Time-frequency domain modelling for ultrasonic nondestructive testing

This thesis endeavours to develop and implement new and improved methods for the characterisation of defects embedded in steel welds through the analysis of data collected by ultrasonic phased array inspections. A factor common to the existing imaging techniques used for flaw characterisation is the subjective thresholding required to estimate the size of the flaw. The work contained in this thesis uses the mathematics of inverse problems and scattering theory to extract information about such defects and puts forward an objective approach which employs a mathematical model. A relationship between the pulse-echo response curve of a scattering matrix and the size and orientation of a flaw is derived analytically via the Born approximation and results in a completely objective approach to crack sizing. Further expansion of these relationships allows for expressions to be formulated concerning the minimum resolvable crack length and the effects of array pitch and flaw depth on the accuracy of the algorithm. The methodology is then extended and tested on experimental data collected from welded austenitic steel plates containing a lack of fusion crack. In the latter part of this thesis, work focusses on the exploration of the fractional Fourier transform and coded excitations. The fractional Fourier transform allows for retention of both time and frequency domain information simultaneously and permits the in homogeneous wave equation (with a forcing function prescribed as a linear chirp modulated by a Gaussian envelope) to be solved in time-frequency space. This in turn facilitates a comparison between a gated continuous wave excitation and a Gaussian modulated linear chirp. It is observed that the Gaussian modulated linear chirp results in a marked increase in the scattering amplitude.

Name dc:type.qualificationname

phd

Level dc:type.qualificationlevel

doctoral-pg

Grantor dc:publisher.institution

University of Strathclyde

Year dc:date.issued

2014

Author dc:creator

• Tant, Katherine M. M.

DOI

10.48730/1wye-b612

Identifier

T13919

OAI identifier oai:identifier

oai:strathclyde:3x816m604