Abstract

INTRODUCTION: One of the main limitations of the traditional eddy-current method is the skin effect, i.e. the fact that the induced eddy currents concentrate into a more or less thin surface layer of the metallic object to be tested. This means that the inspections are confined to a certain material thickness characterized by the so-called skin depth. Since the skin depth relates reciprocally to the magnetic permeability (precisely: to the square root of the permeability), testing the whole volume of thick ferrous objects is pratically impossible.

The great step to overcome the aforesaid problem was done more than 45 years ago by constructing a non-conventional eddy-current probe (referred to as remote field probe) for inner tube inspection and using the logarithm of the signal amplitude as the measure of tube wall thickness. The new concept was later successfully introduced for corrosion testing of oilwell casing pipes. The outer corrosion on these thick-wall steel pipes was well detectable with remote field probes and it revealed that signal phases correlated linearly with wall thickness, Schmidt (1). Due to intensive development works the RFEC technique has become a both theoretically and experimentally well-based method for inside testing of tubes (of small-bore tubes, too) in the last two decades, Mackintosh et a] (2). Moreover, this technique will also applicated for corrosion testing of ferromagnetic sheets (e.g. tank floors) nowadays.

RFEC is a low-frequency through-transmission technique offering the possibility that the whole wall cross-section can be inspected with almost equal sensitivity even in case of relatively thick-wall ferromagnetic tubes or sheets. The signal presentation and interpretation stand already at a high level just like for conventional eddy-current testing, Atherton et al (3).

Contrary to the inspections mentioned above, there are situations where and when tubes are not accessible or can be accessed only in a very complicated and too expensive manner from inside. Serious maintenance and operation problems may arise if corrosion craters on the inner surface of such tubes will not found in time with any appropriate testing procedure.

The present work has been aimed to solve the outside through-wall eddy-current testing of ferromagnetic tubes using the remote field technique.

Summary: A new RFEC-type probe was built for outside detection of inner wall thinnings in relatively thick-wall ferromagnetic boiler tubes. Laboratory tests proved that the probe detected both local craters and extended thinnings with a quite good sensitivity. Since the sensitivity does not decrease appreciably if the lift-off increases moderately, thin non-magnetic scale on the outer tube surface does not affect the inspection provided it does not hinder the probe movement along the tube.

Further developments have been planned in order to separate the flaw effects from the disturbing magnetic effects by phase discrimination te

Keywords: Eddy Current Testing

Source: Conference Proceedings of the 4th International Conference of the Slovenian Society for Nondestructive Testing, 24 - 25 April 1997, Ljubljana, Slovenia.
Pages:205 - 210

Publisher:
Slovenian Society for Nondestructive Testing, Faculty of Mechanical Engineering, Askerceva 6, SLO 1000 Ljubljana, Slovenia. Tel +386 61 177 12 03, Fax +386 61 218 567, Email: janez.grum@fs.uni-lj.si.