NDTnet 1998 Aug, Vol.3 No.8

Abstract

Long range NDE is getting more and more interesting due to different reasons: Save of time and costs for the inspection by covering a long range, inspection of areas with no possibility of access and inspection of areas under extreme environmental conditions with heavy difficulties to apply any sensor. One tool to approach the requirements of long range ultrasonic NDE are guided ultrasonic waves in the frequency range of some 100 kHz.
Up to now only guided waves with polarization in the plane of incidence (Rayleigh-, Lamb-, Longitudinal and Flexural tube- and rod-waves) were considered for long range NDT. The reason is the possibility to excite and detect them by conventional piezoelectric probes. However by Electromagnetic acoustic transducers (EMATs) as well guided waves with polarization in and perpendicular to the plane of incidence (SH-waves) are transmitted and received.
Guided SH-waves have one interesting feature in contrast to guided waves with in-plane polarization: the lowest order mode has no dispersion and the dispersion of the higher order modes is weak. Therefore they are well suited for long range inspection.The contribution will deal with examples of the application of guided shear (SH-) waves in plates and tube walls using EMAT's:
The detection of outside corrosion in the bottom plates of large storage tanks by conventional UT or magnetic stray flux techniques requires a scanning of the whole inside surface of the plate. By guided waves inspection distances between 500 and 1000 mm are possible by which the inspection time is significantly reduced.
Tubes and pipes in chemical/petrochemical plants are often supported by plates etc.. Outside corrosion may occur which is usually hidden. Guided SH-waves with circumferential and/or axial propagation were used to detect this type of corrosion.
The wall thickness of street lamp posts is essential for their safe operation. Inside corrosion also below the ground can occur which is difficult and time consuming to inspect form outside by simple wall thickness measurements. Guided waves in the post tube wall with propagation in axial direction were used to detect and localize inside and outside corrosion also on parts of the post below the ground by scanning the probe in circumferential direction.

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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