Magnetic Particle Inspection is a widely used method to detect defects. The method is popular since it is a non-destructive technique that requires less surface preparation than some other methods. The material to be inspected must be capable of being magnetized so that the inspection will be effective. These metals are known as ferromagnetic materials. Such materials include iron, cobalt, nickel, and some alloys.
History of Magnetic Testing
As early as 1868, magnetizing was used to inspect cannon barrels. After a barrel was magnetized, a compass was moved along its length to detect defects. The movement of the needle indicated an irregularity in the surface.
In the 1920s Williams Hoke discovered that defects in a surface caused the magnetic field to extend past the edge of the part. Hoke found that metallic grindings would form patterns on the surface of the magnetized part that matched the cracks in the surface.
Principles of Magnetic Inspection
Magnetic fields will flow freely and not change directions on surfaces without flaws. If a field encounters a flaw or nonmagnetic material, the flow will come outside the material and then go back inside. Positive and negative magnetic poles are created when a magnetic field leaves the material. The flaw appears to have a magnetic field of its own. As a result of this event, magnetic particles are attracted to the point on the flaw with the greatest magnetic lines, which will cause the particles to form chain structures along the magnetic field lines of the flaw.
Dry Particle Inspection
Dry particle inspection is suitable for rough surfaces such as rough cast metals or welds that have not been ground. A surface is magnetized, and dry magnetic particles are dusted onto the surface. A half-wave DC current can be used to create a pulsating magnetic field and check for lack of penetration in welds. A half-wave DC current using prods can detect cracks and heat tears in large castings.
Wet Particle Inspection
The particles in wet magnetic particle inspection are suspended in a liquid carrier while they are applied to a surface. The particles can be more quickly and uniformly applied with this method than dry application. The particles are more mobile in the liquid carrier and will float to smaller magnetic leakage fields. However, the particles in the wet suspension are much smaller and can become immobile on a rough surface, which means that wet particle inspection is better suited to smooth surfaces and dry particle inspection is better for rough surfaces.
Continuous and Residual Magnetism
Magnetic particles may be introduced to a surface either while a part is being magnetized or used for residual magnetism after the process has ended. Introducing the particles while the part still being magnetized has the advantage that the leakage field will be the strongest. However, continued magnetizing can increase heat on the part and may result in surface damage. If it is a finished part, this could create problems. On parts that have more processing to undergo, this may be less of an issue.
Applications of Magnetic Particle Inspection
Magnetic particle inspection is a useful technique in rail transportation, aviation, construction, pipelines, chemical facilities, and shipbuilding. The procedure has high efficiency and sensitivity. Cracks resulting from metal fatigue can be detected early in their development.