute the load placed on the structure. For wind turbine blades this means changing the shape of the blade profile as it encounters different wind conditions. In order to have active control surfaces functioning on wind turbine blades, the existing blades would have to be retrofitted, and the new blades being manufactured would have to be redesigned. There are different control surfaces to consider: gurney flaps and false wall flaps are two that can perturb the boundary layer across the low pressure side of the wing. A flat plate and blade section test bed will be manufactured in order to gather empirical data from wind tunnel testing. For actuation of the control surface there are many choices: electrical, hydraulic, pneumatic, and electro-hydrostatic. These actuator types will be investigated under a set of criterion to determine the best one for turbine blade application. Sensors will be investigated with respect to their use in sensing strain, temperature, acceleration, humidity, and delamination. Sensors are also used for health monitoring. This helps engineers design under a damage tolerant philosophy as opposed to a safe life structure philosophy. These sensors will be placed into laminates and different surface treatments will be reviewed to find the best configuration for each sensor. The sensor will be cleaned with isopropyl alcohol, dipped in a 20% by mass solution of nitric acid, and submerged in a 20% by mass solution of nitric acid for 10 seconds. Detailed surface images will be taken of sensors with different surface treatments in order to better understand the bonding between the sensor and laminate. These images indicate that submerging the sensors into 20% by mass solution of nitric acid is the best surface treatment.