Magnetic Particle Inspection MPI Coils

We refer to AC Coils as “toys” as they have limited effective applications. Why? AC Coils only introduce a Skin Effect Field, which means they can only detect surface breaking defects. The surface affect on an AC field stops at the end of a workpiece, and does not create a return field on the inside surface of a pipe.

Industry demanded Strong DC Coils to detect Subsurface Defects, simply because DC or a Permanent Field is better for MPI. Traditional applications for Coils is for Fasteners, Shafts, Bars, and Tubes, where stress and fatigue cracks are masked by surface metal smear, such as under bearings races, or thread cracks. AC fields will not detect these slightly subsurface cracks, but a DC field will. Conversely a DC Field is just as capable to detect Surface Breaking Defects as an AC Field. There is a tremendous amount of MPI performed on Pipe ends, and a DC field is induced to both the OD and ID surfaces of a hollow end, such as Female Threads.

DC is may times stronger that an AC field, and a DC field is permanent, while an AC field is only temporary. Most magnetism found in Ferro Magnetic Material has been induced by a Permanent Field. A Permanent Field can only be removed by a reversing and reducing DC Field. An AC Field will not remove a Permanent Field, only the skin, and soon magnetism returns to the surface by magnetic creep. Thinking that an AC Field works for difficult inspections, simply demonstrates a lack of knowledge of the affects of a magnetic field on Magnetic Particle Inspection.