T.O. 33B-1-1
3-1
CHAPTER 3
SECTION I
INTRODUCTION TO MAGNETIC PARTICLE INSPECTION
3
MAGNETIC PARTICLE INSPECTION.
3.1
INTRODUCTION.
This section explains magnetic particle inspection, its purposes, and capabilities. This method is used for detecting
discontinuities in ferromagnetic parts. The part is magnetized by using an electrical current that induces a magnetic
field in the part. A discontinuity, which crosses the magnetic field, creates north and south poles on either side of the
defect area. When magnetic particles are applied to the part, the poles attract the particles and an indication of the
discontinuity is formed. Magnetic field characteristics are described, as well as the various techniques and equipment
used to magnetize and demagnetize components under inspection.
3.1.1
Purpose Of MPI.
NOTE
The 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 method
used 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
Limitation of Magnetic Particle Inspection.
Magnetic particle inspection can detect discontinuities only in parts made of ferromagnetic materials. The magnetic
particle inspection method will detect surface discontinuities, including those that are too fine to be seen with the naked
eye and those that lie slightly below the surface.
3.1.3
Discontinuity versus Defect.
Discontinuities may exist in raw materials, be formed during processing or fabrication of parts, or result from service
use. Discontinuities are considered defects only if their existence is detrimental or harmful to the usefulness of the
parts.
3.1.4
Magnetization.
a. A part is said to be magnetized when it is ferromagnetic and contains a magnetic field. An electric
current can be used to create or induce magnetic fields in ferromagnetic materials. The direction of the
magnetic field is perpendicular to the direction of the magnetizing current. The current direction is
selected to induce a magnetic field that is transverse to the orientation of a suspected discontinuity. The
strength 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 necessary
for the proper application of magnetic particle testing.
