Monte Carlo simulation techniques have emerged as widely accepted computing tools in tackling many problems in modern structural mechanics. Apart from developments in computational hardware, which have undoubtedly made simulation strategies practically feasible, the success of Monte Carlo simulations has also resulted equally significantly from the methodological developments aimed at controlling sampling variance of the Monte Carlo estimates. The study reported in the present thesis is aimed at developing and validating Monte Carlo simulation based approaches with inbuilt variance reduction capabilities to deal with problems of time variant reliability modeling, random vibration testing, and updating reliability models for statically/dynamically loaded instrumented structures. The relevant literature has been reviewed in Chapter 1. Time variant reliability analysis of randomly parametered and randomly driven non-linear vibrating systems has been tackled by combining two Monte Carlo variance reduction strategies into a single framework (Chapter 2). The first of these strategies is based on the application of the Girsanov transformation to account for the randomness in dynamic excitations and, the second approach is fashioned after the subset simulation method to deal with randomness in system parameters. A novel experimental test procedure to estimate the reliability of structural dynamical systems under excitations specified via random process models has been proposed (Chapter 3). The samples of random excitations to be used in the test are modified by the addition of an artificial control force. An unbiased estimator for the reliability is derived based on measured ensemble of responses under these modified inputs based on the tenets of Girsanov’s transformation. The study observes that an acceptable choice for the control force (that can reduce the sampling variance of the estimator) can be made solely based on experimental techniques. This permits the proposed procedure to be applied in the experimental study of time variant reliability of complex structural systems which are difficult to model mathematically. Illustrative example consists of a multi-axes shake table study on bending-torsion coupled, geometrically non-linear, five-storey frame under uni/bi-axial, non-stationary, random base excitation. The first order reliability method (FORM) and inverse FORM have been extended to handle the problem of updating reliability models for existing, statically loaded structures based on measured responses (Chapter 4). The proposed procedures are implemented by combining Matlab based reliability modules with finite element models residing on the Abaqus software. Numerical illustrations on linear and non-linear frames are presented. A solution strategy within the framework of Monte Carlo simulation based dynamic state estimation method and Girsanov’s transformation for variance reduction has been developed to tackle the problem of updating the reliability of instrumented structures based on measured response under random dynamic loading (Chapter 5). For linear Gaussian state space models, the solution is developed based on continuous version of the Kalman filter, while, for non-linear and (or) non-Gaussian state space models, bootstrap particle filters are adopted. Results from laboratory testing of an archetypal five storey bending-torsion coupled frame under seismic base motions form the basis of one of the illustrative examples. A set of three annexures contain details of numerical methods for discretizing Ito’s differential equations (Annexure 1), working of the Girsanov transformation through Kolmogorov’s equations (Annexure 2) and tools for interfacing Matlab and Abaqus codes (Annexure 3).

This dissertation focuses on two specific problems related to the reliability of the modern power grid. The first part investigates the economic dispatch problem with uncertain power sources. The classic economic dispatch problem seeks generator power output levels that meet demand most efficiently; we add risk-awareness to this by explicitly modeling the uncertainty of intermittent power sources using chance-constrained optimization and incorporating the chance constraints into the standard optimal power flow framework. The result is a dispatch of power which is substantially more robust to random fluctuations with only a small increase in economic cost. Furthermore, it uses an algorithm which is only moderately slower than the conventional practice. The second part investigates “the power grid attack problem”: aiming to maximize disruption to the grid, how should an attacker distribute a budget of “damage” across the power lines? We formulate it as a continuous problem, which bypasses the combinatorial explosion of a discrete formulation and allows for interesting attacks containing lines that are only partially damaged rather than completely removed. The result of our solution to the attack problem can provide helpful information to grid planners seeking to improve the resilience of the power grid to outages and disturbances. Both parts of this dissertation include extensive experimental results on a number of cases, including many realistic large-scale instances.

The objective of the dissertation is to combine the recent Mixed-Mode reliability stress studies into a single text. The thesis starts with a review of silicon-germanium heterojunction bipolar transistor fundamentals, development trends, and the conventional reliability stress paths used in industry, after which the new stress path, Mixed-Mode stress, is introduced. Chapter 2 is devoted to an in-depth discussion of damage mechanisms that includes the impact ionization effct and the selfheating effect. Chapter 3 goes onto the impact ionization effect using two-dimensional calibrated MEDICI simulations. Chapter 4 assesses the reliability of SiGe HBTs in extreme temperature environments by way of comprehensive experiments and MEDICI simulations. A comparison of the device lifetimes for reverse-EB stress and mixed-mode stress indicates different damage mechanisms govern these phenomena. The thesis concludes with a summary of the project and suggestions for future research in chapter 5.

The purposes of this study were to: 1) determine absolute intrarater reliability of using the cervical range of motion device (CROM) for measuring cervical movements in older adults, and 2) determine the intrarater reliability and concurrent validity of the Candrive protocol, which uses a universal goniometer to measure rotation. Forty older adults (75.7 + 4.7 years of age) were tested in two sessions, one week apart, by two raters. Intrarater reliability scores were good for the CROM protocol (coefficient of variation (CV) values were 5.5% and 6.2 % for cervical rotation). The Candrive protocol values were higher (CV = 7.9 and 9.4%). Concordance analyses suggested that the Candrive protocol was less than good in terms of its validity, particularly when order effects were taken into consideration. In conclusion, the CROM protocol demonstrated good reliability for either group or individual analyses, whereas the Candrive protocol was less reliable and its validity marginal.

The purposes of this study were to: 1) determine absolute intrarater reliability of using the cervical range of motion device (CROM) for measuring cervical movements in older adults, and 2) determine the intrarater reliability and concurrent validity of the Candrive protocol, which uses a universal goniometer to measure rotation. Forty older adults (75.7 + 4.7 years of age) were tested in two sessions, one week apart, by two raters. Intrarater reliability scores were good for the CROM protocol (coefficient of variation (CV) values were 5.5% and 6.2 % for cervical rotation). The Candrive protocol values were higher (CV = 7.9 and 9.4%). Concordance analyses suggested that the Candrive protocol was less than good in terms of its validity, particularly when order effects were taken into consideration. In conclusion, the CROM protocol demonstrated good reliability for either group or individual analyses, whereas the Candrive protocol was less reliable and its validity marginal.

This thesis presents a reliability analysis of a technical system. It is divided into three main sections. The first section introduces some of the most significant problems of reliability. It deals with a definition and an expresion of reliability, a reliability diagram selection and a detailed description of the reliability analysis. This part also includes an overview of reliability analysis types. Some of the most widely used reliability analyses are briefly described; further advantages and disadvantages of using each method are listed. Failure Modes and Effects Analysis - FMEA is then described in a greater detail. The second section contains an analysis of aviation conditions as well as a design of a reliability analysis that concerns a selected digital system; the system under analysis is a pitch trim indicator. The main design issue lies in a choice of a most suitable method. This choice stems from the overview of reliability analyses presented in the first section of the thesis. In the last section, a FMEA reliability analysis of the pitch trim indicator is conducted. This part includes a discussion of the results as well as a design action that is to lead to an increase in reliability of the analyzed system.

An efficient reliability technique has been developed based on Response Surface Methodology (RSM) in conjunction with the First Order Second Moment (FOSM) reliability method. The technique is applied when the limit state function cannot be obtained explicitly in terms of the design variables, i.e., when the analysis is performed using numerical techniques such as finite elements. The technique has proven to be efficient because it can handle problems with large numbers of design variables and correlated as well as nonnormal random variables. When compared with analytical results, the method has shown excellent agreement. The technique contains a sensitivity analysis scheme which can be used to reduce the computation time resulting in nearly the same accuracy. This technique allows the extension of most finite element codes to account for probabilistic analysis, where statistical variations can be added to the design variables. An explicit solution for rocket motors consisting of propellant and steel case under environmental temperature variations is compared to the RSM technique. The method is then used for the analysis of rocket motors subjected to mechanical loads for which the stress analysis is performed using the finite element method. The technique is also applied to study the reliability of a laminated composite plate with geometric nonlinearity subjected to static and time dependent loadings. Different failure modes were considered as well as different meshes. Results have shown that when the relative size of the element is introduced into the probabilistic model, the same reliability value is obtained regardless of the number of elements in the mesh. This is good because it allows the technique to be used for problems where the failure region is unknown. / Ph. D.

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Although power system reliability analysis is a mature research area, there is a renewed interest in updating available network models and formulating improved reliability assessment procedures. The main driver of this interest is the current transition to a new flexible and actively controlled power supply system with a high penetration of distributed generation (DG) and energy storage (ES) technologies, wider implementation of demand-side management (DSM) and application of automated control, monitoring, protection and communication infrastructures. One of the aims of this new electricity supply network (’the smart grid’) is an improved reliability and power quality performance, realised through the delivery of an uninterrupted and high-quality supply of electrical energy. However, there is currently no integrated methodology to measure the effects of these changes on the overall system reliability performance. This PhD research aims to update the standard power system simulation engine with improved numerical software models offering new capabilities for the correct assessment of quality of supply in future electricity networks. The standard reliability analysis is extended to integrate some relevant power quality aspects, enabling the classification of short and long supply interruptions by the correct modelling of network protection and reconfiguration schemes. In addition, the work investigates the formulation and analysis of updated reliability indicators for a more accurate validation and benchmarking of both system and end-user performance. A detailed database with typical configurations and parameters of UK/European power systems is established, providing a set of generic models that can correctly represent actual distribution networks supplying a mix of residential, commercial and industrial demand for different load sectors. A general methodology for reducing system complexity by calculating both electrical and reliability equivalent models of LV and MV distribution networks is also presented. These equivalent models, based on the aggregation of individual component models, help to reduce calculation times while preserving the accuracy assessment of network’s reliability performance at bulk supply points. In addition, the aggregated counterparts (same and mixed-type) of different ’smart’ component models (DG, ES and DSM) are also included in the analysis, showing how their co-ordinated implementation and control could improve quality of supply. Conventional reliability assessment procedures are also extended in this thesis to include accurate reliability equivalent models, network contingency statistics, actual load profiles and empirical fault probability distributions, which are employed to assess the frequency and duration of interruptions in the supply system for different scenarios. Both analytical and probabilistic simulation techniques (Monte Carlo method) are developed to include up-to-date security of supply legislation, introducing a new methodology for calculating the standard set of indices reported annually to energy regulators.

Carbon nanotubes (CNTs) are known for their exceptional electrical, thermal, mechanical, optical, and chemical properties. With the significant progress in recent years on synthesis, purification and integration challenges, CNT network/array based thin-film transistors (TFTs) are likely to play a critical role as the building blocks of future electronics. CNT-TFTs can find applications in flexible, transparent and energy-efficient circuits, e-displays, solar cells, RFID tags, e-paper, touch screens, implantable medical devices and chemical/bio/optical sensors. CNTs in CNT-TFTs are deposited on low thermal conductivity substrates which can impede the heat dissipation resulting in high temperature. The excessive self-heating in CNT-TFTs can degrade the electrical and thermal performance and could potentially lead to failure of the devices. Therefore, the issues related to operational reliability of CNT-TFTs arising from the self-heating effects need to be examined and studied. In the present work, a computational approach is developed and employed to study the electrical and thermal transport in CNT-TFTs. The modeling framework can predict the current and temperature profile of CNT network/array and the supporting structure. The model is validated against the experimental results. In case of CNT network TFTs, the computational method allows us to examine the role of various device parameters such as network morphology (i.e., network density, CNT junction topology, and CNT length and alignment distribution) and channel geometry (i.e., channel length and width) on heat dissipation and thermal reliability. The simulation results help interpret experimental data and provide the quantitative information about the thermal boundary conductances at CNT junctions and CNT-substrate interfaces in CNT-TFTs. The findings suggest that the structure of CNT junctions on substrate can become very critical in CNT network TFTs as the lack of contact with the substrate at these junctions can lead to junction temperatures hundreds of degrees higher than the rest of the device, which will severely deteriorate the performance of these devices. High-field breakdown study of CNT network TFTs is also conducted which provides guidelines for the design and optimization with respect to aforementioned parameters in order to enhance the performance and reliability. Dense CNT arrays are preferred for better electrical performance in CNT array TFTs, but they also experience electrostatic and thermal cross-talk which can adversely affect the device performance. These effects have been studied in details. The role of trap charges in CNT array TFTs is also investigated to understand and mitigate hysteresis. Lastly, CNT-liquid crystal composites are studied using dissipative particle dynamics (DPD) technique with the aim to understand how the CNT concentration in composite affects the alignment of liquid crystals and to explore the method of CNT alignment using liquid crystals.