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Keith Brewer
Keith Brewer
23:33 Sep-06-2019
Magnetic particle AC versus DC magnetization

NDT literature including training materials and example exam questions frequently refers to a superiority of AC magnetization over DC magnetization when inspecting for surface breaking (in service) defects frequently with no offer of supporting scientific or empirical evidence.

Supporting evidence often includes a statement that the skin effect "concentrates" the magnetic field at the surface, implying that the total field has moved to the surface. The reality is that the skin effect is a loss to the system caused by the induced eddy currents.

An often quoted reference is the Carl Betz classic "Principles of Magnetic Particle Testing". A demonstration comparing wave forms for magnetizing a hollow pin sporting induced quench cracks of random direction and magnitude is magnetized using a central conductor. The applied current is increased by root mean square (RMS) ratio (1.41:1) and photographs of the indications are tabulated. A declaration the AC case is RMS value, the magnetizing value not having been measured and used for comparison with the DC cases. This places all the AC cases in the wrong lines in the table. If a correction is made to compare magnetizing currents in the process of magnetizing the item the DC indications are remarkably stronger than with the AC indications. This result would reflect the skin effect attenuation (concentration?) occurring from the inner surface close to the magnetizing force through the part to the observed surface.

Can anyone advise other explanations which may support claims of waveform superiority and in some cases requirement in standards or procedures to use AC wave form to inspect for surface breaking defects?

    
 
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Anmol Birring
Consultant,
Birring NDE Center, Inc., USA, Joined Aug 2011, 738

Anmol Birring

Consultant,
Birring NDE Center, Inc.,
USA,
Joined Aug 2011
738
20:58 Sep-08-2019
Re: Magnetic particle AC versus DC magnetization
In Reply to Keith Brewer at 23:33 Sep-06-2019 (Opening).

see https://en.wikipedia.org/wiki/Skin_effect

    
 
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Keith Brewer
Keith Brewer
01:30 Sep-09-2019
Re: Magnetic particle AC versus DC magnetization
In Reply to Anmol Birring at 20:58 Sep-08-2019 .

Thank you for your response Anmol.

I checked the reference, seems mainly to deal with current flow, but magnetic field is referred to: "Since the skin effect causes a current at high frequencies to flow mainly at the surface of a conductor, it can be seen that this will reduce the magnetic field inside the wire, that is, beneath the depth at which the bulk of the current flows."

This is claiming a reduction with depth rather than an increase at surface and does not really justify the claims for enhancement with AC for surface defects.

My understanding has always been that the field at the surface is dependent on the magnitude of the current flowing within a body and independent of the distribution of that current? See various diagrams for field distribution for instance for hollow and solid conductors both magnetic and non magnetic. So it is the same for AC as for DC?

I do notice that a diagram for distribution in hollow magnetic cylinders magnetized by AC current with central conductor has not been featured in the literature? This analysis would help people to analyse the Betz experiment. I also notice that he refers to the comparison of the results rather that a direct claim that AC is superior?

    
 
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Rick Lopez
R & D,
John Deere - Moline Technology Innovation Center, USA, Joined Jul 2011, 186

Rick Lopez

R & D,
John Deere - Moline Technology Innovation Center,
USA,
Joined Jul 2011
186
22:41 Sep-10-2019
Re: Magnetic particle AC versus DC magnetization
In Reply to Keith Brewer at 01:30 Sep-09-2019 .

Keith,
NDT practitioners are rarely electrical engineers or physicists who might determine the "scientific" theory behind results (for example, deriving from Maxwell’s equations), and there are several examples of conflicting opinions even when these scientific theories are offered. We can turn to empirical results, but unfortunately these often vary hugely based on specifics of the application, nature of the flaw under study, human factors, and other aspects. An example of this detection variation that comes to mind is a comparison of a 1998 American Petroleum Institute recommended practice that stated an AC electromagnetic yoke could reliably detect surface-breaking defects 0.236" long, while a 2000 preliminary report-out of an Association of American Railroads study suggested that a flaw needed to be longer than 3" to be reliably detected with MT.

From what I've read, the most convincing argument seems to be that AC current’s superiority for surface-breaking flaws is due to its enhanced particle mobility – caused by the strong pulsation in the waveform. Some have concluded that skin effect is a poor argument for AC's surface-flaw superiority, and surface tangential magnetic field strength was independent of waveform. One researcher published a paper showing how the tangential magnetic leakage field at a discontinuity was weaker than that caused by other waveforms (thus supporting the pulsing / particle mobility argument).

Ammeters on wet benches come in a few flavors (peak, RMS, average), so direct comparison of waveforms isn’t trivial. From an empirical standpoint, several authors have compared detectability from AC and various forms of “DC”. One example was done on pipe to seek surface-breaking stress corrosion cracking. That team used a variety of surface preparation techniques, color contrast and fluorescent particles, and two magnetizing waveforms with portable yokes. In their study they found that in general AC could detect flaws of around half the length possible with "pulsed DC". A couple of others discussed how AC required about half the current used with HWDC (FWDC required higher current setting than HWDC).

Taking the pulsation idea a bit further, 1-phase rectified current has more than 3-phased rectified current. In 2006 work published by one equipment manufacturer they showed a strong difference in indication formation between 1-phase and 3-phase rectified current when the wet bench's ammeter showed 500 amps.

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