Lift-off invariance transformations for eddy current nondestructive evaluation signals

Eddy current (EC) testing methods are widely used in a variety of applications including the inspection of steam generator tubes in nuclear power plants, aircraft parts and airframes. A key factor that affects the EC signal is lift-off. In practice, it is difficult to keep track of the actual value of the lift-off, which is essential for accurate interpretation of the signal. Hence it is necessary to have a scheme to render the EC signal invariant to the effects of lift-off. This paper describes a new method for compensating EC signals for variations in lift-off. The signals are transformed to obtain a zero lift-off equivalent signal that can be subsequently used for defect characterization.