<p>  </p> <p>Structural health monitoring is more efficient than traditional visual interval-based structural inspection because structural assessments are implemented when a sensor, such as an accelerometer, measures the vibration of the structure and detects any abnormal readings outside of a safety threshold. These vibrations tend to be atypical when there is damage to the structure. Processing the collected data from an accelerometer using Fast Fourier Transformation (FFT) allows for a graphical visualization of visualizing these atypical measurements in the frequency domain. The comparison and analysis of vibration frequency incurred from three different scenarios (damage, no damage, and impact) in the steel truss prototype has resulted in fundamental knowledge necessary to differentiate an abnormality in accelerometer readings resulting from a vehicular crash against one in which there is actual structural damage. The primary outcome of this work will lead to avoiding unnecessary inspection costs due to possible faulty diagnostics and determining the reliability of the structural health monitoring method.</p>

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Composite materials, constructed from a combination of fibre and resin, have rapidly emerged as a high performance alternative to conventional materials for new constructions as well as strengthening and repair of existing structures. However, the use of such materials may be accompanied by various types of damages and failure modes, including delamination, debonding, fibre rupture, and matrix cracking. This thesis presents a new nondestructive evaluation (NDE) technique for damage detection in composite structures. The concept, based on detecting local dielectric permittivity variations, was employed to design capacitance sensors with high sensitivity to detect such damages. An analytical and 2D finite element models were used to assess the influence of the sensor geometrical parameters on the output signals and to optimize the sensor design. Concrete and wood specimens wrapped with glass-fibre composites containing pre-induced defects with different types and sizes were constructed and inspected. The sensors were also used to detect the delaminations and water intrusion defects in pultruded composite members. The principles behind the sensor operation were also applied to detect other damages in other structures; the capacitance sensors were designed and used to locate ungrouted cells in a concrete masonry wall. The proposed sensors, coupled with a commercially available portable capacitance meter, facilitate employing this technique in the field for rapid inspection of composite structures without the need for sophisticated data analyses that are usually required by other more expensive and time consuming NDE techniques. / Thesis / Master of Applied Science (MASc)

La presencia inadvertida de daños en elementos estructurales representa un aspecto crítico en la seguridad de los mismos. Tales fallas pueden causar irregularidades de funcionamiento e incluso conducir al colapso catastrófico. Por ese motivo, la detección de daños en forma temprana es de fundamental importancia. Un tipo de falla que puede pasar peligrosamente inadvertida es aquélla producida por fatiga, ya que resulta muy difícil de detectar a simple vista. Si bien existen técnicas adecuadas para la detección localizada, la aplicación de las mismas puede resultar impráctica en base a la dificultad en la revisión de estructuras complejas, que inclusive pueden pre-sentar sectores inaccesibles. En consecuencia, se han investi-gado otro tipo de procedimientos de carácter más global, entre los que ha adquirido gran importancia la identificación basada en la respuesta dinámica. Esencialmente, este método se basa en la comparación entre valores teóricos y experimenta-les de parámetros dinámicos de estructuras dañadas. Las incógnitas del mencionado problema corresponden a las magni-tudes asociadas al daño (localización, intensidad). Por lo tan-to, a los valores más próximos a los reales les corresponderá una mínima desviación entre los valores teóricos y experimen-tales. Este tipo de técnica inversa hace uso de un modelo teó-rico de la estructura dañada. En esta tesis se presenta un nuevo modelo unidimensional que permite reproducir el comportamiento estructural de vigas de pared delgada con presencia de fisuras debidas a fatiga. En dicho modelo, se admite la posibilidad de considerar el efecto no lineal de cerra-miento parcial de fisura o breathing. El modelo asocia un enfoque de daño estructural por fisura basado en conceptos de mecánica de fracturas, que se desarrolla íntegramente en este trabajo, con una teoría de vigas de pared delgada recien-temente desarrollada para el caso de vigas intactas. Esta teo-ría debe ser extendida para considerar la presencia de fisuras. La premisa global de la teoría es que la presencia de una fisu-ra genera una reducción localizada en la rigidez de la viga. El modelo unidimensional de viga de pared delgada dañada es aplicable a vigas construidas con materiales isótropos, ortótro-pos y compuestos con laminación cross-ply simétrica o espe-cialmente ortótropa. Considera además flexibilidad por corte debido a flexión y alabeo. Los parámetros asociados a las fisuras son identificados mediante la minimización de una fun-ción objetivo, que se define en términos de diferencias norma-lizadas entre valores de indicadores de fisura obtenidos experi-mentalmente y calculados con el modelo viga. Como indica-dores de daño estructural se consideran los desplazamientos producidos por excitación forzada y también las frecuencias de vibración natural. En orden de resolver el problema inverso, se emplea el algoritmo de optimización Evolución Diferencial. / The unnoticed presence of damage in structural elements represents a critical issue in their security. Such flaws may generate malfunctions and even leading to catastrophic collapse. Thus, early detection of damage represents a topic of fundamental importance. A kind of flaw that can be dange-rously unnoticed is that one produced by fatigue, in terms of the difficulty to detect it with the naked eye. Although there are adequate techniques for local detection, their application may result impractical based on the difficulty represented by complex structures, which can have inaccessible parts. Consequently, other procedures of more global character have been topic of research. Among them, identification ba-sed on dynamic response has acquired major importance. Essentially, this method is based on comparisons among theo-retical and experimental values of dynamic parameters related to damaged structures. The unknowns of the mentioned problem correspond to the magnitudes associated to damage (location, severity). Thus, a minimal deviation among theore-tical and experimental results will correspond to the values closer to real ones. This kind of inverse technique makes use of a theoretical model of the damaged structure. In this the-sis, a new one-dimensional model is presented, which allows replicating the structural behavior of thin-walled beams with the presence of cracks generated by fatigue. In this model, the possibility of considering the nonlinear effect of crack partial closure or breathing is allowed. The model associates a structural damage approach based on fracture mechanics concepts, developed entirely in this work, with a recently developed thin-walled beam theory for the case of intact beams. This theory must be extended in order to consider the presence of cracks. The global premise of the theory is that the presence of a crack generates a localized reduction on the beam stiffness. The one-dimensional thin-walled da-maged beam model is applicable to beams made of isotropic material beams, orthotropic materials and composite materials with symmetric cross-ply or specially orthotropic stacking sequences. In addition, shear flexibility debt to flexure and warping is considered. Damage parameters associated to cracks are identified by means of the minimization of a target function, defined in terms of normalized differences among values of damage indicators obtained experimentally and calculated with the beam model. As indicators of structural damage, displacements debt to forced excitation and also frequencies of natural vibration are considered. In order to solve the inverse problem, the optimization algorithm Differential Evolution is employed.

Maintenance of the structural integrity and damage detection are critical for all massive and complicated new and aging structures. A structural health monitoring (SHM) system intends to identify damage on the structure under monitoring, so that necessary action can be taken in advance to avoid catastrophic results. Impedance-based SHM utilizes a piezoelectric ceramic as a collocated actuator and sensor, which measures the electrical impedance of the piezoelectric ceramic over a certain frequency range. The impedance profile of a structure under monitoring is compared against a reference profile obtained from the healthy structure. An existing approach called the sinc method adopts a sinc wave excitation and performs traditional discrete Fourier transform (DFT) based structural condition assessment. The sinc method requires rather intensive computing and a digital-to-analog converter (DAC) to generate a sinc excitation signal. It also needs an analog-to-digital converter (ADC) to measure the response voltage, from which impedance profile is obtained through a DFT. This dissertation investigates system design approaches for impedance-based structural health monitoring (SHM), in which a primary goal is low power dissipation. First, we investigated behaviors of piezoelectric ceramics and proposed an electrical model in order to enable us to conduct system level analysis and evaluation of an SHM system. Unloaded and loaded piezoelectric ceramics were electrically modeled with lumped linear circuit components, which allowed us to perform system level simulations for various environmental conditions. Next, we explored a signaling method called the wideband method, which uses a pseudorandom noise (PN) sequence for excitation of the structure rather than a signal with a particular waveform. The wideband method simplifies generation of the excitation signal and eliminates a digital-to-analog converter (DAC). The system form factor and power dissipation is decreased compared to the previously existing system based on a sinc signal. A prototype system was implemented on a digital signal processor (DSP) board to validate its approach. Third, we studied another low-power design approach which employs binary signals for structural excitation and structural response measurement was proposed. The binary method measures only the polarity of a response signal to acquire the admittance phase, and compares the measured phase against that of a healthy structure. The binary method eliminates the need for a DAC and an ADC. Two prototypes were developed: one with a DSP board and the other with a microcontroller board. Both prototypes demonstrated reduction of power dissipation compared with those for the sinc method and for the wideband method. The microcontroller based prototype achieved an on-board SHM system. Finally, we proposed an analytical method to assess the quality of the damage detection for the binary method. Using our method, one can obtain the confidence level of a damage detection for a given damage distance. / Ph. D.

Although nondestructive evaluation techniques have been implemented in many industry fields and proved to be useful, they are generally expensive, time consuming, and the results may not always be reliable. To overcome these drawbacks, structural health monitoring (SHM) systems has received significant attention in the past two decades. As structural systems are becoming more complicated and new materials are being developed, new methodologies, theories, and approaches in SHM have been developed for damage detection, diagnosis, and prognosis. Among the methods developed, the guided Lamb wave based SHM can be a promising technique for damage evaluation since it provides reliable damage information through signals propagating over large distance with little loss of amplitude. While this method is effective for damage assessment, the guided Lamb wave contains complicated mode characteristics, i.e. an infinite number of wave modes exist and these modes are generally dispersive. For this reason, a minimum number of wave modes and various signal processing algorithms are implemented to obtain better signal interpretations. Phased array beamsteering is an effective means for damage detection in guided Lamb wave SHM systems. Using this method, the wave energy can be focused at localized directions or areas by controlled excitation time delay of each array element. In this research, two types of transducers are utilized as phased array elements to compare beamsteering characteristics. Monolithic piezoceramic (PZT) transducers are investigated for beamsteering by assuming omnidirectional point sources for each actuator. MacroFiber Composite (MFC) transducers with anisotropic actuation are also studied, considering the wave main lobe width, main lobe magnitude, and side lobe levels. Analysis results demonstrate that the MFC phased arrays perform better than the PZT phased arrays for a range of beamsteering angles and have reduced main lobe width and side lobe levels. Experiments using the PZT and MFC phased arrays on an aluminum plate are also performed and compared to the analysis results. A time-frequency signal processing algorithm coupled with a machine learning method can form a robust damage diagnostic system. Four types of such algorithms, i.e. short time Fourier transform, Wigner-Ville distribution, wavelet transform, and matching pursuit, are investigated to select an appropriate algorithm for damage classification, and a spectrogram based on short time Fourier transform is adopted for its suitability. A machine learning algorithm called Adaboost is chosen due to its effectiveness and high accuracy performance. The classification is preformed using spectrograms and Adaboost for crack and corrosion damages. Artificial cracks and corrosions are created in Abaqus® to obtain the training samples consist of spectrograms. Several beam experiments in laboratory and additional simulations are also performed to get the testing samples for Adaboost. The analysis results show that not only correct damage classification is possible, but the confidence levels of each sample are acquired. / Ph. D.