Lay-up characterization and elastic property determination in composite laminates

This dissertation focuses on two important nondestructive evaluation and materials characterization problems related to composite laminates: ply lay-up characterization and elastic property determination. For ply lay-up characterization, we have developed a shear wave transmission technique to effectively detect ply lay-up errors in composite laminates. The effects of fiber orientation on normal-incident shear waves propagating through a composite laminate have been investigated both theoretically and experimentally. To facilitate rotation, EMATs (electromagnetic acoustic transducers) were used to generate and receive the shear waves. It was found that the transmitted shear waves when the EMAT transmitter and receiver were perpendicular to each other had a great sensitivity to ply lay-up errors. This technique has been successfully demonstrated on both cured and uncured composite laminates. For elastic property determination, we have first applied the simultaneous velocity and thickness imaging technique to map out small changes in ultrasonic velocity (hence elastic constant) when the material thickness was unknown or varied spatially. Applications to several industrial materials have demonstrated the usefulness of this technique for both materials characterization and flaw detection in metals and composite laminates. We have also extended this technique to generate images of sample surface contours and cross-sectional profiles when the velocity was unknown. Next, we have extended the synthetic aperture scanning method using planar transducers in an immersion leaky wave reflection or transmission measurement to allow the use of focused transducers. The complex transducer point approach has been used to model the receiver output voltage and to analyze the transducer beam effects on the result of a synthetic aperture scan. It was found that the large angular beam spread of focused transducers can be used for rapid mapping of the reflection or transmission coefficient and the associated dispersion spectrum. A novel stepwise, targeted procedure has also been developed to allow efficient reconstruction of material elastic property with only minimal use of the highly redundant dispersion spectrum data. Experiments on both isotropic and anisotropic plates showed that this method can be used for rapid evaluation of the elastic behavior of composite laminates and other plate materials with a reasonably good accuracy

Time of flight diffraction and imaging (TOFDI)

Time of flight diffraction and imaging (TOFDI) is based on time of flight diffraction (TOFD), adding cross-sectional imaging of the sample bulk by exploiting the scattering of ultrasonic waves from bulk defects in metals. Multiple wave modes are emitted by a pulsed laser ultrasound ablative source, and received by a sparse array of receiving electromagnetic acoustic transducers (EMATs), for non-contact (linear) scanning, with mode-conversions whenever waves are scattered. Standard signal processing techniques, such as band-pass filters, reduce noise. A B-scan is formed from multiple data captures (A-scans), with time and scan position axes, and colour representing amplitude or magnitude. B-scans may contain horizontal lines from surface waves propagating directly from emitter to receiver, or via a back-wall, and angled lines after reflection off a surface edge. A Hough transform (HT), modified to deal with the constraints of a B-scan, can remove such lines. A parabola matched filter has been developed that identifies the features in the B-scan caused by scattering from point-like defects, reducing them to peaks and minimising noise. Multiple B-scans are combined to reduce noise further. The B-scan is also processed to form a cross-sectional image, enabling detection and positioning of multiple defects. The standard phase correlation technique applied to camera images, has been used to track the relative position between transducer and sample. Movement has been determined to sub-pixel precision, with a median accuracy of 0.01mm of linear movement (0.06 of a pixel), despite uneven illumination and the use of a basic low resolution camera. The prototype application is testing rough steel products formed by continuous casting, but the techniques created to facilitate operation of TOFDI are applicable elsewhere

Ultrafast Ultrasound Imaging

Among medical imaging modalities, such as computed tomography (CT) and magnetic resonance imaging (MRI), ultrasound imaging stands out due to its temporal resolution. Owing to the nature of medical ultrasound imaging, it has been used for not only observation of the morphology of living organs but also functional imaging, such as blood flow imaging and evaluation of the cardiac function. Ultrafast ultrasound imaging, which has recently become widely available, significantly increases the opportunities for medical functional imaging. Ultrafast ultrasound imaging typically enables imaging frame-rates of up to ten thousand frames per second (fps). Due to the extremely high temporal resolution, this enables visualization of rapid dynamic responses of biological tissues, which cannot be observed and analyzed by conventional ultrasound imaging. This Special Issue includes various studies of improvements to the performance of ultrafast ultrasoun

A Structural Health Monitoring Concept on Ultrasonic Based Assessment of Aged Structures with Isotropic and Anisotropic Material Properties

Ultrasound is a technique widely applied in classical NDT where a lot of progress has been made in terms of signal processing, resolution, visualisation, and possibly more. With the introduction of the phased array technique, advanced computation and sensor signal processing also the characterisation of anisotropic materials has been facilitated. In par-allel SHM has emerged as a field of research very much looking into the automation of NDT in view of taking enhanced advantage of an engineering structure’s damage toler-ance and hence light weight potential. Ultrasonic guided waves are a field very much explored currently within the context of SHM and as such it is worth to determine what of the guided wave related techniques developed in NDT may be inheritable for SHM applications. Reverse Phase Matching (RPM) referred to anisotropic material properties and ray tracing are an interesting combination to be pursued in terms of finding an opti-mum sensor configuration for SHM through numerical simulation based on FEM, allowing to take better advantage of a polymer based composite materials potential such as CFRP by taking advantage of those materials’ inherent non-linear responses. The approach to be taken is illustrated with respect to an example, explaining on how to assess an ageing CFRP component in terms of its damage condition and an SHM system to be configured in the end. Finally a systematic is presented on how knowledge generated in NDT can be linked into a ‘SHM-toolbox’ to be used for the advancement of SHM systems to be de-veloped in the longer term.Ultraschall ist eine in der klassischen zerstörungsfreien Prüfung (ZfP) weit verbreitete Technik, bei der vielfältig Fortschritte hinsichtlich Signalverarbeitung, Auflösungsvermö-gen, Visualisierung und vermutlich weiterer Dinge erzielt wurden. Mit der Einführung des Phased Array sowie fortgeschrittener Rechner- und Signalverarbeitungstechnik ist auch die Charakterisierung anisotroper Werkstoffe erleichtert worden. Parallel dazu hat sich das Gebiet der Zustandsüberwachung (Structural Health Monitoring (SHM)) als For-schungsfeld entwickelt, dies im Hinblick auf die Automatisierung der ZfP und der erwei-terten Nutzung des Potenzials der Schadenstoleranz bzw. des Leichtbaus von Bauteilen. Geführte Wellen auf der Basis des Ultraschalls sind im Bereich des SHM ein derzeit breit untersuchtes Forschungsfeld und es ist somit wert zu ermitteln, inwieweit sich auf ge-führte Wellen beziehende Techniken aus der ZfP auf SHM-Anwendungen übertragen werden können. Reverse Phase Matching (RPM) in Verbindung mit anisotropen Werk-stoffeigenschaften und Ray Tracing sind eine in diesem Sinne interessante, sich fügende und verfolgungswerte Kombination, um möglicherweise über die Betrachtung inhärenter nicht-linearer Werkstoffantworten auf der Basis einer numerischen Simulation (FEM) ein optimales Sensornetzwerk für SHM zu bestimmen, womit das Potenzial eines Faserver-bundwerkstoffs wie CFK besser genutzt werden kann. Ein hierzu gewählter Ansatz wird am Beispiel einer gealterten CFK-Struktur im Hinblick auf deren Schädigungszustand er-läutert, für den dann diesbezüglich am Ende ein SHM-System konfiguriert werden kann. Abschließend wird eine Systematik vorgestellt, über die in der ZfP generiertes Wissen mit einem ‘SHM-Werkzeugkasten’ so verknüpft wird, dass längerfristig eine Weiterentwick-lung von SHM-Systemen ermöglicht wird