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Technical Discussions
Liu.
Engineering,
China, Joined Jul 2012, 34

Liu.

Engineering,
China,
Joined Jul 2012
34
18:17 Jul-19-2015
relationship of lifting force,pie field indicator and notched shims

Dear all,
i would like if someone can clarify a confusion for me of the relationship of lifting force,pie field indicatior and notched shims

we are use Yoke AC method, we confirm the lifting force with the 4.5Kg block, that means we meet the magenatic field strength when we can lift up the block, why we need the pie field indicator to confirm the magenatic strength on site ? especially we use the Yoke AC method, the magenatic field orientation is consistent along two legs,
thats means the pie field indicatior also is useless to the field orientation

ok, i know maybe we need pie indicator or notched shim to establish the system field strength , so when we performing MPI on site, we only need pie indicator or notched shim , or we need take both of them???

my client said , both of they are required, but i am got confusion!

Much thanks to anyone can post your professional views to my e-mail or on board

 
 Reply 
 
Lalu Nair
Engineering, Sr. QA/QC Engineer
IIT NDT, United Arab Emirates (UAE), Joined Mar 2013, 4

Lalu Nair

Engineering, Sr. QA/QC Engineer
IIT NDT,
United Arab Emirates (UAE),
Joined Mar 2013
4
12:10 Jul-20-2015
Re: relationship of lifting force,pie field indicator and notched shims
In Reply to Liu. at 18:17 Jul-19-2015 (Opening).

Yes you need both. Pie gauge can only be used for field direction, ASTM E1444-11 Para. 6.2.5 states "The pie gauge or flexible laminated strips shall not be used to determine adequate field strength". Para. 6.3 directs you to the notched shims or QQI's as well as a few other options.

 
 Reply 
 
Bruce McPherson
Consultant
MFL Services, United Kingdom, Joined Mar 2009, 26

Bruce McPherson

Consultant
MFL Services,
United Kingdom,
Joined Mar 2009
26
12:12 Jul-20-2015
Re: relationship of lifting force,pie field indicator and notched shims
In Reply to Liu. at 18:17 Jul-19-2015 (Opening).

Liu,
You are not the only person to be confused by this situation. Your client is also confused!
Pie gauges and notched shims are only useful for indicating the strength and direction of magnetic fields in steel which is:
a. Previously magnetised and the pie gauge or shim is being used to show the strength and direction of any residual magnetism which may be present
b. Being actively magnetised using a direct induction method such as prods or a head shot.

Pie gauges and slotted shims provide no useful information about the levels of magnetism in an object being actively magnetised using indirect induction methods such as a yoke or a coil because not all of the magnetism produced by the coil or yoke goes into the material being magnetised. Some of the magnetism produced by the yoke or the coil is present in the air around the object being magnetised and this magnetic field can be sufficient to produce indications on the pie gauge or slotted shim.
To prove this point, try the following
Place the AC yoke on a ferrous steel plate and energize the yoke. Place a slotted shim or pie gauge on the surface of the steel plate between the poles of the yoke and apply magnetic particles.
Clear indications on the artificial flaws on the shim or pie gauge which are at 90 degrees to the direction of the magnetic field should be visible.
Repeat the process, but this time, instead of a ferrous steel plate, use a block of wood.
Obviously the wood cannot be magnetised, but the indications on the shim will still be visible because of the magnetism in the air between the poles of the yoke.
Since the shim or pie gauge produces indications when placed on a magnetic material and a non magnetic material, it is clear that, in this particular situation, the shim (or pie gauge) is not a suitable method for indicating the strength or direction of the magnetic field.

Hope that helps

Bruce McPherson

 
 Reply 
 
Liu
Engineering,
China, Joined Jul 2012, 34

Liu

Engineering,
China,
Joined Jul 2012
34
16:59 Jul-20-2015
Re: relationship of lifting force,pie field indicator and notched shims
In Reply to Bruce McPherson at 12:12 Jul-20-2015 .

Dear Sir,

thanks for you professional reply, sorry i am not very clear of your ideas

if i am got u, when AC yoke were utilized in steel fabricaiton welds , using of Pie field indicator or notched shim is a absolutely a mistake?

what we are going to do in order to confirming the strength and direction of magnetic fields when AC yoke were used? gaussmeter????

 
 Reply 
 
Diego
Consultant,
Freelance, Spain, Joined May 2013, 188

Diego

Consultant,
Freelance,
Spain,
Joined May 2013
188
19:30 Jul-20-2015
Re: relationship of lifting force,pie field indicator and notched shims
In Reply to Liu at 16:59 Jul-20-2015 .

Mr. Liu.
The field strength of a yoke can be empirically determined by measuring
its lifting power. It is sufficient.

 
 Reply 
 
Gerald Reams
Gerald Reams
20:22 Jul-20-2015
Re: relationship of lifting force,pie field indicator and notched shims
In Reply to Diego at 19:30 Jul-20-2015 .

I agree with Diego, "The field strength of a yoke can be empirically determined by measuring its lifting power. It is sufficient."

Also, an AC Yoke produces a longitudinal field and is located between the 2 legs of the yoke.

 
 Reply 
 
Bruce McPherson
Consultant
MFL Services, United Kingdom, Joined Mar 2009, 26

Bruce McPherson

Consultant
MFL Services,
United Kingdom,
Joined Mar 2009
26
21:24 Jul-20-2015
Re: relationship of lifting force,pie field indicator and notched shims
In Reply to Liu at 16:59 Jul-20-2015 .

Liu,
You are correct, when using an AC yoke, using a pie gauge or slotted shim to determine field strength and direction is absolutely a mistake.
I agree with Diego and Gerald Reams that the ability of the yoke to lift the 4.5Kg test weight is only thing you need to demonstrate to your client that the yoke is sufficiently powerful to perform a satisfactory inspection

Lalu Nair,
The relevant parts of ASTM E-1444 relating to the use of electromagnetic yokes and this discussion are paras 6.2.3 and 7.1.5.4
Para 6.2.3 " Adequate field strength shall be established in accordance with 7.1.5.4"
Para 7.1.5.4 "Dead Weight Check - AC yokes shall be capable of lifting 10lb with the legs at 2-4 inch spacing"

For the reason given in my response to Liu's initial enquiry, there is no requirement to use a pie gauge or slotted shim to check field strength and direction when using an electromagnetic yoke.

Regards

Bruce McPherson

1
 
 Reply 
 
Liu
Engineering,
China, Joined Jul 2012, 34

Liu

Engineering,
China,
Joined Jul 2012
34
05:30 Jul-21-2015
Re: relationship of lifting force,pie field indicator and notched shims
In Reply to Bruce McPherson at 21:24 Jul-20-2015 .

Dear Bruce,

following up your statement, how to determin the AC yoke system sensitivity, only using lifting force demonstration?

i mean we need considerate the particle suspension concentration, we can control by settling test

but when they combined, they are a system, notched shim is desiged to confirming the system sensiticity

what about AC yoke method? no need ??

 
 Reply 
 
Bruce McPherson
Consultant
MFL Services, United Kingdom, Joined Mar 2009, 26

Bruce McPherson

Consultant
MFL Services,
United Kingdom,
Joined Mar 2009
26
11:05 Jul-21-2015
Re: relationship of lifting force,pie field indicator and notched shims
In Reply to Liu at 05:30 Jul-21-2015 .

Liu,
My previous replies have only discussed magnetic field strength and not overall system sensitivity.
The overall sensitivity of MPI using an AC yoke depends on several other factors, only one of which is particle concentration.
Other factors which affect the overall sensitivity include, but are not limited to:
Visible or fluorescent particles
Surface finish of the component being tested
Light intensity at the surface being inspected
Visual acuity of the person performing the examination.

Imagine an inspection performed on a weld with an AC yoke which can lift the 4.5Kg test weight, and with a magnetic particle suspension which has the correct concentration.
If the surface of the weld is not clean, the light level is too low, or the person performing the test has poor eyesight, then the system would not be sufficiently sensitive to detect potential defects in the weld.
I respectfully suggest that you familiarise yourself with ASTM E-709 and ASTM E-1444 since all of the factors affecting the performance of an MPI are discussed in these documents

Best regards

Bruce

 
 Reply 
 
Jared
Circle Systems, Inc., USA, Joined Mar 2014, 5

Jared

Circle Systems, Inc.,
USA,
Joined Mar 2014
5
15:35 Jul-21-2015
Re: relationship of lifting force,pie field indicator and notched shims
In Reply to Bruce McPherson at 11:05 Jul-21-2015 .

Liu,

I believe that to confirm sensitivity/reliability of your chosen method rather than using flawed shims, use a component that has previously been inspected with a known discontinuity or discontinuities(ASTM E1444 7.1.1). As Mr. McPherson has already stated there are other factors that must be controlled and measured, with emphasis on the particle concentration of the magnetic particle solution.

Hoping this helps.
Jared

 
 Reply 
 

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