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M Bayray:
"Crack Growth Investigations in Pressure Vessels using Acoustic Emission Technique";
Supervisor, Reviewer: J. Zeman, H. Jörgl; Institut für Apparate- und Anlagenbau, 2002.

This dissertation is a result of an extensive experimental and theoretical study conducted on crack growth in pressure vessels due to high pressure test and cyclic pressurisation. Acoustic Emission (AE) was utilised as the main experimental monitoring technique.
Literature on the effect of overload on fatigue life and AE technique in monitoring crack growth were reviewed. Lamb's wave theory has been studied and applied to analyse AE signal waveforms. Simulated AE sources were employed to investigate AE signal characteristic Dispersion of AE signals, attenuation of AE signal amplitude and in-plane (IP) and out-plane (OOP) AE sources were the characteristics studied, utilising electrical pulse (EP) and pencil lead breaks (PLB). A method, which is based on the Window Fourier Transform (WFT), has been developed to identify the symmetric and asymmetric AE signal wave modes. Result of the simulated AE tests show it was possible to identify the two wave modes.
Experimental investigation of crack growth due to cyclic pressurisation of vessels was conducted in two series of vessels: series BC and series BM. In vessel series BC the experiments included two vessels with welded-inspection openings. AE data analysis was carried out using traditional and the waveform analysis method. Most of the AE events due to crack growth occur near to the final failure. Those events occur during the increasing part of the load cycle and most events were recorded at 30-120° phase of the pressure cycle, well below the peak pressure. Waveforms of the AE events were analysed using the WFT method. The events were categorised among which most were clearly in-plane type of AE source. Pressure tests were conducted in vessel series BM, after a specified number of cycles of pressurisation (where the crack grew for some length), to investigate the effect on the crack growth, and to detect the existence if the crack using the AE data. The intermittent pressure test (about 1.5 times the peak cyclic pressure) had no significant effect on the rate of crack growth. However, the pressure tests caused crack blunting which was clearly visible on the fracture surface.