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Technical Discussions
NDT Inspector
Norway, Joined Aug 2009, 204


NDT Inspector
Joined Aug 2009
00:10 Nov-05-2009
Eddy Current V's DPI/MPI

Would Eddy Current find everything DPI/MPI could?

Godfrey Hands
PRI Nadcap, United Kingdom, Joined Nov 1998, 295

Godfrey Hands

PRI Nadcap,
United Kingdom,
Joined Nov 1998
08:30 Nov-05-2009
Re: Eddy Current V's DPI/MPI
In Reply to anjafo at 00:10 Nov-05-2009 (Opening).

Hi Anjafo,
In theory, Eddy Current should find everything that DPI or MPI will, but there are certain limitations.

MPI is field direction sensitive, so you need to be certain that you have covered the component or area with crossing magnetic fields.
The same is applicable to Eddy Current. This will not easily detect a crack when the probe runs along the length of the crack, so once again, you need to double inspect at right angles to the first inspection to be certain of detecting the cracks.

Also when it comes to borderline defect dimensions, there are some limitations.
In a laboratory environment with experimental Eddy Current equipment, we have been sucessful in reliably detecting cracks of 0.020mm deep, whilst with a very sensitive penetrant or magnetic test, detection of smaller defects could be possible. Here however surface finish is critical for all three methods.

Eddy current can also detect defects under a thin non-conductive coating (like MT, but unlike PT).



Jim Draper
Consultant, ASNT Level III MT PT RT VT
Draper NDT, USA, Joined Jul 2009, 99

Jim Draper

Consultant, ASNT Level III MT PT RT VT
Draper NDT,
Joined Jul 2009
20:32 Nov-05-2009
Re: Eddy Current V's DPI/MPI
In Reply to Godfrey Hands at 08:30 Nov-05-2009 .

DPI will is not limited to electricaly conductive material. Also, I have used direct current fluorescent MPI to locate flaws 1/4" deep. Of course that is limited to ferromagnetic material, while eddy current can be used to test other electricaly conductive materials.


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