In this study, a tool was developed for incorporating crashworthiness into the preliminary design of grid-stiffened composite fuselage structures. The crash analysis of a fuselage structure was simplified through the development of a global-local procedure, which reduces the computational requirements of a crash simulation while facilitating the calculation of local stresses. The method was implemented in concert with a progressive failure model to model the entire crash sequence, including failure events and subsequent response. Several examples were used to validate this method. This method was then implemented, along with simple models for assessing the ability of a fuselage to meet other load requirements, into a genetic algorithm optimization procedure. The resulting preliminary design tool permits a designer to optimize a fuselage for minimum weight, maximum crashworthiness or any combination of these parameters. An illustrative example was utilized to demonstrate the use of the tool for an aluminum fuselage as well as a composite fuselage. Several designs were found for both materials that substantially increased crashworthiness without a significant penalty on structural weight. / Ph. D.

Soft-switching techniques can significantly reduce the switching loss and switching stresses of the power semiconductor devices in a power converter. This work presents several soft-switching topologies for high power PWM converters. These new topologies achieve soft-switching functions with minimum increase of device voltage/current stresses and converter circulating energy, and thus have advantages over conventional techniques in efficiency, power density, reliability, and cost of power converters. The improved zero-current transition (ZCT) converters achieve zero-current switching at both turn-on and turn-off for all main switches and auxiliary switches. These converters significantly reduce the switching loss and stress of the power semiconductor devices, while have a voltage/current stress and circulating energy similar to a PWM converter’s. The analysis, design, and experimental verification are presented. The three-phase zero-voltage transition (ZVT) boost rectifiers/voltage source inverters are developed with simple auxiliary circuits. Unlike most existing three-phase soft-switching techniques, these new topologies achieve soft-switching functions without overcharging the resonant inductors, and realize the benefits of soft-switching operation with minimum extra main switch turn-offs and fixed auxiliary circuit control timing. The operation principles of the developed techniques are experimentally verified, and their efficiency performances are evaluated with experiments and computer simulation. The three-phase ZVT buck rectifier topologies developed in this work achieves zero-voltage turn-on for all main switches with an optimum modulation schemes and simple auxiliary circuits. The auxiliary circuits, which are connected directly to each main switch, can also absorb the parasitic resonance of the bridge arms, and keep the voltage stress of the power devices at the minimum. The analysis and simulation results are presented to verify the converter operation. New ZVT dc-link schemes for three-phase ac-dc-ac converters are investigated. With coordinated control of the ac-dc converter and the dc-ac converter, a set of simple auxiliary circuit can provide soft-switching function for all switches in both the ac-dc converter and the dc-ac converter. The power loss in the auxiliary circuit is also significantly lower than existing dc-link soft-switching schemes. Simulation with experimentally obtained device switching loss data proves that significant efficiency improvement can be achieved with the new ZVT dc-link techniques. New ZVT and ZCT techniques for three-level converters are also developed. The auxiliary circuits are not in the main power path, and allow the converters to be controlled with optimum PWM schemes. Analysis and simulation results are presented to demonstrate the operation principles and advantages of soft switching in three-level converters. / Ph. D.

Ecological parameters of hydropsychid caddisflies are very important for production dynamics of stream ecosystems, due to the abundance and biomass of these collectorfilterers in many stream types. I studied life history and secondary production of Cheumatopsyche spp. (Trichoptera: Hydropsychidae) in Stroubles Creek, a 3rd order stream near Blacksburg, Virginia, in order to compare life histories and production dynamics in two reaches with different land-use in a watershed. The upstream section was influenced by urban and agricultural activities. The channel showed evidence of disturbance related to both geomorphological processes and agricultural activities. There was higher annual accumulated degree days, and stream substrate particles were mostly mid and coarse gravel. The downstream section was surrounded by forest, with little evidence of human activities and disturbance and with a more diverse distribution of particle size ranges in the streambed. Cheumatopsyche pettiti (Banks) was the only species of the genus that developed the whole life cycle in the upstream site, while C. pettiti and C. oxa Ross were both abundant downstream. There were three other Cheumatopsyche species collected with light traps in the area. Cheumatopsyche larvae and other aquatic insects were more abundant downstream, probably in relation to higher habitat availability. C. pettiti was determined to be bivoltine at the upstream site, perhaps related to high thermal input. However, only a fraction of the two Cheumatopsyche species could develop a 2nd generation downstream, overlapping with continuous emergence of the overwintering larvae during the summer. Cheumatopsyche production in Stroubles Creek was higher than in forested, low-order streams of North America but similar to some values obtained at streams in agricultural areas and large rivers. Production was less than from sites below food-enriched reservoir releases or from snag habitat in southeastern, coastal-plain rivers. Detritus was the greatest component of the diet of Cheumatopsyche at the two sites, but detritus and animal food had similar contribution to production downstream. The adjustment in life history and production dynamics in the two reaches showed the ecological plasticity of Cheumatopsyche and the importance of hydropsychids in the energy flow of moderately disturbed streams in mountainous regions. / Ph. D.

In this dissertation, an experimental and theoretical investigation into the nonlinear vibration of beam and beam-like structures with rectangular cross sections is presented. Two structures, a cantilever beam subject to a harmonic external excitation and a portal frame subject to a harmonic base motion, are the objects of study. For the cantilever beam, we present experimental results regarding multimode behavior. The beam was tested in both a vertical and horizontal configuration. Our experiments show that. for a forcing frequency near the fourth natural frequency of the beam, a low-frequency mode can be activated through a nonlinear mechanism. The nonlinear mechanism responsible for the transfer of energy to a low-frequency mode of the beam in the horizontal configuration was a subcombination internal resonance. However, for the same beam in the vertical configuration, both a subcombination internal resonance and a nonresonant modal interaction were observed to transfer energy to a low-frequency mode. The subcombination internal resonance consisted of contributions from the directly excited fourth mode, the fifth mode, and the low-frequency second mode. The response due to the nonresonant modal interaction consisted of contributions from the directly excited fourth mode and the indirectly excited low-frequency first mode. Both of these interactions are the result of a system with a dominant cubic nonlinearity. The single-mode response of the cantilever beam in the horizontal configuration was the subject of study. A comparison between the theoretically and experimentally obtained frequency-response curves revealed a discrepancy for an assumed ideal clamp. The model was brought into agreement by incorporating a quadratic damping term modeling the effect of air damping and a nonlinear rotational spring to model the flexibility of the clamp. The portal frame is a structure with a dominant quadratic nonlinearity. Experimental results are presented for the cases of a single combination resonance and multiple combination resonances. For the multiple combination resonances, excitation of a single mode was found to eventually activate contributions from six other modes, most of them possessing lower frequencies. The amplitudes of these lower-frequency modes were at times larger than that of the directly excited mode. The final topic is parameter identification for nonlinear systems. A scheme of experiments is designed that in conjunction with a multiple scales analysis can accurately estimate the nonlinear coefficients of a single-degree-of-freedom model. Parameters for a portal frame were ascertained by activating a subharmonic resonance of order one-half. / Ph. D.

This dissertation presents two different three-dimensional theories that examine the interaction between light and "strong" sound fields. Strong refers to a sound field strength sufficient to deflect most of the incident light into the first diffracted order, but not strong enough to induce nonlinear behavior in the acousto-optic cell. Primary emphasis is placed on deriving and experimentally verifying theories that predict the interaction process. All strong interaction theories to date consider only two transverse dimensions, where sound and light travel predominantly in the x and z directions, respectively. Our theories employ "split-step" type numerical methods to solve the theoretical equations derived for the interaction. The first method is called the Fourier-Optics approach and applies directly to a nondiffracting sound column. An incident light beam propagates through the cell by alternating between interaction and diffraction. The second method is called the Wave-Equation approach because it solves the coupled differential equations derived from Maxwell's equations. This method differs from the Fourier-Optics approach because it includes sound field diffraction into the interaction process. Throughout both theories we assume the sound field represents a bulk acoustic wave. Theoretical predictions show and experimental measurements verify that adding the y dimension into the interaction process enables one to examine acousto-optic interaction effects in a different way. Specifically, we show that the height (measured along the y-direction) of the sound beam contributes to distortions of the zero diffracted order not predicted by other theories. We also show that the y-dimension contributes to a reduction in diffraction efficiency as the light beam is incident at greater distances away from the acoustic transducer. This effect while observed in practice has not been shown in theory by previous acousto-optic interaction theories. Both theoretical predictions and experimental results are presented below. / Ph. D.

The objectives of this research were to correlate benzene biodegradation with soil physical, chemical and biological properties, to determine if biodegradation could be predicted based on measured or observed soil properties, and to investigate the role of nutrients on benzene biodegradation in soil. Benzene disappearance over time was measured in aerobic active and control (autoclaved) microcosms prepared with previously-uncontaminated subsurface soils. Soil microcosm experiments were prepared with initial benzene concentrations of 1, 10 and 50 mg/L. For each soil, logistic, zero-, first-, and 3/2- order kinetic models were fit to benzene disappearance versus time data by regression analysis. The logistic and 3/2-order models fit the data better than zero- and first-order models for experiments prepared with initial benzene concentrations of 1 and 10 mg/L. For an initial benzene concentration of 50 mg/L, experimental data were often better fit by zero- or first-order kinetic models. To obtain predictive equations, logistic kinetic model rate constants were related to soil properties using multiple linear regression (MLR). The “best” MLR models and their regression coefficient estimates were statistically significant at p<0.05. For experiments prepared with an initial benzene concentration of 1 mg/L, the resulting predictive equation contained soil phosphorus concentration and cation exchange capacity (CEC). For experiments prepared with an initial benzene concentration of 10 mg/L, the predictive equation contained soil copper, nitrate-N and phosphorus concentrations, CEC, and % sand. A comparison was made between benzene biodegradation in unamended soil microcosms and soil microcosms amended with ammonium and potassium phosphates (11 mM nitrogen, 6 mM phosphorus and 0.6 mM potassium). Benzene disappearance over time in soil microcosms was stimulated by nutrient addition in one (11%), 6 (50%), and 5 (45%) soils at initial benzene concentrations of 1, 10 and 50 mg/L, respectively. In general, nutrient addition had the greatest affect on benzene biodegradation in low pH soils. / Ph. D.

This dissertation involved a series of in vivo experiments to measure growth, immunoresponsiveness, and disease resistance in several genetic stocks maintained under different nutritional regimens. Genetic stocks included (1) a commercial layer line, (2) commercial broiler lines, (3) a White Rock line selected for high juvenile body weight, (4) a White Leghorn line selected for low antibody response to sheep erythrocytes, (5) a fast growing White Rock dwarf line, (6) a slow growing White Rock dwarf line, (7) a cross of 3 X 4, and (8) reciprocal crosses of 5 X 6. The nutritional regimens were either alternate-day (AD) and ad libitum (AL) feeding or high (E) and low (A) nutrient density. As expected, within a feeding regimen males were heavier than females and within a sex, AL chicks were heavier than AD chicks. Also, chicks fed Diet E were heavier than those fed Diet A. For traits associated with body weight there were significant feeding regimen by sire family interactions which was due to differences among sires in the magnitude of AD:AL relationship. Heterosis for body weight was consistently higher in males than females regardless of feeding regimens and higher for AD than AL chicks suggesting that the degree of heterosis was age and environment dependent. Immunocompetence as measured by production of antibody against sheep erythrocytes (SRBC) was higher for chicks fed AD than AL. In one experiment there was a significant diet by stock interaction for SRBC antibody titers. The interaction was because the commercial broilers fed Diet A had higher SRBC antibody titers than those fed Diet E, while the pattern for diets reversed in commercial layers. This suggested that the allocation of nutritional resources towards growth and immune response was not independent of the previous selection history of the population. Also, influence of genetic factors on antibody production was evident as there were differences among sire families in antibody titers to SRBC. In addition, heterotic effects for SRBC antibody were evident as titers were higher for the 3 X 4 crosses than lines 3 and 4 and titers were higher for the 5 X 6 crosses than lines 5 and 6. In faster growing genetic stocks responses to E. coli inoculation as measured by mortality and severity of cardiac and air sac lesions were greater for chicks fed AL than AD. For chicks fed AD lesion scores were lower for those without access to feed for the 24-h period immediately after challenge. When AD chicks were released to ad libitum feeding for several days and then inoculated, lesions were more severe than for AL and AD chicks. This result demonstrated that chicks from lines selected for rapid growth tended to allocate a greater proportion of resources to growth which reduced their ability to adjust rapidly to a disease challenge. Genetic factors influencing the response to E. coli inoculation were also evident as lesions were less severe for cross chicks than for their parental lines. / Ph. D.

The purpose of this research is to present privatization in Romania, and in particular to analyze Management and Employee buyouts as a variety of insider privatization that is unique to Romania. The institutional setting and a short chronology of privatization from 1991 to 1995 is presented. The question of why outsiders consistently pay premiums above book value when acquiring state-owned companies while insiders pay only approximately book value is investigated. Based on the available evidence it is contended that adverse selection prevents uninformed outsiders, foreign companies, and national residents as well, from investing on a large scale in the companies offered for sale. Outsiders are willing to buy the companies only when they have enough information, acquired either from insiders or using their own business skills. It is very likely that outsiders seek investment opportunities rather than assets already in place, because information on assets already in place is relatively more difficult to obtain. When outside investors are interested in taking over a company, they bid up the price and crowd out capital constrained insiders. In the absence of competition from outsiders, book value is the default price, and workers and managers are the only potential buyers. Workers and managers are granted preferential financing from the government in order to acquire state-owned firms. When interpreted in the broader context of mass privatization, this approach might be preferred by the government because it is populist and relatively expedient. When other methods failed to produce satisfactory results in Romania, Management and Employee Buyouts appeared to be the only method that keeps privatization going. / Ph. D.

The main purpose of this dissertation is to examine the effect of managerial attitudes on program standardization in international marketing. Three attitudinal variables have been identified as potential predictors of program standardization decisions: managers’ cultural distance, ethnocentrism, and quality-of-life (QOL) orientation. This dissertation empirically examines the effects of these managerial attitude variables on program standardization. It is hypothesized that managers’ ethnocentrism is directly related to program standardization, whereas cultural distance and QOL orientation are inversely related to program standardization. Furthermore, it is hypothesized that both ethnocentrism and QOL orientation are likely to moderate the relationship between cultural distance and program standardization. That is, cultural distance is likely to affect program standardization more for managers who have a high QOL orientation (or low ethnocentrism) than for managers who have a low QOL orientation (or high ethnocentrism). Cross-cultural comparisons of the three attitudinal variables and degree of standardization between U.S. managers and South Korean managers also have been explored. Specifically, it is hypothesized that compared with South Korean managers, U.S. managers are more likely to be characterized by high cultural distance, low ethnocentrism, and a high QOL orientation, and by a low degree of commitment to program standardization. Hypothesis were tested through an experiment using convenience samples of American and Koreans who were enrolled in MBA programs in the United States and South Korea. Results provide moderate support for the hypotheses. Implications are discussed. / Ph. D.

A national teacher shortage in Family and Consumer Sciences (FCS) prompted the development of this study. This study examined career motivations of recent graduates of FCS to determine target audiences and marketing strategies for recruitment to address the teacher shortage. Personal, educational, and employment demographics, career choice decisions, and contextual factors of recent graduates of FCS teacher licensure programs in 14 southern states were examined. The study also examined differences between first and second career graduates. A survey instrument developed by Serow (1994) was adapted for use in this study. Subjects were asked to indicate which of 35 career choice factors and personal values influenced their decision to become a teacher and to choose FCS as a field of study. The instrument was mailed in May 1996 to 494 graduates, whose names were obtained from teacher educators. The final sample consisted of 396 graduates. A total of 262 respondents completed the survey for a return rate of 66%. All but five of the respondents were female. The average age was 30 years. Ten percent were from minority groups. Over 40% lived and worked in rural areas or towns with under 10,000 population. Eighty-eight percent completed their teacher licensure through a bachelor's degree program. Most made their career choice decision while in college. Employment demographics indicated 43% of respondents were second career graduates, who indicated they had various first careers before seeking teacher certification. A total of 31% of respondents, although certified, were not teaching. Reasons given included not being able to find a job in the geographical area where they wanted to live or being employed in another field. Interest in FCS subject matter, enjoyment of working with young people, interest in families, helping people, professional satisfaction, and creativity were the reasons most often given as influencing career choice decisions. Cluster analysis of graduates responses on career choice factors revealed five groupings. These are (1) altruistically motivated, (2) interest, flexibility, and variety of career, (3) secondary school experience influenced, (4) socially conscious and friend influenced, and (5) college experience influenced. / Ph. D.