Chair: John A. Shaw

Shape Memory Alloys are a unique and valuable group of active materials. NiTi, the most popular SMA, has a power density orders of magnitude greater than any other known material, making it valuable in the medical and transportation industries where weight and space are at a premium. In the nearly half-century since its discovery, the adoption of NiTi has been slowed primarily by the engineering difficulties associated with its use: strong thermal coupling, material level instabilities, and rapid shakedown of material properties during cycling. Material properties change drastically with minute changes in alloy composition, so it is common to require a variety of experiments to fully characterize a new SMA material, all of which must be performed and interpreted with specialized techniques.

Several of these techniques are then applied to a systematic characterization of NiTi tubes in tension, compression, and bending. Particular attention is given to the nucleation and propagation of transformation fronts in tensile specimens. The fronts take the form of either a ring of small “branched” inclusions, or of a helical inclusion which at high loading rates wraps around the entire tube. A rate study shows that the shorter ringed front is energetically preferred near isothermal conditions, but at higher rates, where thermomechanical coupling favors a long transformation front, the helical front is preferred.  Compression experiments show dramatic asymmetry in the uniaxial response, with compression characterized by a lower transformation strain, higher transformation stress, and 1 uniform transformations (no fronts).

A very simple SMA actuator model is introduced. After identifying the relevant nondimensional parameters, an analytical solution to the governing equations is developed, the first of its kind. The power of the analytical solution is exercised in a series of design studies examining spring sizing, power requirements, response time, and energy efficiency.

Plasma actuators and various forms of volumetric energy deposition have received a good deal of research attention recently as a means of hypersonic flight control.  Unfortunately, ground-based and flight experiments are extremely expensive and potentially dangerous, thus creating a need for computational tools capable of quickly and accurately modeling these devices and their effects on the flow-field. This thesis addresses these limitations by developing and incorporating several new features into an existing parallelized three-dimensional flow solver to accurately account for chemical reactions and electromagnetic effects.

A phenomenological heating model is developed and coupled to the fluid solver to investigate whether a practical level of pitch moment control can be achieved from volumetric energy deposition for a realistic hypersonic vehicle.  
The results imply that the shape of the deposition does not have a noticeable effect on the flow structure, whereas the amount of energy deposited greatly influences the flow-field. The results suggest that these systems are adequate replacements for traditional mechanical flaps.

While the phenomenological heating model sufficiently characterizes the downstream flow properties, the approximation is nonphysical. To improve the physics and near-device results, a three-dimensional magnetohydrodynamics (MHD) solver is developed and coupled to the fluid solver. This solver accurately computes the current density, electric, and magnetic fields, and accounts for their effects on the flow-field.

A particularly important parameter in the MHD solver is the electrical conductivity.  Although several semi-empirical models exist in the literature, none provide generality across different flight regimes or gas compositions. Boltzmann?s equation provides the necessary generality, but directly coupling a Boltzmann solver to a fluid solver is computationally prohibitive, even in the most modern CFD tool suites. A surrogate model of solutions to Boltzmann?s equation is developed and coupled to the fluid solver to provide the accuracy and generality of the Boltzmann solver without the computational expense.  

With this accurate electrical conductivity module, the coupled MHD-fluid solver is used to investigate the effectiveness of a MHD-parachute, a device that uses a magnet positioned near the bow of the vehicle to reduce the amount of heat transferred to the vehicle.