Editing Magnetic particle inspection (section)

== Types of electrical currents used ==

There are several types of electrical currents used in magnetic particle inspection. For a proper current to be selected one needs to consider the part geometry, material, the type of discontinuity one is seeking, and how far the magnetic field needs to penetrate into the part. 

*[[Alternating current]] (AC) is commonly used to detect surface discontinuities. Using AC to detect subsurface discontinuities is limited due to what is known as the [[skin effect]], where the current runs along the surface of the part. Because the current alternates in polarity at 50 to 60 cycles per second it does not penetrate much past the surface of the test object. This means the magnetic domains will only be aligned equal to the distance AC current penetration into the part. The frequency of the alternating current determines how deep the penetration.
*[[Rectifier#Full-wave_rectification|Full wave DC]] (FWDC) is used to detect subsurface discontinuities where AC can not penetrate deep enough to magnetize the part at the depth needed. The amount of magnetic penetration depends on the amount of current through the part.<ref name="betz">{{Citation | last = Betz | first = C. E. | title = Principles of Magnetic Particle Testing | page = 234 | publisher = [[American Society for Nondestructive Testing]] | year = 1985 | url = http://wiki.magwerks.com/wiki/images/c/c6/Waveform_to_Depth_Comparison.pdf | isbn = 978-0-318-21485-6 | postscript = . | access-date = 2010-03-02 | archive-url = https://web.archive.org/web/20110714021042/http://wiki.magwerks.com/wiki/images/c/c6/Waveform_to_Depth_Comparison.pdf | archive-date = 2011-07-14 | url-status = dead }}</ref> DC is also limited on very large cross-sectional parts in terms of how effectively it will magnetize the part.
*[[Rectifier#Half-wave_rectification|Half wave DC]] (HWDC, [[pulsating DC]]) works similar to full wave DC, but allows for detection of surface breaking indications and has more magnetic penetration into the part than FWDC. HWDC is advantageous for inspection process as it actually helps move the magnetic particles during the bathing of the test object.  The aid in particle mobility is caused by the half-wave pulsating current waveform.  In a typical mag pulse of 0.5 seconds there are 15 pulses of current using HWDC. This gives the particle more of an opportunity to come in contact with areas of magnetic flux leakage. 

An AC [[electromagnet]] is the preferred method for find surface breaking indication.  The use of an electromagnet to find subsurface indications is difficult.  An AC electromagnet is a better means to detect a surface indication than HWDC, DC, or permanent magnet, while some form of DC is better for subsurface defects.