Spectral Analysis of Experimental Ultrasonic NDT Signals

Abstract

Non Destructive Testing (NDT) plays an important role in different fields. It allows materials control or structures to check their condition, as well as detect defects without damaging inspected parts. The practical difficulty of extracting information to characterize materials led to use different methods of signal processing. Real ultrasonic signals considered in this paper are collected from Steel plate with defect and Aluminum cubic sample without defects prepared in LEND laboratory of Jijel University in Algeria. Two experiments are used: pulse-echo by contact and immersion techniques wherein ultrasonic energy is transmitted by piezoelectric transducer. The reflected signals are received by the same transducer where the energy is converted into an electrical signal. Signals are treated by three techniques adapted for nonstationary signals, and based on energy distribution in time-frequency plan; namely: Continuous Wavelet Transform (CWT), Wigner Ville (WVD) and Choi-Williams (CWD) distributions. This energy distribution allows an easy flight of time’s measurement of different echoes, and thereafter velocity’s calculation and defect’s localization. Advantages and limits of each method are as follows: WVD achieves good resolution of interfaces; however, its capacity remains limited by the appearance of non-desirable terms which may limit results readings. CWD avoids interferences phenomenon. It allows exactly extraction and clearly representation of signal components in time/frequency. Application of CWT clearly shows that temporal resolution is improved by contrast frequency resolution is degraded for high frequency terms. Also the disadvantage of this method comes from the absence of criterion of mother wavelet choice. Comparative study shows that CWD makes it possible to have a velocity closer to that given in theory. While the CWT and WVD give more accurate results regarding the defect’s position. This justifies the use of these efficient methods for non-destructive ultrasonic testing for defect localization and material characterization (Velocity and Young's modulus).