Resonance frequency detuning (RFD) is a piezoelectric-based vibration reduction approach that applies to systems experiencing transient excitation through the system*s resonance—for example, turbomachinery experiencing changes in rotation speed, such as on spool-up and spool-down. This technique relies on the inclusion of piezoelectric material and manipulation of its electrical boundary conditions, which control the stiffness of the piezoelectric material. Resonance frequency detuning exploits this effect by intelligently switching between the open-circuit (high stiffness) and short-circuit (low stiffness) conditions as the excitation approaches resonance, subsequently shifting the natural frequency to avoid this resonance crossing and limit the response. The peak response dynamics are then determined by the system*s sweep rate, modal damping ratio, electromechanical coupling coefficient, and, most importantly, the trigger (represented here in terms of excitation frequency) that initiates the stiffness state switch. This thesis identifies the optimal frequency-based switch trigger over a range of sweep rates, damping ratios, and electromechanical coupling coefficients. With perfect knowledge of the system, the optimal frequency-based switch trigger decreases approximately linearly with the square of the coupling coefficient. Furthermore, phase of vibration at the time of the switch has a very small effect; switching on peak strain energy is marginally optimal. In practice, perfect knowledge is unrealistic and an alternate switch trigger based on an easily measurable parameter is necessary. As such, this thesis also investigates potential methods using the open-circuit piezoelectric voltage response envelope and its derivatives. The optimal switch triggers collapse to a near linear trend when measured against the response envelope derivatives and, subsequently, an empirical control law is extracted. This control law agrees well with and produces a comparable response to that of the optimal control determined using perfect and complete knowledge of the system.

Seismic designs of building structures are currently made based on the design criterion of life-safety and this requires that the structures do not collapse to compromise safety of people in the structure, but they can be designed to experience some damage. However, this design approach has allowed large economic losses primarily due to the damage to the nonstructural components at relatively moderate levels of seismic intensities. This led to a new thinking about design approach called performance-based design approach that satisfies the life-safety objective at the same time, reduces the economic loss to an acceptable level. The performance-based design approaches are multi-level design that addresses several different levels of structural performances under different levels of seismic intensities. In this study, we have investigated the use of energy dissipating damping devices to achieve the performance of a building structure in a desirable manner over all levels of seismic intensity. Since the initial motivation of performance-based design was reducing economic loss, the life-cycle cost-based optimization is considered in this study to obtain the optimal designs with different damping devices. For the optimal design, three types of devices are used in this study: fluid viscous dampers, solid visco-elastic dampers, and yielding metallic dampers. The combinations of two different types of dampers are also examined in this study. The genetic algorithm (GA) approach is adopted as an optimizer that searches for the optimal solution in an iterative manner. Numerical results from the application of the optimal design to the selected model building are presented to demonstrate the applicability of the developed approach and to estimate the effectiveness of the obtained optimal design with each device. It is shown in the results that the optimal design with each individual damping devices or the combination of two different types of damping devices are very effective in reducing the expected failure cost as well as the displacement response quantities and fragilities. The results also show that the optimal designs focus relatively more on reducing economic losses for the lower but more frequent excitation intensities as these intensities contribute most to the failure costs. / Ph. D.

Bone growth and remodeling are complex phenomena that are influenced by a variety of factors including mechanical stimuli. However, it is still unclear how to identify and quantitatively characterize the mechanical stimuli responsible for bone cell growth. The objective of this study is to design and simulate an experimental apparatus to cyclically apply pressure and shear stresses to bone cells and observe their growth (or lack thereof) as a function of the applied loads.

Classified for decades as a “least risk medical device,” surgical staplers have been recently associated with at least 41,000 injuries and 360 deaths in the last ten years (FDA Letter to Healthcare Providers, 2019). This shocking development has generated calls for a broad investigation into the errors involved in surgical stapler use and reform of the regulatory protocol for medical devices. Current regulatory infrastructure and framework operate with understandings that combine risk inherent to the device and that which is born by the operator (FDA Classification Call, 2019). This thesis explores the aforementioned classification error and its adverse outcomes from an epistemological standpoint. Social epistemic analysis is applied to FDA regulation and to the comparison of two scenarios in reference to the current status-quo classification and to the proposed risk reclassification of surgical staples. Expert versus novice error avoidance surgical performance capabilities are discussed under these two different classificatory scenarios and epistemic social roles.

Silicon Carbide (SiC) is a wide bandgap semiconductor that is currently of major interest for power electronics applications. SiC-based semiconductor devices and circuits are presently being developed for use in high-temperature, high-power, and/or high-radiation conditions under which conventional semiconductors lose their efficiency. However, the blocking capabilities of SiC power rectifiers and transistors are yet to approach their impressive theoretical limit due to so called edge effects at the device periphery. Surface passivation, which addresses many issues related to surface electric fields, is an extremely important fabrication step for high performance semiconductor electronic devices. Surface passivation can influence the surface recombination velocity, surface charge, interface trap density, and other surface characteristics. In this work, two-dimensional device simulations are used to establish the trends and the extent of the influence of charges, present in surface passivation dielectrics, on the reverse bias characteristics of SiC devices. Actual charges and charge instability are experimentally evaluated in a few common types of passivation dielectrics used in SiC device technologies. Device simulations are used to predict the corresponding improvement (or degradation) of the breakdown conditions at the device periphery, associated with the experimentally measured dielectric charges.

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