depth of detection for MT for cracks

By RANDER

Date 05-16-2008 14:54

It is taught that MT will only detect subsurface discontinuities up to 1/4" depth max.

By hogan

Date 05-16-2008 15:50

try
http://www.ndt-ed.org/EducationResources/CommunityCollege/MagParticle/cc_mpi_index.htm

or ASNT E709, ASME V

By jmdugan10 (*)

Date 05-16-2008 16:28

It also depends on the MT meathod.

Are you using a yoke, prods, or a bench?
Are you using AC, DC, Halfwave rectified?

On a bench I've gotten sensitivity on a Ketos ring @ 1/2". I'll also say it was a really fuzzy indication, but it was there.

I'll not dispute 1/4" but I recall the book I learned from said somthing like 0.100" max.

UT would be a better detection method on thick materials if the configuration allows it.

By CHGuilford (****)

Date 05-16-2008 16:31

What I have been taught is not to count on MT to find subsurface discontinuities. It might indicate some subsurface conditions but it might not, depending on numerous variables and circumstances.

The link cited in hogan's response is an excellent resource for information.

By jmdugan10 (*)

Date 05-16-2008 16:37 Edited 05-16-2008 16:58

Good point. No one NDT method is a magic bullet.

By NDTIII

Date 05-17-2008 13:43

If you're working to ASME it is a surface examinaton. Of course DC current has a better depth ofpenetration but it is not recognized by ASME.

By CWI555

Date 05-19-2008 16:17

NDT III has stated correctly that ASME doesn't consider MT to be any form of volumetric. If you have concerns of Subsurface, it would be better to us UT or RT, preferably UT when it comes to planars. (cracks)

By Mikeqc1 (****)

Date 05-19-2008 16:32

Isn't There Examples of near Surface indications in SEC V?

By CWI555

Date 05-19-2008 17:06

T-720 GENERAL
The magnetic particle examination method may be
applied to detect cracks and other discontinuities on or near
the surfaces of ferromagnetic materials. The sensitivity is
greatest for surface discontinuities and diminishes rapidly
with increasing depth of subsurface discontinuities below
the surface. Typical types of discontinuities that can be
detected by this method are cracks, laps, seams, cold shuts,
and laminations.

SE-709
1. Scope
1.1 This guide describes techniques for both dry and
wet magnetic particle examination, a nondestructive
method for detecting cracks and other discontinuities at
or near the surface in ferromagnetic materials.

These are the only direct mention of "near surface" that I can find in Section V or it's referenced docs as it relates to MT. (2007)
In short, they are still only refering to surface. "near surface" is poorly defined, but getting below .1" and calling it "near surface" is asking for trouble.

Regards,
Gerald

By dmilesdot (**)

Date 05-21-2008 14:09

In my experience the surface condition has a lot to do with being able to detect an indication with Mt that is "near surface". For instance if you are doing mag particle on an as welded situation, I seriously doubt that most level II's would recognize a subsurface indication. The same goes for rusty or pitted surfaces. Machined or reasonably smooth surfaces would be about the only way you would detect a subsurface indication and then only with DC current. JMHO.
Dave

By CWI555

Date 05-21-2008 15:59

I would agree with that assesment.