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ALOK-imaging and -reconstruction of surface defects on heavy plates with E.M.A.-Rayleigh wave transducers

: Rieder, H.; Salzburger, H.J.

Review of Progress in Quantitative Nondestructive Evaluation. 8A
New York: Plenum Publishing, 1989 (Review of Progress in Quantitative Nondestructive Evaluation 8A)
ISBN: 0-306-42837-7
Annual Review of Progress in Quantitative Nondestructive Evaluation <15, 1989, La Jolla/Calif.>
Conference Paper
Fraunhofer IZFP ()
ALOK; defect imaging; defect reconstruction; EMAT; heavy plate; rayleigh-wave; surface inspection

High quality heavy plates have to be free of any defects. The UT-inspection for internal defects is introduced as a standard. However, until now the inspection for surface defects is limited to visual inspection, magnetic stray flux - or eddy current inspection. The most sensitive technique is the magnetic stray-flux technique, but it is very time consuming because of the necessity to scan the whole surface. Using Rayleigh waves the inspection can be automated and performed at rolling speed. The sensitivity of frequency optimized Rayleigh waves is sufficient to detect a 0.3mm deep model defect at a distance of 300mm. The use of piezoelectric probes is limited by coupling problems. These problems are overcome by the use of EMAT's, which have been optimized with respect to probe size, sensitivity and wear resistance. For the automated inspection the ALOK-system has been adapted in a laboratory system to the surface inspection of heavy plates by Rayleigh waves. This equipment consisits of an ultrasonic hardware device for fast data-acquisition and-preprocessing (essential data reduction) according to the ALOK-algorithm, and a computer to realize noise- and ghost- elimination as well as defect imaging and -reconstruction. Examples of the surface inspection of heavy plates using electromagnetically excited Rayleigh waves in connection with the ALOK data-acquisition and evaluation system are reported. The investigations were supported by the ECSC (European Community for Steal and Coal).