Gibbs-like phenomena in chaos-based frequency modulated signals

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Sub-MHz ultrasound for thick section and high attenuation materials.

This thesis described research on materials that exhibit a high degree of attenuation over the sound energy, especially those that are thick and bulky, where penetration of ultrasonic signals becomes difficult. Materials such as viscoelastic polymers, composites (especially multi-layered structures) and also concrete are among the so-called high ultrasonic attenuation materials. Hence, there is a huge need to develop advanced tools, devices and post-processing algorithm that can help to evaluate their integrity. This research thus focused on designing an ultrasonic system which is capable of examining the internal integrity of highly attenuating materials. To reach this aim, an innovative combination of Sub-MHz frequencies (below 1 MHz) excitations in the form of coded waveforms such as chirps and binary coded, together with specifically realized piezo-composite transducers was used for the research activity. The system was then used in combination with advanced signal processing techniques, i.e. pulse compression and other algorithms to enhance the acquired signal quality. Industrial samples of polyurethane structure used in deep-sea oil and gas industries, ceramic bricks used in furnaces, multi-layered structures used in the aerospace industries were tested using the developed system. All the tested samples had different properties that require different approaches during the experiment as well as the data analysis. As a consequence, the research activity focused not only on the use of the innovative sub-MHz inspection system above-described, but also on developing novel algorithms for the data processing tailored for each particular inspected material. It was shown that the system, is capable of revealing anomalies (i.e. cracks, manufacturing and artificial defects) within the tested samples. Furthermore, the advanced signal processing and image reconstruction techniques exploited helped in retrieving the correct shape and dimensions of those defect with respect to the standard imaging procedure. This research work can hopefully be a meaningful contribution towards solving the NDT problems within these industries