MT sensitivity

By Joseph P. Kane (****)

Date 05-14-2011 13:04

wet

By Mikeqc1 (****)

Date 05-16-2011 14:05

U SURE ITS NOT DRY?

By jwright650 (*****)

Date 05-16-2011 14:15 Edited 05-16-2011 14:18

Mike,

wet *fluorescent is the most sensitive MT

*edited the spelling for clarification

By jwright650 (*****)

Date 05-16-2011 14:24

http://www.ndt-ed.org/EducationResources/CommunityCollege/MagParticle/Equipment/Particles.htm

By Mikeqc1 (****)

Date 05-16-2011 15:36

i have a book that really is confusing me with this.

By Mikeqc1 (****)

Date 05-16-2011 14:37 Edited 05-16-2011 15:44

per book.......Mt methods from least sensitive to most sensitive.
Least sensitive is AC wet, then AC Dry, DC Wet, DC dry, ( most sensitive) HWDC Dry.
Says that dry particles are attracted to weaker fields therefore its more sensitive.

By CWI555

Date 05-16-2011 15:37

fluorescent wet. Where are you reading dry particles are more sensitive??

By Mikeqc1 (****)

Date 05-16-2011 16:01

Right here in front of me NDT general dynamics.
im confused at why they are saying that, thats why im bringing it up.

By jwright650 (*****)

Date 05-16-2011 16:22

The wet particles are smaller and are more mobile due to the wet medium.....they have to be more sensitive than dry.

By Mikeqc1 (****)

Date 05-16-2011 16:37

but the wet particles are less permeable.... so they are less effective for subsurface dicontinuities, and on rough welds dry powder is the favoured method over wet.
Im going to go over this book good.........

By CWI555

Date 05-16-2011 18:36 Edited 05-16-2011 18:51

For subsurface defects????
MT is a surface method and is not reliable for subsurface defects.
A trip down memory lane may help with that: Edit: to understand why General dynamics says what it does.

ASME Section V 1974 Edition
T720
Magnetic particle examination provides for the
detection of cracks and other linear discontinuities
and shall be applicable only to ferrormagnetic materials.

Its sensitivity is greatest for surface discontinuities
and diminishes rapidly with depth below the surface.

In T-725 It mentions SE 109 and SE 138. 109 for dry, and 138 for wet.

Historically, any mention of dry was assumed to be prods unless thereafter modified.
Reading 138 shows that there was not a focus on the magnetizing technique for wet inspection.
Section 3 of E138 current of the 74 ASME 5 list all normal methods.

By contrast, E109 specifically calls out prods under section 6.
Under section 7 it states the following:
7.1 Over-all magnetization (as specified in Appendix A1) or the wet method of magnetic
particle inspection may be used if such methods are more practical for certain cases. If
such a method is used it shall be by mutual agreement of the manufacture and the purchaser.

By default, dry powder 40 years ago assumed prods. This has changed in later years of most codes due to:
1. Propensity of arc strikes from prods
2. The general unreliability for detection of subsurface discontinuities.

There are exceptions to that, but as a general rule of thumb, most people stay away from trying to
detect subsurface flaws with MT. That’s what the volumetric methods are for.

In saying that, and if you absolutely must for some reason I cannot fathom; then the dry prod method
is the most sensitive for subsurface. (reference table 1 SE 709 article 25 ASME V 2010 edition) EDIT:(109 and 138 rolled together)
However, your still going to be stuck with relatively small inspection areas, high risk of arc strikes,
and the need to be able to prove your getting down to where you say you are.

That last bit is a killer.

In all, 'surface' indications by florescent wet method is your most sensitive method for 'surface' indications.
The exception to that rule is Prod dry powder.

As for wet being less permeable, that can be argued as well. They are relatively the same particles with a coating.

By js55

Date 05-17-2011 21:51

Gerald,
Don't be such a stranger around here.

By CWI555

Date 05-18-2011 16:21

Jeff,

I am here most every day, but lately haven't had time to do much but a quick perusal.
My boy has been taking up more time lately, he's at the age of playing ball, and climbing trees. It's hard on an old fat guy trying to keep up.

By js55

Date 05-19-2011 11:27

Oh yeah. I know about old fat guys trying to keep up. :lol:

By qcrobert (***)

Date 05-16-2011 16:37

Here are other opinions for consideration:

According to West Penn NDT book, "Magnetic Particle Inspection (MT) - Wet & Dry
A non-destructive test method used to detect surface and near-surface discontinuities in ferromagnetic material. A magnetic field is induced in a test piece and iron filings are applied. These filings will collect around anomalies in the magnetic field producing a visible area of possible defects.

Dry - An inspection technique where ferromagnetic (iron based) materials are magnetized and dyed iron filings are broadcasted over the material under test, breaks in the surface (cracks, seams) act as magnetic poles attracting the iron filings outlying the defects for detection.

Wet - In the wet method the same magnetizing principal is utilized, but the particles are dyed with a fluorescent coating and suspended in a liquid. The liquid flows over the part and the particles are attracted as in the dry method and viewed under ultraviolet light, this results in a more sensitive test.

According to Charles Hayes, CHARLES HAYES, International Sales/Support Manager, The Lincoln Electric Co., Cleveland, Ohio. He holds NDT Level III certification from the American Society of Nondestructive Testing and is a member of the AWS D1D Subcommittee on Inspection.
"Magnetic powders may be applied dry or wet. The dry powder method is popular for inspecting heavy weldments, while the wet method is often used in inspecting aircraft components. Dry powder is dusted uniformly over the work with a spray gun, dusting bag or atomizer. The finely divided magnetic particles are coated to increase their mobility and are available in gray, black and red colors to improve visibility. In the wet method, very fine red or black particles are suspended in water or light petroleum distillate. This can be flowed or sprayed on, or the part may be dipped into the liquid. The wet method is more sensitive than the dry method, because it allows the use of finer particles that can detect exceedingly fine defects. Fluorescent powders may be used for further sensitivity and are especially useful for locating discontinuities in corners, keyways, splines and deep holes."

By Mikeqc1 (****)

Date 05-16-2011 19:05

well i guess a book from 1977 isnt the best to study with.

By qcrobert (***)

Date 05-16-2011 19:47

It also helps me to change the oil in the lamp and extend the wick.....

By jwright650 (*****)

Date 05-16-2011 19:54

LOL....@ the oil lamp comments.

By Mikeqc1 (****)

Date 05-17-2011 18:28

http://mech.vub.ac.be/teaching/info/Damage_testing_prevention_and_detection_in_aeronautics/PDF/magnetic.pdf

By maxilimiano (**)

Date 06-06-2011 00:09

Could we open this case again... :)
May be you make definition for "sensitivity"...that could be "our eye sensitivity"...or "tool sensitivity"....or something else..

My eyes more sensitiv if I use WET..

Best Regard

By Mikeqc1 (****)

Date 06-06-2011 16:01

It really depends on if you are looking for Surface or subsurface indications and the surface of the object being tested.
Dry particles primarily used with Moble or portable equipment during the inspectio of rough or unground welds and sand castings.
Wet method is not effective for subsurface indications or rough or unground welds.
there are so many things that play part in the decision on what method to use, and to say the wet method is the most sensitive all around in my opinion is not true.

And if you can see the florecent glow in the wet suspension it would do no good if the suspension is not performing its job by locating the discontinuity.
MDK