Electromagnetic Testing - Eddy Current Testing Applications Chapter 5 & 6

5.1.4.2 Probe and frequency selection
The essential requirements for the detection of subsurface flaws are,
sufficient penetration for sensitivity to the subsurface flaws sought, and
sufficient phase separation of the signals for the location or depth of the flaws
to be identified. As standard depth of penetration increases, the phase
difference between discontinuities of different depth decreases. Therefore,
making interpretation of location or depth of the flaws difficult. Example: If the
frequency is set to obtain a standard depth of penetration of 2 mm, the
separation between discontinuities at 1 mm and 2 mm would be 57°. If the
frequency is set to obtain a standard depth of penetration of 4 mm, the
separation between discontinuities at 1 mm and 2 mm would be 28.5°.
Keywords:
As standard depth of penetration increases, the phase difference between
discontinuities of different depth decreases.
Charlie Chong/ Fion Zhang