Quantitative Problems in Magnetic Particle Inspection

Because it can be effective, rapid, and inexpensive, possibly more nondestructive evaluation is performed on magnetic steels by magnetic particle inspection (MPI) than by any other method. In industrial use for over fifty years, it has long been considered a mature technology. However, a number of questions remain on how to obtain reproducible, quantitative results when using MPI. It is possible to make the method too sensitive, in which case an obscuring background forms, or not sensitive enough, in which case important defects are missed. The primary factors that must be controlled to obtain reproducible and predictable MPI are: 1) magnetization level, 2) concentration, magnetic properties, and shapes of the particles used, 3) method of particle application, and 4) method of illumination and interpretation of the indications. A number of these factors has recently been addressed by Skeie and Hagemaier1. We briefly mention some of these here and discuss in some detail the nature of the magnetic leakage field and how it affects MPI, and outline a procedure by which the field level required to produce indications for a given defect may be estimated.</p