Non destructive testing (DC or AC)

[321GO]

Hi Guys, hope you're all well

So, in ND testing of cracks in metallic materials most seems to be with (high freq.) AC current powered primary coils and eddy current induced secondary fields (which are probed one way or another).

But why is this done with AC since DC is also suitable for primary field as far as I know.

Is the AC method more accurate or does it have some practical reason?

 

[ScottyUK]

You answered your own question without knowing it: the AC coils induce currents in the object under test as you said. DC won't induce anything because induction requires a change in the field. DC might magnetise the object if it is ferrous, either temporarily or permanently, but it won't induce any currents in the object.