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MT magnetizing techniques

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Written by Administrator
Saturday, 10 December 2011 12:30

January 2012

We have the pleasure to publish on our Website a contribution by a French MT expert, Stéphane GRAVELEAU, from the R and D Department of SREM TECHNOLOGIES (France).

Magnetizing techniques and current waveforms are described.
Very often, the wave forms displayed in training courses, even in standards, are those got on weakly inductive loads. As this paper explains, on highly inductive loads, the curves do not have the form that one would expect.
In fact, waveforms are very dependent on the electric characteristics of the load.

So far, this connection is rarely highlighted.

By releasing this document, we think we fill in a gap; furthermore, we hope it will be of some interest for you, and that it will give you a top - quality piece of information.

Patrick DUBOSC and Pierre CHEMIN


Magnetic particle testing requires that parts under inspection be magnetized. Several techniques of magnetization are available, some of them using different electric wave forms.

1- Traditional techniques

• Transverse magnetization by the current flow technique

To produce a transverse magnetization, the current flow technique is generally used. It gives rise to a circular magnetic field that circles the part.

Transverse magnetization allows for the longitudinal discontinuities detection.

• Longitudinal magnetization through either magnetic heads or solenoid

To produce the longitudinal magnetization, either two magnetic heads, each set at one end of the part, or a solenoid (coil) circling the part, is generally used.

Longitudinal magnetization allows for the transverse discontinuities detection.

 

2- Other techniques

There are many other techniques to magnetize parts. They are preferably used when the electric arc produced during the current flow may be detrimental to the part. Then, non-contact magnetization techniques are performed. They may be used also when the need is to lower the time-for-inspection when using a swinging field process. Two examples are described underneath:

• Magnetization using a threaded bar and the magnetic core of the induced current transformer

This technique, processed with sinusoidal AC allows for both the transverse and longitudinal magnetizations of the part, while there is no contact between the part and the magnetization equipment.
Thanks to out-of-phase electric currents, the transverse and longitudinal magnetizations can be simultaneously applied and produce a swinging field in the part.

The swinging field magnetization produces on the entire surface of the part under inspection a magnetic field whose direction is constantly changing, without any contact with the part. This technique shall use only sinusoidal AC and has some limits that shall be thoroughly known.

When sinusoidal AC is not allowed, it is still possible to use a threaded bar or a solenoid to magnetize a part without contact. Nevertheless, no swinging field can then be produced.

Other less common techniques, such as the residual magnetization or permanent magnets(4), may also be useful in specific applications. However, these techniques are not described in the ISO standards.

3- Electric current main wave forms

Almost all magnetizations are produced by an electric current. There are several current wave forms, each with its own advantages or limits:

• Sinusoidal alternating current (AC).

• Half wave rectified direct current (HWDC) (single phase) or pulsating direct current.

• Full wave rectified direct current (FWDC) (single phase).

• Three phase half-wave rectified current (HWDC) (three phases).

• Full wave rectified direct current (FWDC) (three phases).

• Direct current (DC).

The sinusoidal alternating current is suitable for the detecting open-to-surface discontinuities. DC and rectified currents allow for the detecting sub-surface discontinuities, with some limits. The higher the DC component of a rectified current, the easier the detection of discontinuities deep inside the part. With some limits, too: it is impossible to determine accurately the detection depth because it also greatly depends on the defect size and on the magnetic-field strength in the part.

Rectified currents are generally produced by using more or less complex electronic circuits that include diodes, depending on the type of the target current:

The rectified currents are generally obtained using electronic assemblies with diodes, more or less complex, depending on the nature of the wished rectification:

• Half wave rectified direct current (HWDC) (single phase)

• Full wave rectified direct current (FWDC) (single phase)

• Full wave rectified direct current (FWDC) (three phases)

It is important to note that the actual current shape is highly dependent on the load induction. In MT, the load may be weakly inductive when using direct current flow in a part or when using a threaded bar. However, the load is far more inductive when using a solenoid or a coil, which has the effect of averaging the current (see the above diagrams). The more turns the coil or the solenoid have, or the better they are coupled with the part under test, the more inductive the load will be and the more averaged the current will be.
Maybe the most extreme example deals with the DC magnetic heads: these magnetic heads comprise a very high number of turns directly wound on a magnetic core. Thus, In that situation, the load is so inductive that it is possible to get a DC-like current, with almost no fluctuation, using a very simple full-wave rectifier.
The form of the magnetic field is directly linked to the current wave form. It is then possible to use a tangential magnetic field strength meter that displays the field form to know the current wave form.

References

(1) Pierre CHEMIN and Patrick DUBOSC, Suggested definitions of some MT terms left out of EN 1330-7:2005 standard, (Document updated in January, 2011). DPCNewsletter N°024, March 2010. On our Website:
http://www.ressuage-magnetoscopie-penetranttesting-magnetictesting-dpc.info/site/en/dpc-news/2010/147-dpcnews-024-definitions-pour-la-norme-en-1330-72005

(2) Pierre CHEMIN and Patrick DUBOSC, Magnetic particle testing and the swinging field technique: it works! DPCNewsletter N°033, February 2011. On our Website:
http://www.ressuage-magnetoscopie-penetranttesting-magnetictesting-dpc.info/site/en/dpc-news/2011/156-dpcnews-033-aimantation-par-champ-tournant-

(3) Pierre CHEMIN and Patrick DUBOSC, Magnetic particle testing: some limits to magnetization by swinging field, DPCNewsletter N°034, March 2011. On our Website:
http://www.ressuage-magnetoscopie-penetranttesting-magnetictesting-dpc.info/site/en/dpc-news/2011/157-dpcnews-034-aimantation-par-champ-tournant-les-limites

(4) Pierre CHEMIN and Patrick DUBOSC, MT using permanent magnets or the residual technique: two left-aside techniques? - Paper presented at the 2011 COFREND Congress on NDT, May 24-27, 2011, Dunkirk (France).

Last Updated ( Saturday, 10 December 2011 13:40 )