Magnetic Particle Testing

Because magnetization of certain metals is possible, small particles that are attracted to magnetic fields are applied to the surface of the test object after or during induction of a magnetic field, thereby detecting certain discontinuities which are present in the material. Since magnetic particle testing is capable of revealing discontinuities (variations in material composition) economically, it is one of the most used nondestructive test methods. However, this test method is limited to disclosing only those methods that are at or near the surface.

Magnetic Particle testing is mainly used to detect fatigue and stress cracks in ferrous materials only. A magnetic field is introduced in the test specimen. Any voids (cracks) will allow the magnetic field to escape the flow of the material, and the applied iron particles will bridge the disturbance.

The objective of magnetic particle testing is to ensure maximum reliability by providing a means of obtaining a visual image of an indication related to a discontinuity in or at the surface of a material. It can also disclose this discontinuity without impairing the material. Magnetic particle testing has the ability to separate acceptable and unacceptable material in accordance with a predetermined set of standards.

Magnetic Particle testing is a simple test method that can be applied to finished products such as billets, hot rolled bars, castings and forgings. Basically it can be used on any ferromagnetic material to find surface and substrate irregularities. It can also be adopted for use on weldments, engine components, shafts and gears.

The medium for Magnetic Particle testing comes in either powder or paste form. In the dry method, the powder is applied in its dry form by dusting or blowing over the specimen. When the wet method is used, the medium is first mixed with a liquid to make a bath, which is then sprayed onto the surface of the test piece.

Since an indication must be readily visible, a good light source is essential. The choice of the particle color is entirely dependent on visibility. The most widely used colors are red, grey and black.

The fluorescent powders and pastes are particles coated with a dye that fluoresces brilliantly under an ultraviolet or black light, thereby increasing visibility.

Some of the limitations involved with magnetic particle inspections are that the specimen must be clean and have a relatively smooth surface. Cracks must be at least 1/64 of an inch in the major dimension. The alignment of the magnetic field and the strength is critical to a proper inspection. Also some metal removal may be required and the demagnetization of the part may be problematic.