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Low frequency eddy current inspection of the tube cladding

: Hübschen, G.; Becker, R.; Kröning, M.

Electric Power Research Institute -EPRI-, NDE Center:
EPRI pipe inspection workshop. Sess. 5
Charlotte/N.C., 1993
Workshop on Pipe Inspection <3, 1993, Charlotte/N.C.>
Fraunhofer IZFP ()
defect; defect analysis; defect characterization; eddy current testing; Fehler; Fehleranalyse; Fehlernachweis; Fehlerprüfung; Oberflächencharakterisierung; Oberflächenfehler; Riß; Rißprüfung; Wirbelstrom

The multifrequency eddy current technique with frequency components greater then 20kHz is the established method for the defect inspection of the nearsurface zone of austenitic cladded components. A special mixing algorithm suppresses the disturbing signals caused by the lift-off above the uneven and rough cladding surface (welds beads) as well as the gradients of the ferrite content and carries out the detection and the sizing of surface breaking cracks. Defects below the surface (covered defects) can only be detected if their upper extension is lying not deeper than 2 mm below the surface. The determination of the remaining layer thickness (ligament) above the defect tip is not possible. The interface zone of the adjacent ferritic base material is inspected by ultrasound. Nevertheless the acoustic properties of the austenitic caldding complicate the detectability of the smaller defects; the discrimination between voluminous and crack-like defects is only possible under restrictions. A deverse inspection technique to prove the integrity of the cladding has to close this gap, i.e. to characterize the defect indications according to the defect type and size. The critical crack-like defects growing from the interface into the cladding or into the base material (sub-cladding cracks) have to be distinguished from voluminious defects due to manufactering process. An approach to tackle this task is below frequency eddy current (LFEC) method.