Introduction
Magnetic Particle Inspection (MPI) is a nondestructive testing technique used to detect surface and near-surface discontinuities in ferrous materials. The technique is mostly carried out using a portable yoke or a permanent magnet, to create a magnetic field in the part under test. Particles of ferromagnetic material, usually iron filings, are applied to the mechanically prepared surface of the part,and where discontinuities are present they will trap the particles. These particle accumulations are then observed and evaluated to determine the presence and location of surface and near-surface discontinuities. It is important to note that MPI is only effective on ferromagnetic materials. The MPI process is relatively simple and inexpensive, making it an ideal NDT tool for the detection of cracks and other surface imperfections.
The Principle of Magnetic Particle Inspection
The basic principle behind Magnetic Particle Inspection is the application of a magnetic field to the part under test, and then the application of iron filings or some other magnetic material on the surface or in the vicinity of the part. The magnetic fields created by the magnet cause the magnetic particles to be drawn to areas of stress, such as cracks. This then causes an accumulation of particles, which create a visible contrast on the part, indicating the presence of a crack or other defect.
Magnetic Particle Inspection Equipment
Before the MPI process can be started, the necessary equipment must be selected and then rigged up in the vicinity of the part being tested. The two basic pieces of equipment are a magnet and a viewing device. The magnet is typically a DC or AC related yoke, and the viewing device usually comprises of either black and white or colour fluorescent lamps. A variety of other tools and accessories such as probes, clamps, back-lighting, and specialised lighting can also be used to gain a better view of the accumulated particles
Application of Magnetic Particle Inspection
Magnetic Particle Inspection can be used to detect a variety of different cracks and defects, as well as voids, lack of fusion, lack of penetration and porosity. The process is also suitable for use on a wide range of surfaces, including rough and finished surfaces on a variety of materials such as steel, cast iron and other ferrous materials.
Advantages and Disadvantages of Magnetic Particle Inspection
The advantages of Magnetic Particle Inspection include:
•The ability to detect a wide range of surface and near surface flaws.
•High degree of sensitivity - even very small defects can be detected.
•A relatively simple and quick process.
•The detection areas are easy to mark for further investigation.
•Inspection can often be carried out in-situ, which results in time and cost savings.
The disadvantages include:
•It cannot be used to detect subsurface defects.
•Inaccurate results can be obtained if the surfaces are contaminated in any way.
•It requires the use of specialised and expensive equipment.
Conclusion
Magnetic Particle Inspection is an effective and efficient nondestructive testing technique used to detect surface and near surface discontinuities in ferrous materials. When applied correctly, the process can give accurate and reliable results. The process is simple and relatively inexpensive, and can be used to detect a wide range of defects on different surfaces and materials.
