Application of time-frequency analysis in the case of ultrasonic imaging in a background of structural noise

Ultrasonic non-destructive testing of components made from austenitic steels is complicated by structural noise and the shape of intergranular stress corrosion cracks. To improve detection, characterisation of such cracks and development of the advanced signal processing techniques, the ultrasonic NDT simulation technique has been applied. The present paper describes the application of a numerical simulation and imaging technique, which enables to take into account direct, mixed and skip reflections from the internal defects of a granular structure. The NDT simulation algorithm is based on an extended diffraction model of the ultrasonic transducer. To simulate intergranular stress corrosion cracks, a fractal theory has been applied. The numerical simulation of the inspection of an austenitic steel structure with an intergranular stress corrosion crack has been performed. In the cases of SNR (signal-to-noise ratio) deviation, the time-frequency analysis based on the Hilbert-Huang transform has been applied. The results obtained by numerical simulation have been verified experimentally and good correspondence has been obtained.

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