Introduction to Magnetic Particle Testing

Magnetic Particle Testing

Magnetic particle testing is a non-destructive testing method for the detection of surface and subsurface discontinuities in ferromagnetic materials.

Ferromagnetic materials are materials that can be magnetized to a level that will allow the inspection to be effective. They are Iron, Cobalt, Nickel and their magnetic alloys.

Magnetic particle testing can detect, surface and near-surface cracks seams, laps, cold shuts in castings lamination, lack of fusion near the surface, undercuts, deep scratches and fatigue cracks are indicated. Linear inclusions and porosity at or very near the surface may produce indications.

The technique uses the principle, that during the magnetization of a ferromagnetic material, magnetic lines of force [magnetic flux lines] pass-through this magnetically conducting medium. If the magnetic flux lines hit an area of different magnetic permeability such as a crack near the surface, a portion of these flux lines gets diverted and leak out above the surface of the material. A magnetic leakage field emerges from the part.

To show this leakage field, colored finely divided iron particles are sprayed to the area under examination. The leakage field attracts and accumulates some of these iron powder particles and essentially creates a powder caterpillar worm-like visual indication for the human eye. The indication is produced directly on the surface of the part and above the discontinuity.

There are variations in the way the magnetizing field is applied, but they are all dependant on the above principle. All surface and near-surface crack-like defects that produce a leakage field at the test surface can be detected. No elaborate precleaning is necessary, and surface defects filled with foreign material can be detected.

Characteristics of a discontinuity that enhances its detection are,

• Its depth is at right angles to the surface
• Width of the surface opening small so that the air gap created  
• Is narrow
• Its length at the surface is large with respect to its width
• It is comparatively deep in proportion to the width of its opening.

In general, reliable detection requires that the width - depth - length dimensions of the discontinuities correspond to the ratio 1: 5:10. The lowest detection limits are a 1µm crack width, with a 10 µm depth of cut.

Optimum crack detection occurs when the magnetic flux lines flow at right - angles to the length of the defect. To form a detectable leakage field, the angle between the field direction and the expected defect’s length shall not be greater than 45°.

Disadvantages

It can be used only on ferromagnetic materials, has a certain application that requires large amounts of electrical current and requires the magnetic field to be properly oriented in relation to the discontinuities anticipated. Paint coatings and nonmagnetic coverings affect the sensitivity of examination. Demagnetization of the parts following examination may be required. Post cleaning to remove the magnetic particle materials from the test surface is required.

Detectability of Defects

Detectability depends on the formation of a strong leakage field which is dependent on surface opening of the discontinuity and its depth through the part thickness. A shallow surface scratch which may be as wide as it is deep usually does not produce an indication. If a crack is wide open at the surface, the reluctance of the air gap in the crack opening reduces the strength of the leakage field. This, combining with the inability of the particles to bridge the air gap, fails to form an indication. Laps emerge at an acute angle to the surface and a wide air gap is created between its lip and the part surface. The leakage field may be quite weak because very little leakage flux takes the path out through the surface lip of the lap to cross this high reluctance gap. If the faces of a crack are tightly forced together under compressive stress, the almost complete absence of an air gap may produce so little leakage field that no particle indication is formed.

The surface structure of a test piece has a significant influence on the detectability of defects. The surface cutting depth of a defect should be at least twice the associated surface roughness. Defect detectability can be further reduced by false indications arising from magnetic stray fields, accumulation of powder due to surface roughness, part configuration, scratches, scales, slots, etc. Cases can occur where It is difficult to generate the force required for a positive defect indication.

Surface irregularities and scratches can give misleading indications. Therefore, it is necessary to ensure careful preparation of the surface before magnetic particle testing is undertaken.

Detectability of Sub Surface Defects

Magnetic particle testing can detect near-surface discontinuities of favorable position and adequate size, but the possibility of an indication rapidly decreases when the discontinuity is more than 2 mm below the surface.

Detection sensitivity increases with an increase in magnetic field strength, but with very high field strength magnetic particles will be attracted to defect-free areas of the surface as well as to defects. The depth below the surface at which a sub-surface the defect may be detected is of the order of 3 to 7 mm when the direct current magnetization is used, but this will also depend on the size, shape, and orientation. Therefore, the deeper the discontinuity lies below the surface, the larger it must be to yield a readable indication and the more difficult the discontinuity is to find by this method.

Cracks below a non-magnetizable surface layer, up to 40 µm is detectable.