Materials testing: how does magnetic particle testing (MT) work?

Like all non-destructive testing (NDT) methods, magnetic particle testing (MT) is the practical application of a physical effect. Here, the key property is magnetic permeability: the ability of a material to allow magnetic field lines to pass through it.

Permeability and material behaviour in a magnetic field

Materials respond differently to an external magnetic field H. This produces a magnetic flux density B inside the component. In simplified terms, permeability can be described as the ratio of B to H, a measure of how strongly a material “takes up” or amplifies an external magnetic field.

Three classes of material are distinguished according to their behaviour:

• Diamagnetic materials take up fewer field lines than are present in the external field. Their permeability is very low. Copper is a typical example.
• Paramagnetic materials take up slightly more field lines than are present in the external field. Their permeability is also low; aluminium and air belong to this category.
• Ferromagnetic materials take up far more field lines than are present in the external field. Iron and nickel have high permeability and significantly amplify external magnetic fields.

This strong concentration of field lines in ferromagnetic materials is the basis of magnetic particle testing.

Magnetisation and the formation of leakage flux

Consider a ferromagnetic component containing a crack. The aim is to detect this crack. To do so, the component is magnetised, effectively “filling” it with magnetic field lines.

In undisturbed material, the field lines run largely within the component. At a crack, however, there is a discontinuity: the crack is not ferromagnetic but may, for example, be filled with air. Air is paramagnetic and can take up only very few field lines.

As a result, some field lines are diverted, and a large proportion leave the component at the crack and re-enter it beyond the crack. This local emergence of field lines is known as magnetic leakage flux. It occurs precisely where the discontinuity is located.

Leakage flux can be detected in different ways. One is to use a probe, such as a coil. Moving it through the leakage field induces an electrical voltage that can be measured. This method is known as the magnetic flux leakage technique. Another, much more readily visualised approach is the magnetic particle technique.

Making indications visible with magnetic particles

In magnetic particle testing, fine ferromagnetic particles are applied to the surface of the magnetised component, usually in a carrier liquid to increase their mobility. If a crack is present, the magnetic particles accumulate along it. But why?

Magnetic poles form in the region of leakage flux: one pole forms where field lines emerge, and the opposite pole forms where they re-enter. These local magnetic poles attract the magnetic particles, which collect along the crack and form a visible indication. At the same time, the accumulated particles themselves act as a magnetically conductive bridge. They make it easier for the field lines to return to the component, stabilising the indication.

To ensure that the fine particles can be reliably detected, they are coloured to provide contrast or given a fluorescent coating. Fluorescent magnetic particle testing under UV light offers particularly high testing sensitivity and is widely used in industrial practice.

The crucial role of magnetisation direction

A key aspect of magnetic particle testing is the orientation of the magnetic field relative to the possible crack orientation. If the magnetic field runs perpendicular to the crack, pronounced leakage flux develops, making the crack readily detectable. If the magnetic field runs parallel to the crack, however, the field lines follow the geometry without leaving the component. No significant leakage flux develops, and therefore no indication appears.

Since the actual crack orientation is usually unknown in practice, the component must be magnetised so that as many potential crack directions as possible are covered. Various strategies are available:

• Magnetisation in two mutually perpendicular directions, for example when testing welds with a handheld yoke.
• Generation of a rotating magnetic field by electronic control (combined testing).
• Mechanical rotation of the component in a constant magnetic field, as used when testing railway wheels with a Minden coil.

Conclusion: magnetic particle testing in materials engineering

Magnetic particle testing and magnetic flux leakage testing do not “recognise” cracks in a geometric sense. They respond solely to differences in magnetic permeability. The ferromagnetic component has high permeability, while the air in the crack has much lower permeability. It is precisely this contrast that generates the leakage flux.

If a crack were hypothetically filled with a material having exactly the same permeability as the component, no leakage flux would develop and the crack would remain invisible.

Magnetic particle testing therefore involves deliberately detecting magnetic inhomogeneities. For the non-destructive testing of ferromagnetic materials, it is a robust method with a clear physical basis and an excellent track record in practice.

W.S. Werkstoff Service – expertise in NDT

If you would like to deepen your knowledge of non-destructive testing, the team at W.S. TrainingCenter will be happy to support you with practical continuing education and training. You can find further information on our course and seminar pages.