<div>Self-sensing materials are engineered to transduce mechanical effects like deformations and damages into detectable electrical changes. As such, they have received immense research attention in areas including aerospace, civil infrastructure, robotic skin, and biomedical devices. In structural health monitoring (SHM) and nondestructive evaluation (NDE) applications, damages in the material cause breakage in the conductive filler networks, resulting in changes in the material's conductivity. Most SHM and NDE applications of self-sensing materials have used direct current (DC) measurements. DC-based methods have shown advantages with regard to sensitivity to microscale damages compared to other SHM methods. Comparatively, alternating current (AC) measurement techniques have shown potential for improvement over existent DC methods. For example, using AC in conjunction with self-sensing materials has potential for benefits such as greater data density, higher sensitivity through electrodynamics effects (e.g., coupling the material with resonant circuitry), and lower power requirements. Despite these potential advantages, AC techniques have been vastly understudied compared to DC techniques. </div><div><br></div><div>To overcome this gap in the state of the art, this thesis presents two contributions: First, an experimental study is conducted to elucidate the effect of different damage types, numbers, and sizes on AC transport in a representative self-sensing composite. And second, experimental data is used to inform a computational study on using AC methods to improve damage detection via electrical impedance tomography (EIT) – a conductivity imaging modality commonly paired with self-sensing materials for damage localization. For the first contribution, uniaxial glass fiber specimens containing 0.75 wt.% of carbon nanofiber (CNF) are induced with five types of damage (varying the number of holes, size of holes, number of notches, size of notches, and number of impacts). Impedance magnitude and phase angle were measured after each permutation of damage to study the effect of the new damage on AC transport. It was observed that permutations of hole and notch damages show clear trends of increasing impedance magnitude with the increasing damage, particularly at low frequencies. These damages had little-to-no effect on phase angle, however. Increasing numbers of impacts on the specimens did not show any discernable trend in either impedance magnitude or phase angle, except at high frequencies. This shows that different AC frequencies can be more or less useful for finding particular damage types.</div><div><br></div><div>Regarding the second contribution, AC methods were also explored to improve damage detection in self-sensing materials via EIT. More specifically, the EIT technique could benefit from developing a baseline-free (i.e., not requiring a ‘healthy’ reference) formulations enabled by frequency-difference (fd) imaging. For this, AC conductivity measurements ranging from 100 Hz to 10 MHz were collected from various weight fractions of CNF-modified glass fiber/epoxy laminates. This experimental data was used to inform fdEIT simulations. In the fdEIT simulations, damage was simulated as a simple through-hole. Simulations used 16 electrodes with four equally spaced electrodes on each side of the domain. The EIT forward problem was used to predict voltage-current response on the damaged mesh, and a fdEIT inverse problem was formulated to reconstructs the damage state on an undamaged mesh. The reconstruction images showed the simulated damage clearly. Based on this preliminary study, this research shows that fdEIT does have potential to eliminate the need for a healthy baseline in NDE applications, which can potentially help proliferate the use of this technique in practice.</div>

L’objectif principal de cette thèse consiste à développer des stratégies de modélisation et de contrôle pour la conversion de fréquence en utilisant un convertisseur multi-modulaire sans utilisation de lien en continu. Ce convertisseur est nécessaire pour l’intégration des énergies renouvelables offshore dans un réseau de transport à basse fréquence. La basse fréquence électrique profite à la fois des avantages des réseaux alternatif et continu. L’utilisation de convertisseurs modulaires dans ces réseaux permet, entre autres, de supprimer le transformateur du côté de la basse fréquence électrique. Dans cette optique, l’énergie est convertie depuis la fréquence nominale à 50 Hz ou 60 Hz (considérée comme la haute fréquence) vers un tiers de 50 Hz. Le convertisseur est aussi capable de transformer d’autres valeurs de fréquence. Dans un premier temps, la méthode de modélisation du convertisseur est reliée à la théorie du transformateur de voltage triphasé avec un couplage en YDY. En deuxième lieu, l’application du théorème de superposition nous permet d’évaluer séparément la contribution de chaque fréquence. Dans un dernier temps, le contrôle principal du convertisseur utilise des correcteurs résonants, capables donc de travailler directement avec les courants internes du convertisseur qui sont à deux fréquences nominales. Grâce au découplage entre l’entrée du convertisseur et sa sortie, les défauts et les situations de déséquilibre ne se propagent pas vers la sortie du convertisseur, fonctionnement requis pour l’application en offshore. / The purpose of this thesis is to develop strategies to model and control a modular multilevel converter in order to transform frequency without passing through a direct current link. This converter is needed for application in offshore systems where the energy, produced by the offshore renewables, is transmitted using low frequency links. Low frequency presents benefits from alternative and direct current technologies. Moreover, a voltage transformer is not required at the low frequency side when the proposed converter is used as a frequency transformer. In this scenario, frequencies are converted from the highest nominal frequency (50 Hz or 60 Hz) to one third of 50 Hz. Conversion to other frequency values has been tested as well using the same converter. A methodology based on the electrical transformer theory when connected in YΔY coupling and, the superposition theorem to separate the contribution from both frequencies, are proposed to link these currents with the two three-phase systems at different nominal frequencies. Resonant controllers are introduced to efficiently correct internal current signals of the converter that nominally are constituted of two frequencies: input and output frequencies. The converter is able to decouple the input side from the output. Therefore, faults and unbalanced conditions on the input side are not propagated to the output-side of the converter, which is required for offshore applications.