Ultrasonic discrimination and modelling for crack-tip echoes

Abstract

The problem of discriminating between the acoustical signatures of an open crack tip and an interface is approached by modelling the various ultrasonic signatures and by a signal processing method. In the modelling study, the Kirchhoff approximation (the physical optics approximation) is used to describe the far field scattered by an arbitrary object with non-negligible dimensions in comparison with the wavelength, and to express this field depending on the incident field. It is established that, under this condition, the impulse response of an open crack tip is proportional to the first-order derivative of the impulse response of the transmitter. In the signal processing study, spectral and wavelet analyses were applied to the discrimination problem. The basic idea was to discriminate signatures simultaneously in time and in frequency (scale) domain. This method was found to be an effective means of testing models for the interactions between waves and flaws. Both aspects (modelling and signal processing) were studied numerically and experimentally, and the validity of the results was tested on an industrial sample.

Keywords:

• ultrasonic
• crack detection
• nondestructive testing
• acoustical signature discrimination
• spectral analysis
• wavelet decomposition

PACS numbers::

• PACS numbers: 43.35.Zc
• 43.60.Ac
• 43.60.Hj