NDT

Magnetic Particle Inspection Explained

March 25, 2008
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Source: Magnaflux


By definition, nondestructive testing is the collective name given to repeatable processes applied to components or structures to interrogate their integrity, in such a way as to not impair their ability to operate or perform.

At the turn of the 20th century, such tests were crude, relatively insensitive and not widely used. There was eyeball examination aided by low-power magnifying glasses. Also, hearing tests were performed by tapping metal parts with a hammer and listening for any difference in the ringing sounds produced.

In the early 1920s, X-ray, which previously had been used only by physicians, was shown to be applicable to inanimate materials. While radiography can detect subsurface or interior defects in metals, a reliable, inexpensive testing method was greatly needed to locate surface cracks and discontinuities in high quantities of machine and structural parts.

That method was magnetic particle testing, based on the principle of using a piece of magnetized metal and applying magnetic particles to find invisible defects, without harming the part in any way.

Magnetic particle inspection is a nondestructive test method that provides for the detection of linear, surface and near surface discontinuities in ferromagnetic (material that can be magnetized) test materials. A linear indication is any indication having a length dimension at least 3-times greater than its width (L= 3 x > W).

This image is a bolt with a longitudinal crack. Source: Magnaflux

FAQs

Here are some frequently asked questions about magnetic particle inspection:

Q: What are the advantages of magnetic particle inspection?
A: The test method process is quick and simple in principle and application. It is highly sensitive to the detection of surface and slightly subsurface linear indications, and indications appear on the actual test part. The test method process may often work through contaminant layers and coating thickness, and the method lends itself to automation and high-volume production inspection. Finally, it is less expensive than other more sophisticated methods of quality assurance.

Q: What are the limitations of magnetic particle inspection?
A: First, the test material must be ferrous. It provides limited and variable potential for detection of subsurface indications. Care is required to avoid burning and arcing of test part surface at points of electrical contact. The magnetic field direction must intercept the major dimension of the discontinuity. Complex test part geometry may sometimes pose problems with correct amperage determination and magnetic field intensity. Finally, demagnetization of the test part following the inspection is often necessary.

There are different types of magnetic particles: dry method non-fluorescent, wet method non-fluorescent and wet method fluorescent.

Q: What are the properties of dry powder and wet suspension magnetic particles?
A: They are iron oxide particles, finely divided in sizes varying between 0.125 and 60 microns with a high permeability (easily magnetized) and low retentivity (ability to stay magnetized). Dry particles are chemically dyed to provide contrast against the background of the test surface. Stock colors are gray, red, black and yellow. Application is either with a squeeze bulb or spray gun. Wet suspension particles are suspended in an oil-based or water-based liquid vehicle or carrier. With water, operators must add conditioners in addition to a wetting agent and corrosion inhibitors.

This illustrates a bolt with a crack under the head. Source: Magnaflux

Q: What is the sensitivity comparison between wet vs. dry methods?
A: Wet method provides improved sensitivity for the detection of very fine surface flaws. Dry method provides improved sensitivity for the detection of subsurface flaws.

Q: What is the maximum depth that magnetic particles can detect a discontinuity?
A: 1/4 inch, however 0.050 to 0.100 inch is a more realistic depth.

Magnetic particle inspection has a lot going for it. Along with being a quick test method, it is also less expensive than other more sophisticated methods of quality assurance. ndt

Glossary

  • Alternating Current (AC). Electric current flows through a conductor in a back and forth direction at specific intervals. Note: It provides the best sensitivity for the detection of surface discontinuities only.

  • Direct Current (DC). Electric current flows through a conductor in only one direction at all times. Note: DC from a battery source has been phased out in favor of rectified forms of AC for surface and subsurface flaw detection.

  • Flux Density (B). Magnetic field strength per unit volume within a ferromagnetic test part measured in gauss.

  • Flux Field Penetration. The ability to establish and drive high-density magnetic lines of force deep into the test part.

  • Full-Wave Rectified (FWDC). Electric current flows through a conductor in one direction only with an increased rate of pulsating surges and drops at specific intervals. Note: FWDC is recommended for effective surface and subsurface flaw detection when using the wet method of inspection.

  • Half-Wave Rectified (HWDC). Electric current flows through a conductor in one direction only with pulsating surges and drops at specific intervals-hence the name half wave. Note: It is most effective for surface and subsurface flaw detection when using the dry powder method of inspection.

  • Linear Indication. Any indication having a length dimension at least three times greater than its width.

  • Magnetic Domains. Ferrous material atoms or molecules normally illustrated as small bar magnets with north and south poles.

  • Magnetism. A form of energy directly associated with electrical current, and characterized by fields or lines of force.

  • Magnetizing Force (H). Magnetic field strength per unit volume in air, measured in oersteds.

  • Particle Mobility. The ability to establish activity or motion to the magnetic particles applied to the test part surface.

  • Permeability. The ease with which a material can be magnetized. The ability of a material to conduct magnetic lines of force.

  • Reluctance. The opposition of resistance by a material to conduct magnetic lines of force.

  • Retentivity. The ability of a material to retain magnetic lines of force following magnetization.

  • Right Hand Rule. Magnetic lines of force will always travel perpendicular or 90 degrees to the direction of electrical current flow.
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