The three standard depths crack has the largest amplitude response. Since its interrupts the eddy currents as far down
in the metal of the test can sense, it looks like a change in the bulk property of area of lower conductivity, and the crack
signal direction (phase) is along the conductivity curve (see Figure 4-27).
Figure 4-27. Three Standard Depths of Penetration.
It is obvious that making the 3 standard depths crack deeper will not change the response signal, since there is no more
eddy current flow for it to interrupt. However, there will be a change in its signal if we submerge it below the surface.
First, eddy currents will flow over the top of the crack (at the surface), it will therefore not be blocking as much of the
EC flow, and the amplitude of the signal must decrease. Second, the crack is now farther away from the surface so its
phase angle must be still farther away from lift-off (see Figure 4-28).
Figure 4-28. Subsurface Crack