Remote techniques for time-of-flight flaw characterisation

This thesis describes the investigations conducted into the characterisation of various defects within solids using laser - generation and interferometric detection of ultrasound. Laser - generated ultrasound has advantages over the more commonly used piezoelectric transducers in that it is a reproducible, wide bandwidth, non - contact source which, by using appropriate optics, can be focussed to a point or line source and easily scanned over a surface. Used in association with an interferometric detector enables the possibility of a remote inspection system which would be useful for some applications. The properties of such a system for the characterisation of both bulk and surface - breaking defects are ascertained. Part of the research project has been concerned with determining the properties of the ultrasound produced in various solids by different laser pulse profiles. The results obtained indicate that the risetime of the ultrasonic compression wave decreases with decreasing incident laser pulse risetime. However the mechanical and thermal properties of the irradiated solid also affects the risetime of the ultrasound. Modelling of the laser - solid interaction was undertaken to understand this phenomenon in more detail. The wide bandwidth nature of laser - generated ultrasound was used in the characterisation of bulk defects. This has involved determining, using an FFT algorithm, the phase change which occurs when laser - generated ultrasound is scattered from such defects. The aim of the investigation was to assess whether a particular type of defect has a unique phase change which will enable it to be classified. The experimental results obtained compared favourably with the results from theoretical modelling of ultrasonic scattering by defects. Finally the interaction of laser - generated Rayleigh ( surface ) waves with surface - breaking defects was considered. Various techniques for the characterisation of surface - breaking defects by analysis of data in the time and frequency domain were utilised. One such technique involved measurement of the Reflection and Transmission coefficients of various surface discontinuities. The phase change of the scattered Rayleigh wave was determined and compared to the theoretical data available. Also, a technique was established whereby certain features on the waveforms recorded on transmission of a Rayleigh wave through a surface - breaking defect could be used to determine the depth of the defect