Electromagnetic Testing - Eddy Current Testing Applications Chapter 5 & 6

Although these blocks can be used for setting up to test parts of any shape,
for testing in threads, grooves, or slots, and for testing for cracks in holes,
where the eddy current field differs from that for a relatively flat surface, it is
strongly recommended that a reference block of the material and
configuration being tested, with one or more known real or artificial surface
flaws (usually a spark-eroded slot) at the location at which flaws are expected
be used. This approach is also often used with other test parts, for example
the bead seat radius of aircraft wheels, because it gives a higher degree of
assurance that the flaws required to be detected can be found.
Charlie Chong/ Fion Zhang