T.O. 33B-1-13-1CHAPTER 3SECTION IINTRODUCTION TO MAGNETIC PARTICLE INSPECTION3 MAGNETICPARTICLEINSPECTION.3.1 INTRODUCTION.This section explains magnetic particle inspection, its purposes, and capabilities. This method is used for detectingdiscontinuities in ferromagnetic parts. The part is magnetized by using an electrical current that induces a magneticfield in the part. A discontinuity, which crosses the magnetic field, creates north and south poles on either side of thedefect area. When magnetic particles are applied to the part, the poles attract the particles and an indication of thediscontinuity is formed. Magnetic field characteristics are described, as well as the various techniques and equipmentused to magnetize and demagnetize components under inspection.3.1.1 PurposeOfMPI.NOTEThe terms MPI, MPT and MT are used interchangeably in this chapter.Magnetic particle inspection (MPI) or magnetic particle testing (MPT or MT) is a nondestructive inspection methodused to reveal surface and near sub-surface discontinuities in ferromagnetic materials. It consists of three basic steps:a. Establish a suitable magnetic field in the part.b. Apply magnetic particles to the surface of the part.c. Examine and evaluate any particle accumulations on the surface of the part.3.1.2 LimitationofMagneticParticleInspection.Magnetic particle inspection can detect discontinuities only in parts made of ferromagnetic materials. The magneticparticle inspection method will detect surface discontinuities, including those that are too fine to be seen with the nakedeye and those that lie slightly below the surface.3.1.3 DiscontinuityversusDefect.Discontinuities may exist in raw materials, be formed during processing or fabrication of parts, or result from serviceuse. Discontinuities are considered defects only if their existence is detrimental or harmful to the usefulness of theparts.3.1.4 Magnetization.a. A part is said to be magnetized when it is ferromagnetic and contains a magnetic field. An electriccurrent can be used to create or induce magnetic fields in ferromagnetic materials. The direction of themagnetic field is perpendicular to the direction of the magnetizing current. The current direction isselected to induce a magnetic field that is transverse to the orientation of a suspected discontinuity. Thestrength and distribution of the field are varied by the changing the nature of the magnetizing current.Understanding how different types of current can change the magnetization within a part is necessaryfor the proper application of magnetic particle testing.
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