NDTnet 1998 Aug, Vol.3 No.8

Abstract

Electromagnetic ultrasonic (EMUS) transducers are widely used in a variety of industrial applications, essentially, in view of their advantages compared to the conventional piezoelectric transducers. One ultrasonic wave mode that has provided answers to the nondestructive inspection problems in anisotropic austenitic and dissimilar metal welds and clads, is the shear wave with polarization perpendicular to the plane of incidence (SH-wave). This is essentially due to absence of mode conversion, beam skewing, distortion, etc.

It is relatively easy to excite and apply SH waves using EMUS transducers, if not impossible by piezoelectric transducers. While amplitude based defect sizing methods are routine in ultrasonic NDE, transit time based methods are considered to reduce errors in the estimation of defect size. One such method recently being used for detection and sizing of defects is time-of-flight-diffraction (TOFD). While, in the recent past, there are significant TOFD contributions using compressional and shear vertical waves, studies with SH waves are scarce. We have carried out investigations in this area with SH waves generated by EMUS transducers. We have designed and used both non-segmented EMUS transducers and segmented EMUS transducers (a maximum of 8). The two possible cases in the pitch-catch mode namely, symmetric and non-symmetric have been studied and various parameters such as angle of insonification, distance between transmitter and receiver EMUS transducers, frequency, number of pulses, etc. have been optimised. Besides signal averaging, we have adopted signal processing methods to enhance Signal to Noise Ratio (SNR) and to improve accuracy in the transit time measurements and in turn the defect sizing. The contribution gives details of the state-of-the-art phased array EMUS instrumentation and EMUS transducers used for the investigations and discusses the experimental results of characterisation of fatigue cracks and machined slots in steels. The studies bring out the potential of using diffracted SH waves for sizing defects. The difficulties that arise due to the presence of undesired side lobes in the directivity patterns and the ways and means to minimise their effect are also being discussed.

Abstract Source:
Book of Abstracts, 7th European Conference on Non-Destructive Testing, 26-29 May 1998, ISBN: 87-986898-0-00

Full-Text Source:
Proceedings of the 7th European Conference on Non-Destructive Testing, 26-29 May 1998, ISBN:

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