Potential drop flaw detection and sizing

What is NDT? 

A special electrical resistance testing method applies an electrical potential between two prods attached to a specimen surface and then measures the difference in resistance across a second pair of prods placed successively across a crack and across adjacent sound material. The increase in resistance due to the crack is a direct measure of the through-thickness dimension of the crack from the surface (the crack height).

The method is used mostly for sizing cracks which have been found by other techniques. It can be applied to any electrically conductive material. The instruments required can be small and portable. The applied electrical potential may be AC or DC.

Accuracy of results depends on the crack not containing any conductive bridging material, which would shorten the electrical path length.