Magnetic
Particle Testing
Magnetic
particle testing is a non-destructive testing method for the detection of surface
and subsurface discontinuities in ferromagnetic materials.
Ferromagnetic
materials are materials that can be magnetized to a level that will allow the inspection
to be effective. They are Iron, Cobalt, Nickel and their magnetic alloys.
Magnetic
particle testing can detect, surface and near-surface cracks seams, laps, cold shuts
in castings lamination, lack of fusion near the surface, undercuts, deep scratches
and fatigue cracks are indicated. Linear inclusions and porosity at or very near
the surface may produce indications.
The technique uses the principle, that during the magnetization of a ferromagnetic material,
magnetic lines of force [magnetic flux lines] pass-through this magnetically conducting
medium. If the magnetic flux lines hit an area of different magnetic permeability
such as a crack near the surface, a portion of these flux lines gets diverted and
leak out above the surface of the material. A magnetic leakage field emerges from
the part.
To
show this leakage field, colored finely divided iron particles are sprayed to the
area under examination. The leakage field attracts and accumulates some of these
iron powder particles and essentially creates a powder caterpillar worm-like visual
indication for the human eye. The indication is produced directly on the surface
of the part and above the discontinuity.
There
are variations in the way the magnetizing field is applied, but they are all dependant
on the above principle. All surface and near-surface crack-like defects that produce
a leakage field at the test surface can be detected. No elaborate precleaning is
necessary, and surface defects filled with foreign material can be detected.
Characteristics
of a discontinuity that enhances its detection are,
- Its
depth is at right angles to the surface
- Width
of the surface opening small so that the air gap created
- Is
narrow
- Its
length at the surface is large with respect to its width
- It
is comparatively deep in proportion to the width of its opening.
In
general, reliable detection requires that the width - depth - length dimensions
of the discontinuities correspond to the ratio 1: 5:10. The lowest detection limits
are a 1µm crack width, with a 10 µm depth of cut.
Optimum
crack detection occurs when the magnetic flux lines flow at right - angles to the
length of the defect. To form a detectable leakage field, the angle between the
field direction and the expected defect’s length shall not be greater than 45°.
Disadvantages
It
can be used only on ferromagnetic materials, has a certain application that requires
large amounts of electrical current and requires the magnetic field to be properly
oriented in relation to the discontinuities anticipated. Paint coatings and nonmagnetic
coverings affect the sensitivity of examination. Demagnetization of the parts following
examination may be required. Post cleaning to remove the magnetic particle materials
from the test surface is required.
Detectability of Defects
Detectability
depends on the formation of a strong leakage field which is dependent on surface opening
of the discontinuity and its depth through the part thickness. A shallow surface
scratch which may be as wide as it is deep usually does not produce an indication.
If a crack is wide open at the surface, the reluctance of the air gap in the crack
opening reduces the strength of the leakage field. This, combining with the inability
of the particles to bridge the air gap, fails to form an indication. Laps emerge
at an acute angle to the surface and a wide air gap is created between its lip and
the part surface. The leakage field may be quite weak because very little leakage
flux takes the path out through the surface lip of the lap to cross this high reluctance
gap. If the faces of a crack are tightly forced together under compressive stress,
the almost complete absence of an air gap may produce so little leakage field that
no particle indication is formed.
The surface structure of a test piece has a significant influence on the detectability
of defects. The surface cutting depth of a defect should be at least twice the
associated surface roughness. Defect detectability can be further reduced by false
indications arising from magnetic stray fields, accumulation of powder due to
surface roughness, part configuration, scratches, scales, slots, etc. Cases can occur
where It is difficult to generate the force required for a positive defect indication.
Surface
irregularities and scratches can give misleading indications. Therefore, it is necessary
to ensure careful preparation of the surface before magnetic particle testing is
undertaken.
Detectability
of Sub Surface Defects
Magnetic
particle testing can detect near-surface discontinuities of favorable position
and adequate size, but the possibility of an indication rapidly decreases when the
discontinuity is more than 2 mm below the surface.
Detection
sensitivity increases with an increase in magnetic field strength, but with very high
field strength magnetic particles will be attracted to defect-free areas of the
surface as well as to defects. The depth below the surface at which a sub-surface the defect may be detected is of the order of 3 to 7 mm when the direct current magnetization
is used, but this will also depend on the size, shape, and orientation.
Therefore, the deeper the discontinuity lies below the surface, the larger it must
be to yield a readable indication and the more difficult the discontinuity is to
find by this method.
Cracks
below a non-magnetizable surface layer, up to 40 µm is detectable.
