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DPCNews 034 - Magnetization by swinging field: the limits

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Written by Administrator
Tuesday, 01 March 2011 21:48

Magnetic particle testing: some limits to magnetization by swinging field

March 2011

Every technique has its own limits, and magnetization by a swinging field is no exception to this rule.

This article explains some of the limits of this technique and therefore is an add-on to our DPCNewsletter N°033(1).

The fact is: non-contact magnetization by swinging field in a chamber decreases sensitivity; further, this magnetization technique shows also some additional limits due to the shape of the magnetic circuits, which are open (the field lines form closed loops in air). In particular, the background level on the surface is very high, which impairs detection of tiny discontinuities. Therefore, this technique may be used only on parts with specific shapes and is suitable only for the detection of large defects on castings.
Another limit comes from the parts’ dimensions. After all, parts have to be put in the magnetization chamber!

This technique is not suitable for the detection of defects such as forging laps.

As evident in our DPCNewsletter N° 033(1), we did not give examples of industrial uses of this technique in aerospace industries. Quite easy to explain.

Indeed, this technique is not allowed for use in the aerospace industry for several reasons:

• It relies upon AC magnetic fields only, which are not approved in the aerospace industry, except for maintenance. Much to say about this point when dealing with detection of defects on gear teeth, as AC would be far better. However, it is true that this does not allow for the detection of sub-surface defects.

• Aerospace standards and specifications unequivocally require performing in separate steps transverse magnetization and longitudinal magnetization; we think the swinging field magnetization was never thought about when these documents have been written.

Quite recently, one of us had the opportunity to informally talk about it with a Member of the Technical Department of a major aerospace equipment manufacturer.

He agreed that this technique is not used in aerospace because it is not specifically approved.

Here are some main reasons:
• Problems on complex shapes (boss, rib): it is difficult to imagine the magnetic field direction in the area of interest relative to the would-be discontinuities direction.
• Insufficient time for fixing the magnetic particles on some defects.

The tests he performed with this technique made him unable to duplicate the detection of defects, depending on their direction. Therefore, repeatability was not assured.

Indeed, if the setting of a machine is a specialist’s job, a complex shape is likely to lead to more difficult adjustments when using swinging field magnetization than standard magnetization, especially because users do not know how to proceed. It is impossible to state the feasibility of an inspection using the swinging field method without being supplied with some pieces of information about the part. We assume, in fact, that a test is needed. As for the time for particles to settle, magnetization time is adjustable.

It is sometimes easy to understand why results are disappointing; for instance in complex shaped parts, the magnetic field, be it swinging or not, does not always go as we would like. It may be parallel to the would-be defects, while, according to the rules, it should be perpendicular. However, the magnetic lines go the easiest way. Indeed, one must take into account the “shape factor” (it is difficult to have magnetic lines go along short areas), the phase difference and the difference of power between the longitudinal magnetization and the transverse magnetization. Experience only will allow for the right setting of the equipment.  Using an Analysse-type(2) magnetic field strength meter to check the magnetic field is quite the same when the probe is turned on 360° is a good idea; but flexible indicators such as the Burmah Castrol®(2) flux strips are also very good indicators to check the magnetic field DIRECTION where defects are looked for. Easy to put 2 of them perpendicular to each other on the area to be inspected and check that the sensitivity is the same on the two indicators. We recommend using always such flux indicators both with the tangential magnetic field strength meter to fully validate the balance!

There may be a problem of balance between the two swinging field vectors. One of us remembers when George DOWNES, a renowned United Kingdom MT specialist, who died in January 2011, using a part of an helicopter rotor, explained that checking the balance of both magnetizations took time and was crucial to ensure the same detection capability, whatever the direction of the discontinuity. This is why on a swinging field machine, we think it very important to have a PLC or a computer which allows for the recording of settings for each part. In fact, this is more and more a standard fitting, especially in the car industry (1 part equals 1 recipe). It is true it is more time-consuming and a bit more complex. However, if the machine  is well designed and installed by the manufacturer (with the right plugging to the phases), the only thing to do is to independently adjust the transverse and the longitudinal magnetizations, to launch a swinging field cycle while  checking the  field is swinging with  a tangential magnetic field strength meter or flux indicators. If the equipment is moved, it is of the utmost importance to check that the equipment has been plugged again to two phases on the mains. A wrong connection, very common indeed, prevents the magnetic field to swing.

By the way, the “Oldies but Goodies” mini-story published this month on our Website is all about the troubles a cogwheels’ (pinions, transmissions) manufacturer faced.

By experience, a 2-second magnetization is generally enough to ‘‘fix’’ the magnetic particles on the part surface and to prevent the carrier liquid "washing-off" the indication.

More and more often, one wants to go too fast.

Nevertheless, parts such as Y-shaped legs of landing gear may be difficult to check with the swinging field technique due to its shape.

Furthermore, non-contact magnetization in chamber leads to limits that are even more stringent.

In a non-contact magnetization chamber, the magnetic flux flows as an open circuit (magnetic lines loop in the air), with some consequences: 

• To properly magnetize the part, the TRMS value of the tangential magnetic field shall be between 3 and 8 kA/m. In some cases, it may be difficult to get such magnetic field values due to a demagnetizing field, which comes in complete opposition to the magnetic lines penetration, especially along the small surfaces of the part and/or inside the short bores.

• The swinging nature of the field produced in a non-contact magnetization chamber allows for the ensuring that at a given moment the field is perpendicular to a would-be defect, thus easing its detection. However, in the same way one can be sure that at another time, this field will be perpendicular to the surface of the part (flow going out) and will give raise to a magnetic pole blocking the movement of the magnetic particles and lowering the detection contrast (hence, a high background).

These limits make this technique unsuitable to detect defects on forgings where defects are often very tight (laps).

Despite all its benefits, the non-contact magnetization by swinging field in chamber shall not be thought of as a universal inspection technique able to solve all the problems.

References

(1) Pierre CHEMIN and Patrick DUBOSC, DPCNewsletter N°033, Magnetization using the swinging field technique: it works! February 2011, on our Website.

(2) Pierre CHEMIN and Patrick DUBOSC, Magnetic particle testing history, June 2009, on our Website.

(3) Pierre CHEMIN and Patrick DUBOSC, Oldies but goodies, Why do not ask the inspector for help? March 2011, on our Website.


We, Pierre CHEMIN and Patrick DUBOSC, welcome any comment, any idea. If you have some examples you would like to see discussed here, please give us all the useful indications. If you require confidentially, we would modify locations, names and some parameters to prevent any traceability.
Nevertheless, we are convinced that our site may be a kind of surge-valve: the topic is NOT to target this company, or that auditor; but it is always to make users think, to make them ask themselves, or others, the right questions.
We may also give advice, once again on a confidential basis if needed: please, feel free to ask questions, to document our data basis: about Material Safety Data Sheets (MSDS), about environment, a chemical name you don't understand, a Penetrant process you have heard about, etc.
We have plenty of examples, some being out of all the specifications/standards, which led to the discontinuities detection, when the "current, normal, processes" prevented discontinuity finding.

Last Updated ( Tuesday, 07 June 2011 20:37 )