Structural health monitoring of concrete structures using diffuse waves

The work presented in this thesis has aimed to investigate and implement techniques for ultrasonic measurements in structural health monitoring applications for civil structures. The focus of the work has been to make these systems practical in real applications, where the large size of the structures, and the changing environments they are exposed to, pose problems for many methods which otherwise fare well in laboratory settings.There is an increasing demand on the safety and reliability of the civil structures that make up our cities and infrastructure. The field of structural health monitoring aims to provide continuous non-destructive evaluation of such structures. Large concrete structures, such as nuclear power plants or bridges, provide a challenge when implementing such systems. Especially if minor damage is to be detected and even located. Methods based on propagating mechanical waves are known to be useful for detecting structural changes, due to the coupling between the properties of such waves and the mechanical properties of the material. The sensitivity of such measurements generally increase with higher frequencies, and ultrasonic waves can be used to detect minor cracks and early signs of damage. Unfortunately, concrete is a complex material, with aggregates and reinforcement bars on the same order of size as the wavelengths of ultrasonic waves. Ultrasonic waves are quickly scattered and attenuated, which makes traditional pitch-catch measurements difficult over long distances. However, multiply scattered waves contain much information on the material in the structure, and have been shown to be very sensitive to material changes.In this project continuous wave excitation has been used when creating the multiply scattered wave fields. This enables narrow-band detection, which is shown to enable the detection of significantly weaker signals, and thus increase the maximum distance between transducers. Techniques for localizing damage using such continuous wave fields, as well as methods for compensating for effects of changing environmental conditions, are demonstrated. Recommendations are also given for future designers of structural health monitoring systems, as to the choice of frequency, when using multiply scattered wave fields

Transducer Arrays for 3D Ultrasound Computed Tomography

Die Ultraschall-Computertomographie (USCT) ist ein vielversprechendes medizinisches Bildgebungsverfahren zur Früherkennung von Brustkrebs. Am Karlsruher Institut für Technologie wird derzeit ein Gerät der dritten Generation (3D-USCT-III) für 3D-Aufnahmen entwickelt. Unter den kritischsten und technologisch anspruchsvollsten Komponenten dieses Geräts sind die Schallwandlerarrays. Diese müssen eine pseudozufällige Positionierung einzelner Wandler ermöglichen, hohe Bandbreiten, große Öffnungswinkel und eine isotrope Schallabstrahlung aufweisen sowie den Vorschriften für Medizinprodukte genügen. In dieser Arbeit wird die Realisierung neuer Schallwandlerarrays (TAS-III) für die 3D-USCT-Bildgebung umfassend vorgestellt. Dies beinhaltet die Definition von Anforderungen, Systemdesign, automatisierte Fertigung, Charakterisierung und Entwurfsoptimierung. Kernelement des TAS-III-Designs sind Scheiben aus piezoelektrischen Verbundwerkstoffen, die 18 in Polymer eingebettete, räumlich verteilte piezokeramische Fasern enthalten. Zusätzliche Scheiben zur akustischenAnpassung und Dämpfung werden auf beiden Seiten angebracht, um die Arrays zu finalisieren. Für die Herstellung der benötigten 256 TAS-III wurde ein teilautomatisierter Fertigungsprozess entwickelt. Quantitative Qualitätsprüfungen ergaben, dass mehr als 96% der produzierten Wandler voll funktionsfähig waren. Die Amplitude und der Phasenwinkel des akustischen Feldes von 54 Wandlern wurden gemessen und ausgewertet. Die meisten der definierten Anforderungen wurden erfüllt. Es wurde eine mittlere Mittenfrequenz von 2,6 MHz, mit einer fraktionellen Bandbreite von 134% bei -10 dB ermittelt. Die Bandbreite resultiert dabei aus zwei unterschiedlichen Schwingungsmoden. Messungen in 3D zeigten isotrope Abstrahlcharakteristiken mit einem mittleren Öffnungswinkel von 42,8°. Für die Analyse und Optimierung des Designs wurden verschiedene Modellierungsansätze entwickelt. Geringfügige Änderungen der Länge und des Durchmessers der piezoelektrischen Fasern sowie eine höhere laterale Dämpfung konnten die Leistung in gewissem Maße verbessern. Der Umfang weiterer möglicher Verbesserungen zeigte jedoch, dass das TAS-III Design nahe am erreichbaren Optimum liegt. Alternative Wandlertechnologien wurden untersucht, umdie grundsätzlichen Grenzen von Verbundwerkstoffen bestehend aus piezoelektrischen Fasern in Bezug auf Öffnungswinkel und Bandbreite zu überwinden. Der Ersatz der Fasern durch einkristalline piezoelektrische Materialien verspricht eine Erhöhung der Bandbreite um 35%, erfordert jedoch umfangreiche Anpassungen in den Herstellungsprozessen. Die Charakterisierung von mikromechanischen Ultraschallwandlern ergab eine signifikante Vergrößerung des Öffnungswinkels, aber geringere erzeugte Schalldrücke. Dennoch machen die Eigenschaften und die verfügbare Designfreiheit diese Schallwandlertechnologie sehr vielversprechend für zukünftige 3D-USCT- Generationen. Die entworfenen und realisierten TAS-III erwiesen sich als rundum geeignet für den vorgesehenen Einsatz und wurden in zwei 3D-USCT-III-Geräten integriert. Ausführliche klinische Tests werden in naher Zukunft durchgeführt, um die Sensitivität und Spezifität dieser neuartigen 3D-Ultraschallbildgebungsmethode zu bewerten

Damage Detection of Submerged Structures Using Linear and Nonlinear Guided Waves

Metallic plates are one of the major components of liquid containment structures and are widely used in petrochemical and civil engineering. In many cases, the metallic plates have one side exposed to liquid and are subjected to different types of loads with varying amplitudes. Corrosion damage and material degradations are the two major concerns. Damage detection of the submerged plate structures plays an important role in maintaining the structural integrity and safety of high-valued infrastructures (e.g. liquid storage tanks and pipes). Guided wave testing is one of the most promising damage detection approaches. Although guided wave based techniques have been extensively studied on different structures in gaseous environments, the design and implementation for the structures immersed in liquid have not been well investigated. This research aims at enhancing the understanding of guided wave propagation and interaction with damage in submerged structures. The focus of this research is on metallic plates that have one side in contact with liquid and the other side exposed to air. The specific objectives of this thesis include the investigation on the propagation characteristics of guided waves in metallic plates with one side exposed to liquid, the development of numerical models to investigate the scattering characteristics of guided waves at corrosion pit damage, the analyses of the influence of the surrounding liquid medium on the linear and nonlinear guided waves features, and the evaluation of the sensitivity of linear and nonlinear guided waves features to different types of damage in the one-side immersed metallic plate. The main body of the thesis consists of four journal articles (Chapters 2-5). Chapter 2 discusses the propagation characteristics and sensitivity to damage of linear guided waves in a metallic plate loaded with water on one side. The targeted damage is local thickness thinning (e.g. corrosion pits) with a size of around a few millimeters. Chapter 3 further investigates and compares the guided wavefields between a plate surrounded by air and the same plate with one side partly exposed to water. The influence of the surrounding liquid medium on the guided wave propagation is demonstrated experimentally and numerically. Chapters 4 and 5 study two different nonlinear guided wave features, which are second harmonic generation and combination harmonic generation, respectively. The nonlinear guided wave features have better sensitivity to microstructural defects that precede the damage in the macroscale. The targeted damage in Chapters 4 and 5 is fatigue degradation in the early stage, where fatigue appears as multiple micro cracks and is distributed in the structural materials. The microstructural defects are too small to be detected by the linear guided wave feature. However, these small defects can distort the guided waves passing through the material, producing new wave components at frequencies other than the excitation frequency of the incident waves. This provides a way for the nonlinear guided wave technique to evaluate the earlystage damage in submerged structures.Thesis (Ph.D.) -- University of Adelaide, School of Civil, Environmental & Mining Engineering, 202

Early Crack Detection of Reinforced Concrete Structure Using Embedded Sensors

International audienceThe damage in reinforced concrete (RC) structures can be induced either by the dynamic or static load. The inspection technologies available today have difficulty in detecting slowly progressive, locally limited damage, especially in hard-to-reach areas in the superstructure. The four-point bending test on the benchmark RC structure was used as a test of the quality and sensitivity of the embedded sensors. It allowed assessment of whether any cracking and propagation that occurs with the embedded sensors can be detected. Various methods are used for the analysis of the ultrasonic signals. By determining the feature from the ultrasonic signals, the changes in the whole structure are evaluated. The structural degradation of the RC benchmark structure was tested using various non-destructive testing methods to obtain a comprehensive decision about structural condition. It is shown that the ultrasonic sensors can detect a crack with a probability of detection of 100%, also before it is visible by the naked eye and other techniques, even if the damage is not in the direct path of the ultrasonic wave. The obtained results confirmed that early crack detection is possible using the developed methodology based on embedded and external sensors and advanced signal processing