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DPCNews 033 - Swinging field technique: it works!

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Written by Administrator
Tuesday, 01 February 2011 20:49

Magnetic particle testing and the swinging field technique: it works!

February 2011

Introduction

Detecting defects in a part by MT, whatever their direction, requires running in a row two magnetizations in somewhat perpendicular directions. Quite often, this is achieved by a transverse magnetization through a direct or an indirect current flow and a longitudinal magnetization through a solenoid or a magnetic head. This NDT method needs two successive operations, which may lead to a significant waste of time.

Swinging field on magnetic benches

To overcome this drawback, the swinging field MT technique used on a magnetic bench was designed in 1972(1), as per the French patent(2) filed by "Fluxo", then a "SREM TECHNOLOGIES"’s subsidiary.
This patent states:
‘‘The patent details an MT process in which the main characteristic of the invention is that the part under inspection is processed simultaneously by a longitudinal magnetic field and a transverse magnetic field, these two magnetic fields being periodic and out-of-phase".

The inventors of this new magnetization technique were precursors for sure, but then, for different reasons, this invention was not understood, as it should have been. For one thing, the equipment as described was not very efficient and did not allow for the getting the anticipated results. Nevertheless, the main drawback came from the habit of using rectified current flow technique and DC magnetic heads i.e. powered with DC, while this new technique relied upon AC magnetic fields. Detection of non open-to-surface defects was then thought of as impossible!

It is only in the mid ‘90s, with "AC magnetic heads" i.e. powered with AC being put on the market, that the idea rebounded in France. Some European MT equipment manufacturers, mainly German ones, accustomed to using and mastering the "AC heads" technology, made SREM TECHNOLOGIES have second thoughts about its 1972 idea.

Easy to understand why this MT technique appealed to car manufacturers, as these were looking for equipments allowing for the increasing of production rates. However, there was some questions arisen and confusion with the “combined magnetization”.

Indeed, the “combined magnetization” wording is used when the part is sprayed with the detection medium while being magnetized by a transverse magnetization followed without a break by a longitudinal magnetization without any inspection between. Doing so, a risk exists that, during the longitudinal magnetization, the detection medium washes off any indication given off by the transverse magnetization, dramatically impairing the quality of the inspection. On the other hand, on a magnetic bench the MT swinging field technique comes from the using of simultaneous, but out-of-phase longitudinal field through magnetic heads and a transverse field produced by current flow, resulting in a swinging magnetization vector. The entire piece is then magnetized in all directions in a single operation. The inspection’s reliability is high along an acceptable background level.

In this technique, it is crucial that both magnetizations are performed through out-of-phase AC. Using two DC or rectified DC magnetizations, or two non out-of-phase AC magnetizations results in a magnetization vector in one direction and not in a swinging field. There is then a high odd not to detect cracks along a given direction.

These misunderstandings and a lack of explanations for a long time have made many people think that inspecting a part in one operation could only lead to a decrease of the quality of the inspection: a false assertion as far as the swinging filed technique is correctly enforced. Indeed, the only limit of this technique is that it does not allow for the detecting of non open-to-surface defects; nevertheless, if performed as per the rules, an MT inspection makes it easy to detect open-to-surface defects, while this is more dubious for under-the-surface defects. Detectability is highly related to the depth of the defect and to its size, making it unlikely to detect a defect even very close to the surface if it is very small. For the same reason, detecting a defect 2 mm deep on the Ketos ring does not mean that any defect the same depth will be detected.

From 1995 on, the French market has recorded an increasing use of AC magnetic heads and a market share growth of the magnetic benches with the swinging field technique.
Non-contact magnetization by swinging field in chambers
Since 1993, this magnetization technique is used in 2D or 3D chambers.

The main point of these chambers: the entire surface of the part is magnetized by a magnetic field whose direction is constantly changing without any contact with the magnetizing equipment. The chamber comes with two or three coils, each producing a magnetic field perpendicular to the other(s) and proportional to the current they receive. The resulting magnetic field is the sum-up of the vectors of each magnetic field. Coils are energized by out-of-phase AC: this makes the resulting vector rotate 50 times per second (for 50 Hz mains).This rotating vector allows for the magnetization of the overall surface and for the detecting of any defect open-to-the-surface in one operation.

A misunderstanding of this technique due to an ambiguity.
This magnetization technique is sometimes either ignored or misunderstood.

Some of them argue that it does not work while they give no any clue.

Others claim that the magnetic field cannot be applied in several directions at the same time. They are right, but this does not apply to the swinging field. As already written, the swinging field has only one direction at a given moment, but this direction rotates and changes very quickly (360° in 20 ms at 50 Hz).

This technique does not use two simultaneous magnetizations perpendicular to each other: this would be wrong.

Where does this ambiguity come from?

In the EN 1330-7:2005 standard, we find the definition of ‘‘Multidirectional magnetization’’:‘‘Use of two or more magnetization techniques simultaneously to produce a directionally varying field for the detection of discontinuities in all directions.’’

This definition may actually lead to some confusion due to the word ''simultaneously''.

As the ''Swinging field technique'' is not described in the EN 1330-7:2005 standard, we gave the following definition(3):

‘‘Technique in which the magnetization vector turns very quickly to allow for the detecting of all defects (regardless of their direction) in a single shot. This is generally achieved by using 2 phases of a 3-phase main power source. A phase gives the longitudinal magnetization while the second phase gives the transverse magnetization. Both magnetizations shall be balanced. Due to the delay between phases, the magnetization vector scans all the directions on 360°. Equipment may be either a magnetic bench with the current flow technique for the transverse magnetization and with the magnetic flow technique for the longitudinal magnetization, or a non-contact unit (multidirectional magnetization chamber or an inspection equipment by induced current).’’

Thus, this definition better reflects the physical phenomenon at stake; if the magnetization system is powered by a 50Hz mains, the magnetization vector rotates through 360°, 50 times per second, and 60 times per second with 60 Hz power supply like in the USA and Canada.

For any further information, we recommend you read the papers(4) (5) (6) (7) (8) mentioned underneath, in the ''References''paragraph.

The limits of use of this technique will be explained in a soon-to-be-published document.
Some examples of industrial uses
The swinging field magnetization technique has been used for many years in the automotive industry, for example, in order to ease the inspection process and decrease time for inspection while keeping the right level of quality.

However, many other industrial areas use this technique. Here some examples are listed:

Conclusion

One may conclude that this technique has a good track record and should no longer be questioned.

References

• Stéphane GRAVELEAU (SREM Technologies), "Magnetic particle inspection at the cutting edge", COFREND (French Confederation for Non-Destructive Testing) Conference held in Toulouse May 20 to 23. 2008.

• EN 1330-7:2005 Non-destructive testing Terminology Part 7: Terms used in magnetic particle testing, European Committee for Standardization, Brussels, Belgium, 2005.

(1) Pierre CHEMIN and Patrick DUBOSC, Magnetic particle testing history, June 2009: On our Website.

(2) French patent titled "Procédé et dispositif de contrôle magnétoscopique de pièces" (Editor's note: "Method and device for magnetic particle inspection of parts", Publication N°2.094.390, Registration N° 70.22624, applied by FLUXO Company (France) and published on February 4, 1972.

(3) Pierre CHEMIN and Patrick DUBOSC, Suggested definitions of some MT terms left out of EN 1330-7:2005 standard, DPCNewsletter N°024, May 2010 (Document updated in January, 2011).

(4) Stéphane GRAVELEAU (SREM Technologies), La magnétoscopie en chambre sans contact par champ tournant (editor's note: Magnetic particle testing in non-contact chamber by swinging field), CONTRÔLES-ESSAIS-MESURES jounal, edited by SOGICOMMUNICATION, 103 rue La Fayette F-75481 Paris Cedex 10 (France), July 2003. Also on this Website.

(5) Stéphane GRAVELEAU (SREM Technologies), Avec le courant induit, la magnétoscopie s’offre de nouvelles applications (Editor's note: With induced current, magnetic particle testing offers new applications) N°780 of the MESURES journal, 15 Rue d’Oradour-Sur-Glane, F-75015 Paris (France), december 2005. Also on this Website.

(6) Marie-Line ZANI-DEMANGE (from a paper of Stéphane GRAVELEAU of SREM Technologies), La magnétoscopie n’a pas dit son dernier mot (editor's note: Magnetic particle testing has not said its last word), N°807 of the MESURES journal, 15 Rue d’Oradour-Sur-Glane, F-75015 Paris (France), September 2008. Also on this Website.

(7)George DOWNES, Faster magnetic crack detection using the multi-directional swinging field method, Insight NDT Equipment Ltd: On this 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, 17 April 2012 17:44 )