published_or_final_version / Electrical Engineering / Master / Master of Philosophy

With an ongoing desire to improve product performance, in combination with the continuously growing complexity of engineering structures, there is a need for well-tested and reliable engineering tools that can aid the decision making and facilitate an efficient and effective product development. The technical assessment of the dynamic characteristics of mechanical systems often relies on linear analysis techniques which are well developed and generally accepted. However, sometimes the errors due to linearization are too large to be acceptable, making it necessary to take nonlinear effects into account. Many existing analysis techniques for nonlinear mechanical systems build on the assumption that the input excitation of the system is periodic and deterministic. This often results in highly inefficient analysis procedures when nonlinear mechanical systems are studied in a non-deterministic environment where the excitation of the system is stochastic. The aim of this thesis is to develop and validate new efficient analysis methods for the theoretical and experimental study of nonlinear mechanical systems under stochastic excitation, with emphasis on two specific problem areas; forced response simulation and system identification from measurement data. A fundamental concept in the presented methodology is to model the nonlinearities as external forces acting on an underlying linear system, and thereby making it possible to use much of the linear theories for simulation and identification. The developed simulation methods utilize a digital filter to achieve a stable and condensed representation of the linear subparts of the system which is then solved recursively at each time step together with the counteracting nonlinear forces. The result is computationally efficient simulation routines, which are particularly suitable for performance predictions when the input excitation consist of long segments of discrete data representing a realization of the stochastic excitation of the system. Similarly, the presented identification methods take advantage of linear Multiple-Input-Multiple-Output theories for random data by using the measured responses to create artificial inputs which can separate the linear system from the nonlinear parameters. The developed methods have been tested with extensive numerical simulations and with experimental test rigs with promising results. Furthermore, an industrial case study of a wave energy converter, with nonlinear characteristics, has been carried out and an analysis procedure capable of evaluating the performance of the system in non-deterministic ocean waves is presented.

There is a need to monitor the existence and effects of damage in structural materials. Aircraft components provide a much publicized example, but the need exists in a variety of other structures, such as layered materials used in food packaging industries. While several techniques and models have been proposed for material characterization and condition monitoring of bulk materials, less attention has been devoted to thin sheets having no flexural rigidity. This study is therefore devoted to the development of a new method for acoustic Non-Destructive Testing (NDT) and material characterization of thin sheets used in food packaging materials or similar structures. A method for assessing the strength in the presence of crack of thin sheets used in food packaging is first presented using a modified Strip Yield Model (SYM). Resonance frequency measurement is then introduced and it is shown, at low frequency range (less than 2kHz), that a change in the physical properties such as a reduction in stiffness resulting from the onset of cracks or loosening of a connection causes detectable changes in the modal properties, specifically the resonance frequency. This observation leads to the implementation of a simple method for damage severity assessment on sheet materials, supported by a new theory illustrating the feasibility of the detection of inhomogeneity in form of added mass, as well as damage severity assessment, using a measurement of the frequency shift. A relationship is then established between the resonance frequency and the material’s elastic property, which yields a new modality for sheet materials remote characterization. The result of this study is the groundwork of a low-frequency vibration-based method with remote acoustic excitation and laser detection, for nondestructive testing and material characterization of sheet materials. The work also enhances the feasibility of the testing and condition monitoring of real structures in their operating environment, rather than laboratory tests of representative structures. The sensitivity of the new experimental approach used is liable to improvement while being high because the frequency measurement is one of the most accurate measurements in physics and metrology.

No description available.

No description available.

A novel system identification (SI)-based structural health assessment (SHA) procedure has been developed integrating several theoretical and implementation aspects. The procedure assesses health of structures using limited noise-contaminated dynamic responses and without using input excitation information. Since most practical structures are three dimensional (3D), the procedure has been developed for general 3D structures, represented by finite elements (FEs). The procedure identifies defects by tracking the changes in the stiffness of the elements in the FE representation. Once a defective element is identified, defect spot can be identified accurately within the defective element. The procedure is denoted as 3D Generalized Iterative Least-Squares Extended Kalman Filter with Unknown Input (3D GILS-EKF-UI) and implemented in two stages. In Stage 1, based on the available responses, substructure(s) are selected and the 3D GILS-UI procedure is used to generate the unknown input excitation, stiffness parameters of the elements in the substructure, and two Rayleigh damping coefficients. Using information from Stage 1, stiffness parameters for the whole structure are identified using EKF with Weighted Global Iteration (EKF-WGI) in Stage 2. The procedure accurately identified defect-free and defective states of various 3D structures using only analytically generated limited responses. To increase the robustness, 3D GILS-EKF-UI has been extended to develop an integrated structural health assessment strategy, denoted as Iterative Least-Squares Extended Kalman Filter with Unknown Input and Advanced Digital Integration Technique (ILS-EKF-UI-ADIT). The procedure has been implemented in three stages. In Stage 1, an advanced digital integration technique (ADIT) is implemented for post-processing of noise-contaminated acceleration time-histories, addressing all major challenges of digital integration. It also overcomes non-convergence issue in Stage 2 that arises due to phase-shift and amplitude errors. In Stage 2, substructure(s) are identified using the least-squares procedure. In Stage 3, stiffness parameters for the whole structure are identified using the EKF-WGI procedure. ILS-EKF-UI-ADIT has been verified in presence of relatively large noise in the acceleration time-histories, measured at small part(s) of defect-free and defective structures, without using excitation information. The SHA procedure is robust and has the potential to be applied for the health assessment, maintenance, retrofitting, and life extension of existing structural systems.

No description available.

No description available.

THz spinduliuotės generavimas iš puslaidininkių paviršiaus turi didelį potencialą puslaidininkių fizikinėms savybėms tirti. Šis darbas skiriamas puslaidininkių tyrimams generuojant THz impulsus iš jų paviršių, apšviestų femtosekundiniais lazerio impulsais. THz spinduliuotė iš puslaidininkių paviršių gali būti generuojama dėl visos eilės fizikinių mechanizmų: paviršinio lauko ekranavimo, foto-Demberio efekto, optinio lyginimo, elektriniu lauku indukuoto optinio lyginimo, plazminių svyravimų, koherentinių fononų ir plazmonų. Tiriant THz spinduliuotės generacijos mechanizmus galima išmatuoti daug svarbių puslaidininkių parametrų, tokių kaip lūžio rodiklis, judris, krūvininkų gyvavimo trukmė, aukštesniųjų laidumo slėnių padėtys. Darbo metu tirti THz spinduliuotės generacijos puslaidininkio paviršiuje mechanizmai keičiant žadinimo sąlygas: aplinkos temperatūrą, magnetinį lauką, žadinančio lazerio bangos ilgį ir intensyvumą, bei impulso trukmę. Ištyrus visą eilę įvairių puslaidininkių nustatyta, kad geriausias THz spinduliuotės emiteris žadinant 800 nm bangos ilgio spinduliuote yra p-InAs. Pirmą kartą THz žadinimo spektroskopijos metodu tiesiogiai išmatuoti tarpslėniniai atstumai InxGa1-xAs , InAs ir InSb bandiniuose. / Generation of terahertz radiation from semiconductor surfaces has great potential for investigation of physical properties of semiconductors. This work focuses on the semiconductor research when generating terahertz pulses from a variety of semiconductor surfaces. THz radiation from semiconductor surfaces can be generated on a whole range of physical mechanisms: the surface field screening, photo-Dember effect, the optical rectification, electric field induced optical rectification, plasma oscillations, coherent phonons and plasmons. A number of important semiconductor parameters such as refractive index, mobility, carrier relaxation time and higher conductivity valley positions can be measured using THz generation from semiconductor surface technique. In this work THz radiation generation mechanisms were investigated when changing excitation conditions: ambient temperature, magnetic field, laser wavelength and intensity, pulse duration. After tests with variety different semiconductors it was found that p-InAs is the best surface emitter when excitation laser wavelength is 800 nm. Using THz excitation spectroscopy the intervalley distances were measured directly, for the first time, in two InxGa1-xAs, InAs and InSb samples.

Generation of terahertz radiation from semiconductor surfaces has great potential for investigation of physical properties of semiconductors. This work focuses on the semiconductor research when generating terahertz pulses from a variety of semiconductor surfaces. THz radiation from semiconductor surfaces can be generated on a whole range of physical mechanisms: the surface field screening, photo-Dember effect, the optical rectification, electric field induced optical rectification, plasma oscillations, coherent phonons and plasmons. A number of important semiconductor parameters such as refractive index, mobility, carrier relaxation time and higher conductivity valley positions can be measured using THz generation from semiconductor surface technique. In this work THz radiation generation mechanisms were investigated when changing excitation conditions: ambient temperature, magnetic field, laser wavelength and intensity, pulse duration. After tests with variety different semiconductors it was found that p-InAs is the best surface emitter when excitation laser wavelength is 800 nm. Using THz excitation spectroscopy the intervalley distances were measured directly, for the first time, in two InxGa1-xAs, InAs and InSb samples. / THz spinduliuotės generavimas iš puslaidininkių paviršiaus turi didelį potencialą puslaidininkių fizikinėms savybėms tirti. Šis darbas skiriamas puslaidininkių tyrimams generuojant THz impulsus iš jų paviršių, apšviestų femtosekundiniais lazerio impulsais. THz spinduliuotė iš puslaidininkių paviršių gali būti generuojama dėl visos eilės fizikinių mechanizmų: paviršinio lauko ekranavimo, foto-Demberio efekto, optinio lyginimo, elektriniu lauku indukuoto optinio lyginimo, plazminių svyravimų, koherentinių fononų ir plazmonų. Tiriant THz spinduliuotės generacijos mechanizmus galima išmatuoti daug svarbių puslaidininkių parametrų, tokių kaip lūžio rodiklis, judris, krūvininkų gyvavimo trukmė, aukštesniųjų laidumo slėnių padėtys. Darbo metu tirti THz spinduliuotės generacijos puslaidininkio paviršiuje mechanizmai keičiant žadinimo sąlygas: aplinkos temperatūrą, magnetinį lauką, žadinančio lazerio bangos ilgį ir intensyvumą, bei impulso trukmę. Ištyrus visą eilę įvairių puslaidininkių nustatyta, kad geriausias THz spinduliuotės emiteris žadinant 800 nm bangos ilgio spinduliuote yra p-InAs. Pirmą kartą THz žadinimo spektroskopijos metodu tiesiogiai išmatuoti tarpslėniniai atstumai InxGa1-xAs , InAs ir InSb bandiniuose.