Eddy current testing can find small flaws that are barely discernable to the human eye. The sensitivity of the eddy current system (probe and instrument) is dependent on the proximity of the probe to the part under test. If you are too close, you will damage the probe, it you are too far away, you lose signal. Angular misalignment has a similar effect.
The photos below show tests done with the Criterion NDT CR-11. An eddy current pencil probe was held at 2 distances (0.010 inches and 0.040 inches) from a spinning test standard. The screen shots of the CR-11 shows the difference.
Finding and Recommendations
The longitudinal test notch was easily identified when the probe was located 0.010 inches from the part. The signal clearly exceeded the red alarm threshold. When the probe was moved to 0.040 inches from the part, the signal was greatly reduced.
For best eddy current detection, we highly recommend using mechanical fixturing that keeps the eddy current probe at a consistent distance close to the part under test. Photos on the right show probes and fixturing that help accomplish that.