Self-Nulling Eddy Current Probe for Surface and Subsurface Flaw Detection

An eddy current probe which provides a null-signal in the presence of unflawed material without the need for any balancing circuitry has been developed at NASA Langley Research Center [1]. Such a unique capability of the probe reduces set-up time, eliminates tester configuration errors, and decreases instrumentation requirements. The probe is highly sensitive to surface breaking fatigue cracks, and shows excellent resolution for the measurement of material thickness, including material loss due to corrosion damage. The presence of flaws in the material under test causes an increase in the extremely stable and reproducible output voltage of the probe. The design of the probe and some examples illustrating its flaw detection capabilities are presented. Probe Design The self-nulling eddy current probe is a driver-pickup type eddy current probe which has a ferromagnetic shield inserted between the drive and sensor coils. The ferromagnetic material focuses the flux of the probe into a tight cylinder about the outer diameter of the pickup coil. In addition, the direct mutual inductance between the drive and pickup coils is eliminated due to the skin depth attenuation of the fields as they penetrate inward from the drive to pickup coils [2]. Therefore, in the presence of an unflawed sample, there is no flux linkage through the pickup coil