Noise analysis of digital ultrasonic nondestructive evaluation system

Being one of the five most commonly used nondestructive testing (NDT) routi nes, ultrasonic testing (UT) is under fast development in recent years, wit h more attention being focused on quantitative testing and nondestructive e valuation (NDE). In the evaluation of pressure vessels and piping, UT is ut ilized not only in manufacturing quality controlling, but also in-service m onitoring and residual life prediction, such as the inspection of welded jo ints, monitoring of crack propagation, evaluation of materials property det erioration. In ultrasonic NDE and quantitative NDT, one of the main factors disturbing the reliability and accuracy of test is noise encountered during inspection . At present, a digitized instrument is increasingly preferred in practice. Considering every step, from wave emitting to digitalization of analog sig nal, the following noises usually emerge-the electronic circuit noise of in strument; structure noise caused by grain boundaries of material under test ing: ringing noise due to oscillation of probe: digital noise of finite-wor d digital system; pulse noise by virtue of fluctuation of inspection circum stance. Among these, the most serious is the structure noise encountered in the testing of coarse-grained austenite stainless steel, which affects the defect signal, making the least detectable defect size increase. In the pr esent paper the characteristics, detriment and elimination algorithms of el ectrical noise, pulse noise, ringing noise and structure noise in a digital ultrasonic NDE system are discussed. A physical model of the digital ultra sonic NDE system is established, and noises are classified into different c ategories from the viewpoint of the model. The characteristics of electrica l noise are analyzed; an algorithm of extremum filtering constructed to eli minate the pulse noise; high-pass filter and wavelet packet are employed to process ringing noise; the features of structure noise are studied and it is de-noised by wavelet transform (WT) and wavelet packet transform (WPT). The results obtained from real-world signal show that the electrical noise can be taken approximately as white noise with a Gaussian distribution. The algorithm of extremum filtering can filter the pulse noise without any eff ect on other information in the signal. Wavelet packet algorithm is more su itable for the elimination of long-term ringing noise than high-pass filter under the condition of less loss of defect echo. Processing the signals of coarse-grained austenite stainless steel samples with defects by use of WT and WPT concludes that as structure noise can be divided into certain freq uency bands, generally different from those of defect echoes by WT and WPT, the defect can be pointed out, and the signal-to-noise ratio enhanced subs tantially after the threshold processing of frequency components of signals followed reconstruction. (C) 1999 Published by Elsevier Science Ltd. All r ights reserved.