Magnetic Particle Testing

_{(Non Destructive Testing)}
Magnetic Particle Testing

Magnetic particle Testing (MPT) is a non-destructive testing (NDT) process for detecting surface and subsurface discontinuities in ferrous materials. The process puts a magnetic field into the part. The piece can be magnetized by direct or indirect magnetization. Direct magnetization occurs when the electrical current is passed through the test object and a magnetic field is formed in the material. Indirect magnetization occurs when no electrical current is passed through the test object, but a magnetic field is applied from an outside source. The magnetic lines of force are perpendicular to the direction of the electrical current which may be either alternating current (AC) or some form of direct current (DC) (rectified AC).

The presence of a surface or subsurface discontinuity in the material allows the magnetic flux to leak. Ferrous iron particles are applied to the part. The particles may be dry or in a wet suspension. If an area of flux leakage is present the particles will be attracted to this area. The particles will build up at the area of leakage and form what is known as an indication. The indication can then be evaluated to determine what it is, what may have caused it, and what action should be taken if any.

Types of electrical currents used

There are several types of electrical currents used in MPI. For a proper current to be selected one needs to consider the part geometry, material, the type of discontinuity you’re looking for, and how far the magnetic field needs to penetrate into the part.

Alternating current (AC) 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 decides how deep the penetration.

Direct current (DC, full wave DC) Used to detect sub surface discontinuities where AC cannot penetrate deep enough to magnetize the part at the depth needed. The amount of magnetic penetration depends on the amount of current passed through the part.DC is also limited on very large cross sectional parts how effective it will magnetize the part.

Half wave DC (HWDC, pulsating DC) work similar to full wave DC with sightly more magnetic penetration into the part. HWDC is known to have the most penetrating ability in magnetic particle testing.HWDC is advantageous for inspection process because it actually helps move the magnetic particles over the test object so that they have the opportunity to come in contact with areas of magnetic flux leakage. The increase in particle mobility is caused by the pulsating current which vibrates the test piece and particles.

Each method of magnetizing has its pros and cons. AC is generally always best for discontinuities open to the surface and some form of DC for subsurface.