TIME-FREQUENCY ANALYSIS IN ULTRASONIC NONDESTRUCTIVE TESTING
C. H. Chen and Mr. Tzu-Hung Cheng University of Massachsuetts Dartmouth, Electrical and Computer Engineering Dept 285 Old Westport Road, N. Dartmouth, MA 02747-2300 USA
Keywords: Ultrasonic nondestructive testing, time-frequency analysis, smoothed short term Fourier transform, Gabor transform, defect classification
ABSTRACT
Even since the digital signal processing was introduced to ultrasonic nondestructive testing (NDT) of materials, almost all studies have been made in time domain or frequency domain alone. Some important advances on signal processing in NDT have been reported [1][2]. The limitation of time domain or frequency domain information is new well understood. However it has been difficult to extract useful information from time-frequency analysis for NDT. For example, the Wigner distribution has the problem with interference terms (see e. g. [3]). In this paper both smoothed short term Fourier transform and the Gabor transform are shown to have many desired properties for use in NDT. Three topics on using such time-frequency analysis techniques are presented in this paper. The first is the use of energy features extracted from time-frequency analysis for greatly improved classification of geometrical defects in aluminum blocks. The second is the accurate measurement of thickness of bond line reflectors using time-frequency analysis. The third is the size determination of geometrical defects from time-frequency analysis. Since the interference or cross terms are suppressed in the techniques used, the information from time- frequency analysis becomes very useful. Detailed computer results are presented to illustrate the effectiveness of the time-frequency analysis in ultrasonic NDT. REFERENCES
C. H. Chen, editor, "Signal Processing and Pattern Recognition in Nondestructive Evaluation of Materials", Springer-Verlag, 1988
X. Maldague, editor, "Advanced Signal Processing Methods in Nondestructive Evaluation of Materials", Kluwer, 1993
C. H. Chen and J. Guey, "Application of Wigner distributions in Ultrasonic NDE", Review of Progress in Quantity NDE, edited by D. G. Thompson, Plenum Press vol. 11, 1992.
Publication Source: Trends in NDE Science & Technology; Proceedings of the 14th World Conference on Non-Destructive Testing, New Delhi, 8-13 December 1996.Vol. 1, pages 161 - 168 Publisher:Ashgate Publishing Company
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