2-DIMENSIONAL ULTRASONIC FLAW DETECTION BASED ON THE WAVELET PACKET TRANSFORM

An important issue in ultrasonic nondestructive evaluation is the dete ction of flaw echoes in the presence of coherent background noise asso ciated with the microstructure of materials. Many signal processing te chniques have proven to be useful for this purpose, but fully 2-D flaw detection techniques remain desirable, In this paper, we describe a n ovel automatic flaw detection method based on the wavelet packet trans form, which is particularly well adapted to B-scan image analysis, Aft er a brief review of the essential elements of the theory of wavelets and wavelet packets, a detailed description of the method is provided, The detection process operates on a set of spatially oriented frequen cy channels, i.e., detail images, obtained from successive wavelet pac ket decompositions of the initial B-scan. A statistical selection proc edure based on the modeling of the detail image histograms retains the useful information-bearing frequency channels, The flaw information i s then extracted fi om these selected channels by means of a specific thresholding scheme. Same experimental detection results in B-scan ima ges of austenitic stainless steel samples comprising artificial flaws are presented.