PRINCIPLE OF OPERATION
With the yoke connected to a suitable battery source, depressing the switch
creates an intense magnetic field between the legs of the unit. When applied to
ferromagnetic materials, cracks or discontinuities occurring across the magnetic
field create leakage fields that will indicate the presence of such discontinuities
when magnetic particles are dusted/bathed on the surface.
Depending on the degree of contact with the test surface, magnetic yokes have a
characteristic leakage field around each leg that becomes visible in the presence
of magnetic particles and this tends to obscure indications in the immediate
vicinity. This area will increase in size as rough surfaces, which result in poor
contact with the legs of the yoke, are tested. For this reason, best indications
obtained midway between the legs where the magnetic field forms a regular
pattern.
The effective magnetic field of the yoke can best be described as a multiplicity
of invisible lines of force extending across the gap between the legs. When
the yoke is applied to a steel plate, the magnetic circuit is closed and the
magnetic field enters the plate and makes its passage through the legs and
across the surface and subsurface of the plate. A discontinuity across the field
cuts the magnetic circuit; the sides of the discontinuity become opposite poles
of a magnet, and a leakage field will occur in the air above the discontinuities.
Magnetic particles are attracted to this leakage field and form an indication.
OPERATION
The operation of the yoke is simple. The legs are positioned and placed against
a part or area to be tested. Make as good a contact as possible at right angles
to the direction of the suspected discontinuities and depress the switch. As an
example: If testing for longitudinal surface cracks in a weld, the legs should
straddle the weld. If the directions of the discontinuities are not known, two
inspections should be done with the yoke turned approximately 90 degrees for
the second inspection.
Using the powder spray bulb (P/N 501232), dust the area between the legs
lightly with a Magnaflux powder of magnetic particles while the yoke is energized.
Inspect area closely for cracks and other defects. This process is repeated until
the entire area of the part is inspected.
The method described should be adhered to if maximum sensitivity is to be
obtained. Experience with different types of parts and/or surfaces will indicate the
extent of the area in which good indications can be obtained.
Because of its portability, dry powder is generally recommended for use
with yokes. The wet method, though, is better for locating fine cracks. Hand
spray guns, and aerosol products, are available for the wet method of particle
application for yoke inspection.
For extreme sensitivity, Magnaflux fluorescent magnetic particles (Magnaglo) are
available in aerosol form also. After bath and current application, a black light
from a portable ultraviolet lamp (EV6000) is used to fluoresce the Magnaglo
particles, producing brilliant yellow-green indications that are easily detected.
Interpretation:
After an indication has been formed, it must be correctly interpreted or identified.
Size, shape, appearance, location and direction of the indication as well as
knowledge of the part history, all assist the experience inspector in correct
interpretation.
Evaluation:
The most important part of an inspection is the decision as to whether a part with
a known discontinuity is acceptable, can be repaired, or must be rejected. This
depends on how the discontinuity affects the serviceability of the part. Many
parts with indications are acceptable.
