A two part study is performed to investigate the application of reliability-based design optimization (RBDO) approach to design elastic-plastic stiffener beams with Tsection. The objectives of this study are to evaluate the benefits of reliability-based optimization over deterministic optimization, and to illustrate through a practical design example some of the difficulties that a design engineer may encounter while performing reliability-based optimization. Other objectives are to search for a computationally economic RBDO method and to utilize that method to perform RBDO to design an elastic-plastic T-stiffener under combined loads and with flexural-torsional buckling and local buckling failure modes. First, a nonlinear elastic-plastic T-beam was modeled using a simple 6 degree-of-freedom non-linear beam element. To address the problems of RBDO, such as the high non-linearity and derivative discontinuity of the reliability function, and to illustrate a situation where RBDO fails to produce a significant improvement over the deterministic optimization, a graphical method was developed. The method started by obtaining a deterministic optimum design that has the lowest possible weight for a prescribed safety factor (SF), and based on that design, the method obtains an improved optimum design that has either a higher reliability or a lower weight or cost for the same level of reliability as the deterministic design. Three failure modes were considered for an elastic-plastic beam of T cross-section under combined axial and bending loads. The failure modes are based on the total plastic failure in a beam section, buckling, and maximum allowable deflection. The results of the first part show that it is possible to get improved optimum designs (more reliable or lighter weight) using reliability-based optimization as compared to the design given by deterministic optimization. Also, the results show that the reliability function can be highly non-linear with respect to the design variables and with discontinuous derivatives. Subsequently, a more elaborate 14-degrees-of-freedom beam element was developed and used to model the global failure modes, which include the flexural-torsional and the out-of-plane buckling modes, in addition to local buckling modes. For this subsequent study, four failure modes were specified for an elasticplastic beam of T-cross-section under combined axial, bending, torsional and shear loads. These failure modes were based on the maximum allowable in-plane, out-ofplane and axial rotational deflections, in addition, to the web-tripping local buckling. Finally, the beam was optimized using the sequential optimization with reliabilitybased factors of safety (SORFS) RBDO technique, which was computationally very economic as compared to the widely used nested optimization loop techniques. At the same time, the SOPSF was successful in obtaining superior designs than the deterministic optimum designs (either up to12% weight savings for the same level of safety, or up to six digits improvement in the reliability for the same weight for a design with Safety Factor 2.50). / Ph. D.

This research compared physical characteristics of bodybuilders to judges' rankings, and compared the judges' rankings across three levels of judges: Elite, Trained, and Untrained. Twenty-nine male and nine female bodybuilding athletes consented to anthropometric and circumference measurements. The independent variables in this study were bodyfat, fat-free weight, and proportionality of muscle. Three groups of judges ranked male and female athletes in the Open, Novice, Collegiate, and Masters divisions. The measurements of proportionality of muscle, bodyfat, girths, and fat-free weight were analyzed using simple and multiple regression. The judges' rankings in each class were compared using reliability coefficients, correlation, repeated measures analysis of variance, and the generalized theory for inter-rater reliability. There were significant correlations between the Elite judges' rankings and bodyfat in the Men's Open lightweight class. The Trained and Elite judges' rankings were significantly correlated with bodyfat in the Women's Novice class. Fat-free weight was significantly correlated with the Untrained and Elite judges' rankings in the Women's Novice division. Proportionality of muscle was significantly correlated with the Elite and Trained judges' rankings in the Men's Open lightweight class. Elite and Trained judges' rankings were significantly correlated with bodyfat and proportionality of muscle. All three groups of judges' were significantly correlated with the Overall rankings. The Trained judges' rankings were more correlated and more reliable with the Elite judges. The inter-rater reliability scores were higher for the Elite and Trained judges than for the Untrained judges. When fat-free weight was substituted for body weight in the calculation of proportionality of muscle, prediction of ranking was enhanced. The education session led to a significant disparity in judges' rankings between Trained and Untrained judges. Bodyfat appears to be a better selection variable when comparing physical measurements to judges' rankings in women. Proportionality of muscle is a better selection variable in predicting judges' rankings in men. Prediction models developed from this investigation need further testing. / Ph. D.

Transportation engineers and planners worldwide are faced with the challenge of improving transit services in urban areas using low cost means. Transit signal priority is considered to be an effective way to improve transit service reliability and efficiency. In light of the interest in testing and deploying transit signal priority on a major arterial in Northern Virginia, this research focuses on the impacts of transit signal priority in the U.S.1 corridor in Fairfax County in terms of benefits to transit and impacts on other traffic. Using a simulation tool, VISSIM, these impacts were assessed considering a ten second green extension priority strategy. The results of the simulation analysis indicated that the Fairfax Connector buses benefit from the green extension strategy with little to no impact on the other non-transit traffic. Overall, improvements of 3.61% were found for bus service reliability and 2.64% for bus efficiency, while negative impacts were found in the form of increases in queue lengths on side streets by a maximum value of approximately one vehicle. Because this research has provided a foundation for the evaluation of transit signal priority for VDOT and Fairfax County engineers and planners, future research can build upon this effort. Areas identified for future research include the provision of priority for the entire bus route; combination of emergency preemption and transit priority strategies; evaluation of other priority strategies using system- wide priority concepts; and the impacts of priority strategies in monetary terms. / Master of Science

Silicon carbide (SiC) is a wide bandgap material capable of higher voltage and higher temperature operation compared to its silicon (Si) counterparts due to its higher critical electric field (E-field) and higher thermal conductivity. Using SiC, MOSFETs with a theoretical high temperature operation and reliability is achievable. However, current bottlenecks in high temperature SiC MOSFETs lie within the limitations of standard packaging. Additionally, there are reliability issues relating to the gate oxide region of the MOSFET, which is exacerbated through high temperature conditions. In this thesis, high temperature effects on current-generation SiC MOSFETs are studied and analyzed. To achieve this, a high temperature package is created to achieve reliable operation of a SiC MOSFET at junction temperatures of 300 °C. The custom, high temperature package feasibility is verified through studying trends in SiC MOSFET behavior with increasing temperature up to 300 °C by static characterization. Additionally, the reliability of SiC MOSFETs at 300 °C is tested with accelerated lifetime bias tests. / M.S. / Electrical devices that are rated for high temperature applications demand a use of a material that is stable and reliable at the elevated temperatures. Silicon carbide (SiC) is such a material. Devices made from SiC are able to switch faster, have a superior efficiency, and are capable of operating at extreme temperatures much better than the currently widely used silicon (Si) devices. There are limitations on SiC certain structures of SiC devices, such as the metal oxide semiconductor field effect transistor (MOSFET), have inherent reliability issues related to the fabrication of the device. These reliability issues can get worse over higher temperature ranges. Therefore, studies must be made to determine the feasibility of SiC MOSFETs in high temperature applications. To do so, industry standard tests are conducted on newer generation SiC MOSFETs to ascertain their use for said conditions.

Yes / This paper proposes a novel approach for the reliability cost-based optimization of Distribution Systems (DS), considering tie line-based network reconfiguration method with integration of Distributed Energy Resources (DER). An optimal Energy not Supplied (ENS) index is proposed, where the capacity is handled by curtailment devices in the network such as sectionalizers and the energy supplied by DERs which considers Flexibility Services (FS) within a market environment. The decision variables include the investment and operation of tie-lines and buying regulation services from DER such as Distributed Generation (DG) and Battery Energy Storage Systems (BESS). The results validate the cost-effectiveness of the proposed method through implementation of these technologies to improve the reliability of the DS, within a comprehensive set of case-study scenarios for a 16-bus UK generic distribution system (UKGDS). The case study results indicate that significant savings can be achieved through the proposed method, ranging between 36%–71% depending on the level of automation in tie-line operations in combination with the settlement price for the power-balance of FS. This illustrates that the proposed DS reliability cost-based optimization method could have a significant impact for real world DG and BESS applications. / This work was supported in-part by Innovate UK GCRF Energy Catalyst Pi-CREST project under Grant number 41358, and in-part by University of Bradford under the SURE Grant scheme (Grant Number 023000/66005).

Small Unmanned Aerial Vehicles (SUAVs) are rapidly being adopted in the National Airspace (NAS) but experience a much higher failure rate than traditional aircraft. These SUAVs are quickly becoming complex enough to investigate alternative methods of failure analysis. This thesis proposes a method of expanding on the Fault Tree Analysis (FTA) method to a Bayesian Belief Network (BBN) model. FTA is demonstrated to be a special case of BBN and BBN can allow for more complex interactions between nodes than is allowed by FTA. A model can be investigated to determine the components to which failure is most sensitive and allow for redundancies or mitigations against those failures. The introduced method is then applied to the Virginia Tech ESPAARO SUAV. / Master of Science

The incorporation of reliability theory into a fatigue analysis program is studied. A thorough background in probabilistic methods and metal fatigue is presented, allowing a full understanding of these processes. An automotive wheel assembly is then introduced as an example of the applications of this durability/reliability design package. A detailed step-by-step procedure is utilized to develop the basic information needed to analyze the wheel assembly: material properties, geometry, and loading; the relationship between applied load and stress; and the degree of variation in specific material properties, wheel thickness, and service loading. An in depth documentation of the effect of these “real world” variations on wheel reliability is then presented in graphical form. Several different approaches in altering the design variables are used in order to thoroughly illustrate the resulting component reliability. Such information is particularly relevant where product quality and warranty formulation are concerned. / Master of Science

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