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05:12 Jun-29-2006

John Brunk

Engineering, NDT Level III
Self employed, part-time,
USA,
Joined Oct 1999
158
Re: MT Depth of penetration ----------- Start Original Message -----------
: : Sir,
: : In magnetic particle testing using Yoke Method (Dry), up to what depth we can detect the discontinuities?
: : Which code/standard specifies the Depth of penetration?
: ------------ End Original Message ----------
: Yes, ASME Section V states this, however, it also states AC Yokes only. If you were to use a DC yoke you will get some penetration. I believe it is about 6mm. You can drill holes at different depths in a c/s block and try to detect them/
------------ End Original Message ------------

The depth at which a discontnuity could be detected depends upon its size and shape, the magnetic permeability of the material, the ability to make good contact with both poles of the yoke, and to a great extent on the particular yoke that is used. Two of the samples I have used to compare yokes are (1) a plate with a circular saw cust on the back side - so that a longer indication shows detection at a greater depth; (2) a pipe elbow that had been in an acid line and has rows of axial furrows in the ID surface. The relative ability to detect subsurface discontinuities is shown by the length and number of parallel indications. I have one AC-only yoke that does better than several DC yokes, but not as well as one model AC-DC (used in DC) that I perhaps should not identify by name in this forum. I can state that one comparison was made because people did not want to use this very heavy AC-DC yoke until after it was shown to be much better for subsurface defects than some lighter weight yokes. It would be a mistake to promise to be able to find discontinuities at any particular depth. I think 6 mm may be possible under ideal conditions but it is better to think in terms of about 2 to 3 mm. I have found lack of fusion at a weld edge that was hidden by the cover pass, at a depth of 2 mm, with a very good AC yoke. I have also been able to see indications to about 1 mm deep in the pipe elbow sample with an AC coil. In most situations it isvirtually impossible to create a test specimen that properly represents the most difficult defect to be detected and the actual field conditions for yoke contact.



 


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