Pulsed eddy-current nondestructive testing of ferromagnetic tubes

In sinusoidal eddy-current (EC) nondestructive testing (NDT) of thick ferromagnetic tubes, such as oil-well casings, average wall thickness is measured with an exciter coil and a detector coil displaced by more than two tube diameters [remote field eddy current (RFEC) technique]. Since RFEC cannot differentiate outer from inner defects, the average tube inner diameter is determined at higher frequency with another pair of coils displaced by around one tube diameter (electrical caliper). It is expected that the key drawbacks of such EC tools (several coils, complex transducer electronics, relatively high power consumption, and length) can be overcome with the application of pulse excitation. In this paper, we give a thorough analysis of the effect of the excitation frequency and the distance between the coils on the detected signal sensitivity to the tube properties, as a basis for usage of pulsed eddy current (PEC). We show that pulse excitation, thanks to its continuous frequency spectrum and time separation of direct and remote zone signals, allows simultaneous measurement of average wall thickness and inner diameter with one detector coil separated from the exciter by one to two tube diameters. The results of our experimental work confirm that PEC technique provides a significant improvement on present EC tools for NDT of ferromagnetic tubes.