Direction and Intensity
As discussed previously, determining the direction
of the field is important when conducting a magnetic particle
inspection because the defect must produce a significant disturbance
in the magnetic field to produce an indication. It is difficult
to detect discontinuities that intersect the magnetic field at
an angle less than 45o. When the orientation of a defect
is not well established, components should be magnetized in a
minimum of two directions at approximately right angles to each
other. Depending on the geometry of the component, this may require
longitudinal magnetization in two or more directions, multiple
longitudinal and circular magnetization or circular magnetization
in multiple directions. Determining strength and direction of
the fields is especially critical when inspecting with a multidirectional
machine. If the fields are not balanced, a vector field will be
produced that may not detect some defects.
Depending on the application, pie gages, QQI's, or a gauss meter
can be used to check the field direction. The pie gage is generally
only used with dry powder inspections. QQI shims can be used in
a variety of applications but are the only method recommended
for use in establishing balanced fields when using multidirectional
The applied magnetic field must have
sufficient strength to produce a satisfactory indication, but
not so strong that it produces nonrelevant indications or limits
particle mobility. If the magnetizing current is excessively high
when performing a wet fluorescent particle inspection, particles
can be attracted to the surface of the part and not be allowed to
migrate to the flux leakage fields of defects. When performing
a dry particle inspection, an excessive longitudinal magnetic
field will cause furring. Furring is when magnetic particles build
up at the magnetic poles of a part. When the field strength is
excessive, the magnetic field is forced out of the part before
reaching the end of the component and the poles along its length
attract particles and cause high background levels. Adequate field
strength may be determined by:
- performing an inspection on a standard
specimen that is similar to the test component and has known
or artificial defects of the same type, size, and location as
those expected in the test component. QQI shims can sometimes
be used as the artificial defects.
- using a gauss meter with a Hall effect
probe to measure the peak values of the tangent field at
the surface of the part in the region of interest. Most specifications
call for a field strength of 30 to 60 gauss at the surface when
the magnetizing force is applied.
- looking for light furring at the ends
of pipes or bars when performing dry particle inspections on these and other uncomplicated shapes.
Formulas for calculating current levels should only be used to
estimate current requirements. The magnetic field strength resulting
from calculations should be assessed for adequacy using one of
the two method discussed above. Likewise, published current level
information should also be used only as a guide unless the values
have been established for the specific component and target defects
of the inspection at hand.
Using a Pie Gage
pie gage is placed copper side up and held in contact with the
component as the magnetic field and particles are applied. Indications
of the leakage fields provide a visual representation of defect
direction within the component. Pie gages work well on flat surfaces,
but if the surface is concave or convex, inaccurate readings may
occur. The pie gage is a flux sharing device and requires good
contact to provide accurate readings.
Using Quantitative Quality Indicator
Quantitative Quality Indicator (QQI) flaw shims are used to establish
proper field direction and to ensure adequate field strength during
technique development. The QQI flaw shim is the most efficient
means of determining balance and effectiveness of fields. The
QQI's are also flux sharing devices and must be properly attach
so as not to allow particles to become trapped under the artificial flaw.
Application using Super glue is the preferred way of attaching
the artificial flaw, but does not allow for reuse of the shims.
Shims can also be attached with tape applied to just the edge
of the shim. It is recommended that the tape be impervious to
oil, not be fluorescent, and be 1/4 to 1/2 inch in width.
The QQI must be applied to locations on the component where the
flux density may vary. One example would be the center area of
a yoke or Y shaped component. Oftentimes, the flux density will
be near zero in this area. If two legs of a Y are in contact with
the pad in circular magnetization, it must be determined if current
is flowing evenly through each leg. A QQI on each leg would be
appropriate under such conditions.
QQI's can be used to establish system threshold values for a
defect of a given size. By attaching a QQI shim with three circles
(40%, 30% and 20% of shim thickness) to the component, threshold values for
a specific area of the component, can be established. Begin by
applying current at a low amperage and slowly increasing it until
the largest flaw is obtained. The flux density should be verified
and recorded using a Hall effects probe. The current is then increased
until the second circle is identified and the flux density is
again recorded. As the current is further increased, the third ring
is identified and the current values are recorded.
Hall Effects Gauss Meter
are several types of Hall effects probes that can be used to measure
the magnetic field strength. Transverse probes are the type most
commonly used to evaluate the field strength in magnetic particle
testing. Transverse probes have the Hall effect element mounted
in a thin, flat stem and they are used to make measurements between
two magnetic poles. Axial probes have the sensing element mounted
such that the magnetic flux in the direction of the long axis
of the probe is measured.
To make a measurement with a transverse probe, the probe is positioned
such that the flat surface of the Hall effect element is transverse
to the magnetic lines of flux. The Hall effect voltage is a function
of the angle at which the magnetic lines of flux pass through
the sensing element. The greatest Hall effect voltage occurs when
the lines of flux pass perpendicularly through the sensing element.
If not perpendicular, the output voltage is related to the cosine
of the difference between 90 degrees and the actual angle. The
peak field strength should be measured when the magnetizing force
is applied. The field strength should be measured in all areas
of the component to be inspected.