amplitude is seen as it gets still deeper. Besides an amplitude increase for deeper cracks, the phase angle of the crack
indication changes. A shallow crack interrupts little of the eddy current flow, so the amplitude of its signal is small.
Also, it is essentially a surface condition, so the direction (phase) of the signal response is very close to that of lift-off
(see Figure 4-25).
Figure 4-25. Shallow Surface Crack
A deeper crack interrupts more of the eddy current flow, so its signal has greater amplitude. Since it extends well
below the surface, the direction (phase) of its signal is farther away from lift-off (see Figure 4-26.)
Figure 4-26. Deeper Surface Crack