Ultrasonic inspections are widely used for the flaw detection of materials. To apply the fracture mechanics to the evaluation of structural integrity, many researchers have proposed various kinds of methods which could classify quantitatively ultrasonic flaw signal. Those methods are mainly based on the signal analysis and apply either analyses of frequency domain or analyses of time domain. However, the accuracy of these methods is still not satisfactory when they are applied to detect the natural flaw. Recently, time-frequency 2-dimensional analysis have been developed by many researchers. More information is included in the analyses than the analysis of either merely frequency domain, or merely time domain. Then, the analyses are expected to apply various fields of the signal processing. In this study, a new method based on the time-frequency analysis is proposed, to determine exactly a flaw type. As the frequency becomes the higher, the ultrasonic longitudinal wave spreads the narrower. In case of the voluminous flaw detection, a high frequency component of the wave reflects from the part of flaw which is just beneath the ultrasonic probe, while a low frequency component reflects from both parts which is apart from, and beneath the center line of ultrasonic probe. As the low frequency component spreads wider, there is a difference in the beam path between low and high frequency components which are reflected from the voluminous flaw. Therefore, an arrival time of low frequency component is longer than that of high frequency. On the other hand, in case of the small flaw detection, there is not a difference in arrival time between low and high frequency components, between the beam path of low frequency component becomes approximately the same as that of high frequency component. Hence, it is possible to determine the flaw type, applying a time-frequency 2-dimensional analysis and analyzing the arrival time of a particular frequency. The method was applied to three types of artificial flaws, five kinds of side-drilled holes, five kinds of flat-bottomed holes and five kinds of slits. Reflected ultrasonic signals were measured by three types of probes. The probes emitted broad band normal incident longitudinal wave (the probes are 'd=10mm, f=5MHz', 'd=10mm, f=2MHz' and 'd=6mm, f=5MHz', where d and f denote diameter and center frequency, respectively). Short Time Fourier Transform was adopted as time-frequency analysis. For the side-drilled holes, the arrival time delay of low frequency reflected component of ultrasonic signals emitted by all probes, can be detectable. For the slits, that of ultrasonic signals emitted by the probe of 'd=6mm, f=5MHz', is detectable. For the flat-bottomed holes, that of ultrasonic signals emitted by any probe, can not be detected at all. Thus, the result obtained by this research support the feasibility of classification of three types of artificial flaws by means of the time-frequency analysis of reflected signal of ultrasonic wave emitted by the probes of different diameter, that is, 'd=10mm and d=6mm'.
Publication Source: Trends in NDE Science & Technology; Proceedings of the 14th World Conference on Non-Destructive Testing, New Delhi, 8-13 December 1996.Vol. 4, pages 2113 - 2116 Publisher:Ashgate Publishing Company