A new multicharacteristic control chart designed to detect shifts in the mean of a multivariate process is proposed. It is assumed that the correlation matrix is known and that the distribution of the data is multivariate normal. The new chart is based on the minimum standardized sample mean (Z_[1]) and the maximum standardized sample mean (Z_[p]) of p correlated quality variables or characteristics. For this reason the chart has been named the Minimax control chart. A method for calculating probabilities for the joint distribution of Z_[1] and Z_[p] is developed. This method is used to determine the position of the four control limits of the chart; the upper and lower control limits of Z_[1], and the upper and lower control limits of Z_[p]. The control limits of the chart are determined such that the chart has a fixed probability of Type I error. The chart’s performance is compared to that of the Chi-squared control chart in terms of the average run length for several combinations of the parameters of the chart. Among these parameters are the sample size, the number of variables, the probability of Type I error, the correlation matrix, and the direction and magnitude of the shift in the mean. The proposed chart outperforms the Chi-squared chart in all the cases studied where the covariance matrix has non-negative elements. The new chart provides an easy way for diagnosing the system when a signal occurs. That is to say, the chart provides a means to identify the source of the problem when a shift in the mean occurs. The criteria established for diagnosing the system is based on the positions of Z_[1] and Z_[p] in the Minimax chart. Thus, to diagnose the signals no further analysis is needed. The diagnosing criteria are shown to be particularly effective when the shifts in the mean are either axial or diagonal. / Ph. D.

In this dissertation class groups and unit groups of number fields with elementary Galois groups of order 4 and 8 are considered. In chapter 3 we consider bicyclic biquadratic extensions K/k and give a method for determining the structure of the 2-class group of K. In chapters 4 and 5 this method is applied to real and imaginary bicyclic biquadratic extensions of Q. In chapter 6 a method for determining the unit group of an imaginary octic field is given. In the final chapter all imaginary octic fields of class number less than or equal to 16 or prime class number are determined. / Ph. D.

The trend for women to follow gender-traditional educational and occupational pathways has been resistant to change, even in the face of decreased income potential. Scholars have tested hypotheses regarding women's avoidance of nontraditional occupations, but have given scant attention to the experience of women who have exited the traditional path. This research employed a qualitative methodology within a feminist theoretical framework to elicit the voices of women in nontraditional programs. The purpose of this research was to elucidate (a) the career choice process and (b) the educational environment experienced by women in gender-nontraditional programs at the community college level. It was performed at a community college in the southeastern United States. Enrollment data revealed five associate degree programs with a female enrollment of 25% or less. Eight women were selected to participate in qualitative interviews to approach an understanding of educational choice and environment from the female students' perspective. Data from interviews with instructors, site visits, and analysis of relevant documents contributed to the research findings. Findings revealed a description of the site where institutional policies, administrators, instructors, and counselors contributed to a positive learning environment for female students in nontraditional programs. Seventeen themes emerged from interviews with research participants in response to the two main research questions. These were: (1) why women work, (2) a habit of individualism, (3) role models/early influences, (4) children: effects of/on, (5) the significant other, (6) instructors: an ethic of caring, (7) instructors: negative, (8) classes and peers, (9) language, (10) earlier educational environment, (11) the subtle nature of discrimination, (12) not so subtle, (13) evolution: educational, (14) evolution: personal, (15) being allowed (16) perseverance, and (17) internalized sexism. The study resulted in a description of the women's educational environment as a place where the infrequent incidents of discrimination were far outweighed by the feeling of welcome and encouragement framed by the instructors and other faculty. The facilitators of instructor support and a positive campus climate were more than sufficient to help the women who demonstrated intrinsic strength and self-reliance overcome barriers of early educational discrimination and negative input from peers. / Ph. D.

Muskmelon seeds were harvested at eight stages of development to determine the optimal harvest time for best seed quality and long-term storage. Several types of osmotic and matric priming treatments were investigated to determine the optimal treatment for maximal seed vigor and to test the effects of priming on seed storage life. The most effective priming treatments for improved germination of muskmelon seeds occurred at water potentials (Ψ) between -1.5 and -1.8 MPa and at priming durations of 4 to 7 days. Osmotic priming in KNO₃ and PEG 8000 solutions was more effective than matric priming with calcium silicate, vermiculite, and Hayter loam soil. Priming increased the storage life of newly matured 40 and 45-days after anthesis (DAA) stored seeds but decreased the storage life of 55 DAA seeds. Fifty, 55, and 60-DAA seeds showed the greatest tolerance to adverse storage conditions and water stress and had the greatest seedling vigor. The highest quality seeds were attained 50 to 60 DAA from fruit harvested after edible maturity but before the onset of severe decomposition. After 6 years of storage, seeds washed in water at harvest were more vigorous and resistant to accelerated aging than unwashed seeds at most stages of development. Primed seeds retained some beneficial effects after 9 years of storage at optimal conditions but lost viability, vigor, and uniformity of germination more rapidly than non primed seeds following controlled deterioration. Instron analysis showed that priming weakened the perisperm envelope tissue prior to radicle emergence. Less force was required to puncture primed seed pieces than non primed. The penetration force and energy required to puncture the perisperm envelope tissue decreased gradually during imbibition, increased steadily during seed development and dry storage, and was negatively correlated with seed vigor. Priming accelerates enzymatic breakdown of perisperm envelope during germination. Endo-β-mannanase-like activity declined steadily during imbibition and was higher in non primed seeds than primed seeds, indicating that enzymatic activity in the perisperm peaked during priming. Flow cytometry revealed that the beneficial effects of priming and seed maturity do not correlate with DNA replication activity in muskmelon seeds. This study has shown that seed maturation inside the fruit until just after edible maturity is necessary for highest muskmelon seed vigor. Priming treatments should be performed as close to the planting date as possible, because primed seeds exhibit reduced storage life compared to non primed seeds. / Ph. D.

The objective of this study was to determine whether molecular orientation has an effect on the rate of physical aging in amorphous glassy polymers. There is already a large body of literature concerning the phenomenon of physical aging, although the vast majority has been directed toward isotropic, unoriented systems. The importance of this study is therefore twofold: first, from a theoretical standpoint, a better understanding of physical aging in oriented systems will help to elucidate the physics of glassy relaxation which is important since the exact mechanism behind physical aging are still unknown. Second, from an engineering standpoint, a knowledge of the orientation aging relationship will help the designer/engineer with product development since many commercially produced plastic items have some degree of orientation present as a result of the processing methods involved. To measure the aging behavior, samples of bisphenol A polycarbonate and atactic polystyrene were hot drawn (i.e. stretched above T_g) to varying stretch ratios and the degree of orientation quantified using birefringence and the Herman’s orientation function, f. Physical aging rates were determined as a function of fusing volume and linear dilatometry, mechanical creep measurements, DSC, and tensile properties. The molecular state, including the free volume, of the oriented polymers was quantified using Positron Annihilation Lifetime Spectroscopy (PALS), oxygen permeability/diffusion measurements, dynamic mechanical analysis, DSC, and density measurements. The data indicate that physical aging rates are influenced by orientation but the degree varies with the method of testing. Volume relaxation rates were approximately 50% higher for the oriented samples, however, mechanical shift rates determined from the creep data showed a slight decrease with orientation. Further analysis shows that the effective relaxation/retardation times decrease significantly with orientation even though the free volume--as determined by density and PALS measurements--also decreases. This implies a serious deficiency in the free volume theory for molecular mobility. Implications for these findings and possible explanations for this behavior are discussed. / Ph. D.

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.