Thesis (M.Sc.(Engineering))--University of the Witwatersrand, Faculty of Engineering, 2000.

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The objective of this work is to develop an evolution equation for the ductile growth of a spherical void in a highly strain rate and temperature dependent material. The material considered in this work is stainless steel 304L at 982 °C. The material is characterized by a physically-based internal state variable model derived within consistent kinematics and thermodynamics — Evolving Microstructure Model for Inelasticity. Through this formulation, the degradation of the elastic moduli due to damage has been naturally acquired. An elastoviscoplasticity user material subroutine has also been developed and implemented into a commercially available finite element software ABAQUS. The subroutine utilizes a return mapping algorithm, where a purely elastic trial state (elastic predictor) is followed by a plastic corrector phase (return mapping). A conditionally stable fully-implicit scheme, derived from the backward Euler integration method, has been employed to calculate the values of the internal state variables in the elastoviscoplasticity integration routine. A repeating unit cell problem is set up by introducing a spherical void inside a matrix material that simulates a periodic array of voids in a component. Using finite element analysis, a database is generated by recording the responses of the unit cell under various combinations of loading conditions, porosity, and state variables. Functional forms of the void growth equations are constructed by utilizing normalization techniques to collapse all the data into master curves. The evolution equations are converted to a form consistent with the continuum damage variable in the complete thermal-elastic-plastic-damage version of the physically-based internal state variable model.

With a growing desire to generate structural characteristics of late-successional conifer forests, managers are commonly seeking an approach to increase structural heterogeneity in otherwise simple second-growth stands. This study examined understory response to experimental thinning on the Olympic Peninsula of Western Washington. Variable-density thinning within the Olympic Habitat Development Study was found to have significant effects on both the density of seedling recruitment as well as early growth of western hemlock (Tsuga heterophylla (Raf.) Sarg.). In additional analysis, pre-commercial understory thinning, implemented by the Olympic National Forest, produced no observable acceleration in basal area growth. Results suggest that understory development is sensitive to previous management history as well as post-treatment stand structure. Better understanding the sources of variability in understory tree response to non-uniform thinning and understory density management will be principle in evaluating the efficacy of these treatments relative to traditional methods.

Mechanical structure-property relations have been quantified for AISI 4140 steel under different strain rates and temperatures. The structure-property relations were used to calibrate a microstructure-based internal state variable plasticity-damage model for monotonic tension, compression and torsion plasticity, as well as damage evolution. Strong stress state and temperature dependences were observed for the AISI 4140 steel. Tension tests on three different notched Bridgman specimens were undertaken to study the damage-triaxiality dependence for model validation purposes. Fracture surface analysis was performed using Scanning Electron Microscopy (SEM) to quantify the void nucleation and void sizes in the different specimens. The stress-strain behavior exhibited a fairly large applied stress state (tension, compression dependence, and torsion), a moderate temperature dependence, and a relatively small strain rate dependence.

This thesis presents an analysis of subjective risk information and participation in the National Flood Insurance Program (NFIP). Data are taken from a survey of residents in flood-prone coastal regions in the southeastern U.S. Regression models are constructed to better understand factors affecting individuals’ perceived risk ambiguity related to flood risk and the role of risk preferences, risk perceptions, and especially risk ambiguity, on the decision to purchase flood insurance. This is the first study not only of the influence of risk ambiguity on NFIP participation, but also of the impact of using different risk perception measures. Results indicate that NFIP participation is significantly affected by mean perceived risk, but the influence of range/variance of perceived risk, which presents one’s perceived ambiguity, is mixed.

Freeway work zones with lane closures can adversely affect mobility, safety, and sustainability. Capacity drop phenomena near work zone areas can further decrease work zone capacity and exacerbate traffic congestion. To mitigate the negative impacts caused by freeway work zones, many variable speed limits (VSL) control methods have been proposed to proactively regulate the traffic flow. However, a simple yet robust VSL controller that considers the nonlinearity induced by the associated capacity drop is still needed. Also, most existing studies of VSL control neglected the impacts of traffic sensor failures that commonly occur in transportation systems. Large deviations of traffic measurements caused by sensor faults can greatly affect the reliability of VSL controllers. To address the aforementioned challenges, this research proposes a fault-tolerant VSL controller for a freeway work zone with consideration of sensor faults. A traffic flow model was developed to understand and describe the traffic dynamics near work zone areas. Then a VSL controller based on sliding mode control was designed to generate dynamic speed limits in real time using traffic measurements. To achieve VSL control fault tolerance, analytical redundancy was exploited to develop an observer-based method and an interacting multiple model with a pseudo-model set (IMMP) based method for permanent and recurrent sensor faults respectively. The proposed system was evaluated under realistic freeway work zone conditions using the traffic simulator SUMO. This research contributes to the body of knowledge by developing fault-tolerant VSL control for freeway work zones with reliable performance under permanent and recurrent sensor faults. With reliable sensor fault diagnosis, the fault-tolerant VSL controller can consistently reduce travel time, safety risks, emissions, and fuel consumption. Therefore, with a growing number of work zones due to aging road infrastructure and increasing demand, the proposed system offers broader impacts through congestion mitigation and consistent improvements in mobility, safety, and sustainability near work zones. / Thesis / Doctor of Philosophy (PhD) / Freeway work zones can increase congestion with higher travel time, safety risk, emissions and fuel consumption. This research aims to improve traffic conditions near work zones using a variable speed limits control system. By exploiting redundant traffic information, a variable speed limit control system that is insensitive to traffic sensor failures is presented. The proposed system was evaluated under realistic freeway work zone conditions in a simulation environment. The results show that the proposed system can reliably detect sensor failures and consistently provide improvements in mobility, safety and sustainability despite the presence of traffic sensor failures.

Kalman-type filtering methods are mostly designed based on exact knowledge of the system’s model with known parameters. In real applications, there may be considerable amount of uncertainties about the model structure, physical parameters, level of noise, and initial conditions. In order to overcome such difficulties, robust state estimation techniques are recommended. This PhD thesis presents a novel robust state estimation method that is referred to as the 2nd-order smooth variable structure filter (2nd-order SVSF) and satisfies the first and second order sliding conditions. It is an extension to the 1st-order SVSF introduced in 2007. In the 1st-order SVSF chattering is reduced by using a smoothing boundary layer; however, the 2nd-order SVSF alleviates chattering by preserving the second order sliding condition. It reduces the estimation error and its first difference until the existence boundary layer is reached. Then after, it guarantees that the estimation error and its difference remain bounded given bounded noise and modeling uncertainties. As such, the 2nd-order SVSF produces more accurate and smoother state estimates under highly uncertain conditions than the 1st-order version. The main issue with the 2nd-order SVSF is that it is not optimal in the mean square error sense. In order to overcome this issue, the dynamic 2nd-order SVSF is initially presented based on a dynamic sliding mode manifold. This manifold introduces a variable cut-off frequency coefficient that adjusts the filter bandwidth. An optimal derivation of the 2nd-order SVSF is then obtained by minimizing the state error covariance matrix with respect to the cut-off frequency matrix. An experimental setup of an electro-hydrostatic actuator is used to compare the performance of the 2nd-order SVSF and its optimal version with other estimation methods such as the Kalman filter and the 1st-order SVSF. Experiments confirm the superior performance of the 2nd-order SVSF given modeling uncertainties. / Thesis / Doctor of Philosophy (PhD)

Cooperative learning refers to teaching methods that encourage students to work together in small groups to help each other learn educational content. In Sweden, cooperative learning methods are used in classrooms as an educational approach to organize classroom activities into academic and social learning experiences. This thesis investigates how lighting can support cooperative learning in Swedish classrooms to enhance students' learning performance while taking into account visual tasks during different pedagogical activities and visual comfort. To answer this question, the existing lighting of a middle school classroom in Iggesund Skola, Sweden, was studied through a methodology based on both qualitative and quantitative methods. Analysis of literature review, personal observations and interviews, measurements of illuminance, luminance, and color metrics, and 3D simulations formed the foundation of the lighting design proposal for the refurbishment of classrooms.The results show that the lighting requirements of students and teachers have changed with time. To include cooperative learning methods, the users ask for a changeable lighting solution that can work with their flexible learning method. Hence, the design proposal focuses on enhancing students' learning performance while emphasizing the flexibility of their learning method.

<p>In the current marketplace, most ducted split-system heat pumps feature single-speed compressors and fans. To meet forthcoming minimum energy rating requirements, reduce operational costs, and increase environmental sustainability, the seasonal heating and cooling efficiencies of heat pump systems must be improved. Variable-speed equipment offers significant advantages for load modulation and has the ability to increase the seasonal performance greatly. Additionally, novel electrical motor technologies, such as permanent magnet (PM) motors, can reduce the power consumption of the motors by up to 25-55% compared to the widely used permanent split capacitor (PSC) motor or electronically commutated motor (ECM). In this study, a low cost ducted single-speed heat pump system with a cooling capacity of 10.55 kW was analyzed to quantify the impact of fan and compressor motor efficiency on seasonal coefficient of performance (SCOP). Furthermore, single-speed components were replaced with variable to evaluate the performance increase. The single-speed heat pump was experimentally tested, and the results were used to tune a detailed model for further performance analyses. The efficiency was evaluated in heating and cooling mode according to AHRI Standard 210/240 and with an energy savings and cost analysis, that details the SCOP and costs for different locations. The conversion of the fan motors to high efficiency PM magnet motors increased the SCOP by up to 6%. The impact was dependent on the initial motor efficiency and the operational mode. The indoor unit fan motor has a large impact on SCOP in cooling mode and a low impact in heating mode because of the motor waste heat’s impact on capacity. The conversion to a fully variablespeed system greatly increased the performance, with a 72% increase in cooling SCOP and a 19% increase in heating SCOP. The energy savings and cost analysis concluded that the fan motor conversion from single-speed to high efficiency motors is economically viable, but the financial benefit of the upgrade to variable-speed depends on the intended location of use.  </p>