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07:24 Jul-14-2003
DJ Kallhof
Eddy Current response changes

Can the response to a defect change as much a four times the original response simply by polishing the affected area and adjoining material. The base metal is 250 Maraging Steel. The area was polished to a mirror finish by working up to a 1 Micron diamond grit polishing operation to facilitate replicating for grain size. The original response on the flying dot display was app. 5-6 divisions diagonally. After the polishing the response was recorded at 24 divisions straight up.


 
08:21 Jul-15-2003

Godfrey Hands

Engineering,
PRI Nadcap,
United Kingdom,
Joined Nov 1998
281
Re: Eddy Current response changes : Can the response to a defect change as much a four times the original response simply by polishing the affected area and adjoining material. The base metal is 250 Maraging Steel. The area was polished to a mirror finish by working up to a 1 Micron diamond grit polishing operation to facilitate replicating for grain size. The original response on the flying dot display was app. 5-6 divisions diagonally. After the polishing the response was recorded at 24 divisions straight up.
.
Hi,
This sounds to be an extreme change in response.
What frequency were you using ? Very High I would guess to get such a change.
I wonder if there was a very thin layer of something over the original surface that was removed during the polishing.
What was the original surface finish like ? If it was not very smooth, perhaps you now also have a reduced lift-off due to the better surface finish.
Was there any possibility of diamond paste or other (possibly conductive) contaminants on the surface after you had polished the part ?
I think it is a significant change, but perhaps not unexplicable.
I assume that the spot response was at the centre of the screen when the probe was off the component, and that the total spot response was from centre to 5 to 6 divisions diagonnaly and after polishing was from centre to 24 divisions vertically. If the spot was not originally at the centre, then we don't know how far the spot had moved to get to your location 5 to 6 diagonally. This depends on the gain and other factors.

Regards,

Godfrey Hands


 


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