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Magnetic Particle Inspection

Magnetic Particle Inspection

Magnetic particle inspection (MT or MPI) is a nondestructive testing method used for defect detection. Magnetic particle testing is fast and relatively easy to apply, and part surface preparation is not as critical as it is for some other NDT methods. These characteristics make Magnetic particle testing one of the most widely utilized nondestructive testing methods.

Magnetic Particle Inspection Methods
  • Wet Fluorescent Continuous Method (Bench)
  • Wet Fluorescent Residual Method (Bench)
  • External Cable Wrap
  • External Clamps
  • Yoke
Advantages of Magnetic Particle Inspection
  • Large surface areas of complex parts can be inspected rapidly
  • Can detect surface and subsurface flaws
  • Surface preparation is less critical than it is in penetrant inspection
  • Equipment costs are relatively low

Magnetic particle testing uses magnetic fields and small magnetic particles to detect flaws in components. The only requirement from an inspection standpoint is that the component being inspected must be made of a ferromagnetic material such as iron, nickel, cobalt, or some of their alloys.

Ferromagnetic materials are materials that can be magnetized to a level that will allow the inspection to be effective.

The method is used to inspect a variety of product forms including castings, forgings, and weldments. Many different industries use magnetic particle inspection for determining a component's fitness-for-use.

Some examples of industries that use magnetic particle inspection are the structural steel, automotive, petrochemical, power generation, and aerospace industries. Underwater inspection is another area where magnetic particle inspection may be used to test items such as offshore structures and underwater pipelines.

In magnetic particle inspection, the magnetic particles can either be applied to the component while the magnetizing force is applied, or after it has been stopped. Continuous Magnetization describes the technique where the magnetizing force is applied and maintained while the magnetic particles are dusted or flowed onto the surface of the component.

In a wet horizontal testing unit, the application of the particles is stopped just before the magnetizing force is applied; but, since particles are still flowing over and covering the surface, this is considered continuous magnetization.

Magnetic Particle Inspection

Residual Magnetization, on the other hand, describes the technique where the magnetizing force is applied to magnetize the component and then stopped before applying the magnetic particles. Only the residual field of the magnetized component is used to attract magnetic particles and produce an indication.

The continuous technique is generally chosen when maximum sensitivity is required because it has two distinct advantages over the residual technique. First, the magnetic flux will be highest when current is flowing and, therefore, leakage fields will also be strongest. Field strength in a component depends primarily on two variables: the applied magnetic field strength and the permeability of the test object.

High permeability materials do not retain a strong magnetic field so flux leakage fields will be extremely weak or non-existent when the magnetizing force is removed. Therefore, materials with high magnetic permeability are not suited for inspection using the residual technique. When the residual technique is used to inspect materials with low permeability, care should be taken to ensure that the residual field is of the necessary strength to produce an indication. Defects should be relatively large and surface breaking to have a high probability of detection using the residual method.

The second advantage of the continuous technique is that when current is used to generate the magnetizing force, it can provide added particle mobility. Alternating or pulsed direct current will cause the particles to vibrate and move slightly on the surface of the part. This movement allows the particles to travel to leakage sites. More particles mean brighter indications compared to those formed using the residual technique.

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