A series of frozen stress experiments were conducted on surface flaws of varying aspect ratios in pressurized cylinder with star-shaped cutout in order to study the stress intensity factor distribution along the flaw border and to estimate the loss of the inverse square root singularity when the crack border intersects the inner star surface at right angles. By applying a refined optical method, the photoelastic data are converted into classical stress intensity factors resulting from the three dimensional stress state existing at the inner surface and compared with a numerical analysis to indicate the nearly uniform distribution of the stress intensity factor along the crack border. Based upon this result a two dimensional weight function approach is demonstrated to yield accurate values of the maximum stress intensity factor for the motor grain test geometry. / Ph. D.

A new small-signal model is developed for pulse-width-modulated converters operating with current-mode control. Four different control schemes are analyzed, all of which use the instantaneous value of the inductor current as part of the control modulation. Continuous-conduction mode and discontinuous-conduction mode models are provided. Sampled-data modeling is applied to the current-mode cell which is common to all converters, and the important results obtained are simplified to give a powerful model for analysis and design. The new model explains all of the observed phenomena of current-mode control, including the subharmonic oscillation which can occur with constant-frequency modulation. It is shown that the model provides transfer functions with three poles for two-state converters. Predictions of the new model are accurate to half the switching frequency. / Ph. D.

Several novel characterization techniques were developed to gain a molecular-level understanding of the effects of exposure intensity and exposure time on photopolymer network formation. These techniques enable detailed characterization of photopolymer behavior in real-time and post-exposure. In situ dynamic mechanical analysis was performed to observe the changes in composite modulus during photopolymerization of thin acrylate films supported on stainless steel grid sheets. Vitrification phase transformations were monitored in real-time via remote sensing dielectric spectroscopy. The relationship between exposure intensity and vitrification time revealed the exposure conditions necessary to shift the crosslinking rate from reaction kinetics controlled to diffusion kinetics controlled. The effect of exposure intensity and exposure time on chemical conversion was ascertained via Fourier transform infrared microspectroscopy mapping experiments. The relationship between exposure intensity and time at a given conversion level revealed an increased occurrence of radical isolation at higher conversions. Furthermore, the exposure necessary to maintain a fixed conversion indicated greater reciprocity failure at lower conversions, indicative of classical radical termination kinetics. / Ph. D.

The objective of this research is to develop analytical approaches for assessment and prediction of the availability of longwall mining systems. After a functional analysis of longwall mining operations, the longwall production system is divided into four subsystems: coal—cutting, face—conveying, roof—support, and outby—haulage. The operating characteristics of the longwall system are then investigated based on the system configuration, component failure and repair processes, and rules of operation. Through use of the techniques of reliability assessment and stochastic systems analysis, five probability models are formulated and solved with respect to different longwall operating logic. The implementation of these models is demonstrated with a number of case studies. Furthermore, three important applications of the results have been identified for improvement of longwall performance: analysis of component importance, assessment and prediction of productivity, and optimization of system operational effectiveness. This investigation provides a systematic methodology for evaluation of longwall operational effectiveness. A number of system effectiveness measures have been developed for longwall systems with various operating characteristics. Some of the measures include system availability, reliability, failure rate, mean time to failure, mean time to repair, the expected average of the number of system failures, and the limiting probabilities of system failure due to any subsystem. Explicit expressions of system availability are obtained for several practical cases. The methodology developed can be used as both an assessment tool and a design tool for improvement of the operational effectiveness of longwall mining systems. / Ph. D.

Switchgrass (Panicum virgatum L.), a warm-season grass, grows most rapidly in mid-summer when cool-season species such as tall fescue (Festuca arundinacea Schreb.) may have limited growth due to high temperature and low soil moisture availability. The objectives of this study were to investigate physiological factors and to determine management strategies that could optimize growth of switchgrass. The influences of two successive drought cycles on performance and water relation parameters of switchgrass and tall fescue were studied in growth chamber conditions. Water was withheld from conditioned plants until elongation of tillers stopped. Then pots were rewatered and a new drought cycle followed. Control plants remained well watered during this time. Both conditioned and control plants were then subjected to a challenge water stress. Total leaf elongation and soil water content (SWC) were measured daily. Leaf water potential (Ψ), osmotic potential (Π), relative water content, and concentrations of K, Na, Ca, and total free sugars were measured at the end of each water-stress cycle. Osmotic potential at full turgor (Π₁₀₀), symplastic water content (SYM) , and modulus of elasticity (∊) were determined from pressure-volume curves at the end of the two conditioning cycles. Conditioned plants of both species elongated more during the challenge water-stress than control plants and had lower SWC and Ψ when their leaf elongation ceased. Conditioned plants exhibited osmotic adjustment, accumulating free sugars and K, as a result of drought stress. Switchgrass SYM did not change, while Π₁₀₀ decreased, suggesting active salt accumulation. Increased ∊ somewhat counteracted the beneficial influence of osmotic adjustment. Tall fescue SYM increased, while Π₁₀₀ did not change. Decreased ∊ improved drought tolerance of tall fescue. Field experiments were conducted to investigate the influence of date of first harvest and cutting height on yield distribution and canopy characteristics of ‘Pathfinder’ (2-yr study), ‘Cave-in-rock’, and ‘Blackwell’ (1-yr study) switchgrass. Seasonal distribution of dry matter production was established by measuring first-harvest yields and regrowth. The canopy of Pathfinder was characterized by determining number and weight of tillers, light penetration, leafiness, specific leaf weight, and leaf area index in profiles of the canopy before harvest and in regrowth. First-harvest yields increased as date of first cut was delayed and cutting height was lowered. Cutting at 20 cm decreased the yield of first cutting in a second growing season. Plots not harvested in the first growing season gave higher yields in the second growing season compared with previously harvested plots, suggesting that any harvest may decrease subsequent yield potential. Regrowth decreased as date of first cut was delayed. A cutting height of 30 cm produced greater regrowth than cutting at 20 cm. Yields and canopy characteristics suggest that removal of growing points decreased second-harvest yields and weakened regrowth potential in the following year. To maximize regrowth to be used for grazing during July and August, switchgrass hay should be cut after 10 June and before 21 June. / Ph. D.

The dissertation explores the determinants of the choice between friendly mergers and hostile tender offers as alternative acquisition methods in the market for corporate control. The theoretical model focuses on the target management's firm specific human capital as a primary determinant of the choice. The model predicts that firm characteristics like low insider holdings and high debt, indicating the presence of incumbent management's firm specific human capital, increase the likelihood of a friendly merger as opposed to a hostile tender offer. Other firm characteristics that influence the choice of acquisition method emerge from Jensen's (1986) free cash flow theory. The empirical testing of the hypotheses uses state-based sampling and conditional maximum likelihood estimation of logit models. The results provide strong evidence in support of the theoretical model developed in the dissertation and Jensen’s free cash flow theory. / Ph. D.

The purpose of this study was to describe the in-service education needs for developing instructional skills of part-time business faculty. The population included part-time business faculty teaching credit courses at 24 community college campuses in Virginia, North Carolina, South Carolina, Georgia, and Florida during fall 1988. A total of 138 (63.9%) respondents returned a 62-item questionnaire. The average age of the respondents was 44.7 years, and 59.2% had a master's degree or higher. The average number of years of full-time teaching experience was 1.6 years, and the average number of years of part-time teaching experience was 5.5 years. Respondents were classified according to Tuckman's Taxonomy of part-time faculty_ Sufficient numbers of respondents could be placed only in three of the seven classifications for analysis purposes. In-service education needs were analyzed within the three classifications of part-time business faculty for the six categories of instructional skills. Respondents in all three classifications expressed a need for 2 of the 57 instructional skills in the top 10 ranking of statements. When analyzing the differences within the three classifications, one significant F-value of 5.11 with a probability of significance at .0001 resulted for the full-mooners. They had a statistically significant higher need for instructional skills in the Miscellaneous and Students categories but no practically different needs. In addition, significant relationships between characteristics of the respondents and the need for instructional skills in categories were found. First, the lower the level of education, the more the respondents had a need for in-service education for skills in the Planning to Teach category. Second, the lower the age, the more the respondents had a need for in-service education for skills in the Planning to Teach category. Third, the lower the age, the more the respondents had a need for in-service education for skills in the Miscellaneous category. Outcomes from these analyses revealed statistically different but not practically different needs. Those planning in-service education for part-time business faculty should offer activities found among the top 10 ranked instructional skill statements for each classification. In-service education activities should not be based on instructional skill categories or part-time business faculty characteristics of educational level, years of full-time and part-time teaching experience, or age. / Ph. D.

For mobile robots to autonomously accommodate dynamically changing navigation tasks in a goal-directed fashion, they must employ navigation plans. Any such plan must provide for the robot’s immediate and continuous need for guidance while remaining highly flexible in order to avoid costly computation each time the robot’s perception of the world changes. Due to the world’s uncertainties, creation and maintenance of navigation plans cannot involve arbitrarily complex processes, as the robot’s perception of the world will be in constant flux, requiring modifications to be made quickly if they are to be of any use. This work introduces Navigation Templates (or NaTs) which are building blocks for the construction and maintenance of rough navigation plans which capture the relationship that objects in the world have to the current navigation task. By encoding only the critical relationship between the objects in the world and the navigation task, a NaT-based navigation plan is highly flexible; allowing new constraints to be quickly incorporated into the plan and existing constraints to be updated or deleted from the plan. To satisfy the robot’s need for immediate local guidance, the NaTs forming the current navigation plan are passed to a transformation function. The transformation function analyzes the plan with respect to the robot’s current location to quickly determine (a few times a second) the locally preferred direction of travel. This dissertation presents NaTs and the transformation function as well as the needed support systems to demonstrate the usefulness of the technique for controlling the actions of a mobile robot operating in an uncertain world. ¹ This work was supported in part by a grant from the Jet Propulsion Laboratory under a contract from the National Aeronautics and Space Administration, and by a grant from the Naval Surface Weapons Center. / Ph. D.

The primary purpose of this study was to measure experimentally the uniaxial (tangential direction) equilibrium moisture profiles in moisture-sealed wood samples subjected to constant but different temperatures T on opposite faces, and to compare these profiles with those predicted by each of several theoretical models. Each test assembly consisted of eight end matched wood laminae, each 0.2 cm thick, for a total thickness of 1.6 cm in the tangential direction. Opposite faces of each moisture-sealed assembly were exposed continuously for approximately five weeks to different but constant temperatures until the original uniform moisture content M redistributed itself to a new constant but non-uniform moisture profile. At moisture equilibrium, the individual wood laminae were removed from the assembly and their moisture contents measured gravimetrically. Both temperature, T vs x, and moisture profiles, M vs x, where x is the distance from the cold face were plotted for each test. The experimental variables considered were wood species (yellow poplar and hard maple), initial moisture content (9%, 12%, and 15%), and temperature range (15°C to 35°C, and 25°C to 45°C). Each condition was replicated 3 or 4 times, giving a total number of 44 different tests. At the steady state, a moisture content profile opposite to the temperature gradient was established. The temperature gradient dT/dx was constant in all cases, with the moisture content profile increasing almost exponentially with decreasing temperature. The absolute magnitude of the calculated ratio dM/dT was found to vary with initial moisture content, temperature range, and species. The Soret coefficient, defined as - ( 1/M )( dM/dT ), was also calculated and was found to vary with the above variables as the dM/dT ratio. These two quantities always increased with wood moisture content. They were also slightly higher at the higher temperature range. There was only a small difference between species, with yellow poplar giving somewhat higher mean values. The Soret coefficient was generally in the range of 0.03 to 0.05 per degree Kelvin, except for the extremely high moisture contents near and above fiber saturation, where the calculated values approached 0.5 per degree Kelvin. The ratio dM/dT was analyzed in terms of five different theoretical models, two of which are based on nonequilibrium thermodynamics (NET) and three on classical thermodynamics. All models require sorption isotherm and heat of sorption data; and in some cases, the activation energy for moisture transport through wood. Adsorption and desorption isotherm data were obtained at 30°C for both species. Heat of sorption and moisture transport activation energy data used in testing the models were taken from the literature. The two NET models provided the best agreement with the experimental values of dM/dT. The Siau model gave the next best prediction, followed by the Stanish model, with the Skaar- Siau model giving the poorest agreement with the experimental results. The heat of transfer was also computed using the two NET models as well as those of Stanish and Siau. Values ranged from 5500 to 17000 cal/mol and from 8100 to 9900 cal/mol based on the Nelson model. The corresponding values for the Stanish for the general NET model and from model range from 15200 to 16500; while those of the Siau model varied between 11700 to 13600 cal/mol. Calculations of other quantities from the information generated in these experiments revealed that at the steady state, vapor pressure and spreading pressure were not constant across the thickness of the material; the chemical potential of water vapor and sorbed water were equal at all points indicating local equilibrium; and that the sorbed water and water vapor entropies were more or less constant across the thickness of the sample. / Ph. D.

The effects of decision task complexity, decision importance, and decision maker impulsivity on decision making behavior were studied in this thesis. Measures involving time and acquisition of information were devised as well as specialized measures of decision strategy complexity. Thirty-six subjects classified as either high- or low-impulsives (eighteen male, eighteen female) performed decision tasks involving the selection of the “best” apartment from a group of apartments that were homogeneous with respect to desirability. Decision task complexity was manipulated by increasing the number of apartments from which the subject had to choose. Decision importance was manipulated by changing the reward associated for selecting the best apartment. A theoretical decision strategy selection mechanism, based on a similar mechanism proposed by Christensen-Szalanski (1978), was developed to explain the relationship between the independent variables in this study and decision strategy selection. Results indicated partial support for the theoretical mechanism and highlighted areas for future research. / Master of Science