Yang, J; (1992) Ultrasonic scattering from volumetric flaws in structural materials and their characterisation. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis is a theoretical and experimental study of ultrasonic scattering from volumetric flaws in structural materials and ultrasonic inversion techniques for nondestructive characterisation of such flaws. For forward scattering problems, the Method Of Optimal Truncation (MOOT) is studied. A large general purposed computer model is developed based on MOOT. The computer model can be used to simulate ultrasonic scattering from different shapes and sizes of voids, with only minor changes. Numerical results for a number of voids are presented in both the frequency and time domains to provide understanding of basic physical mechanism of scattering by volumetric flaws. The simulated forward scattering data are also used to test a new inversion technique developed in this study. A new ultrasonic inversion technique is developed for determining the geometrical features of a volumetric flaw in structural materials, by the inversion of the backscattered ultrasonic signal using the area function formula. The area function formula is derived from a weak scattering approximation, the Born approximation, but it is shown that the area function sizing technique works well for voids which are clearly strong scatterers. The technique extracts the flaw size from the shape of the area function which is evaluated from the backscattering signal. Unlike most of other ultrasonic inversion schemes, this technique has the advantage that it does not require the determination of the flaw centroid (zero-of-time problem). The technique is tested by the inversion of the numerical and experimental scattering data for estimating the sizes of a number of flaws. The results show very good agreement between the true sizes and the estimated sizes. The experimental work is carried out on simulated defects in the immersion and contact modes. Several techniques for processing experimental signals are investigated, including deconvolution techniques.

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