Eddy Current Inspection
Electromagnetic analysis is a term describing the broad
spectrum of electronic test methods involving the intersection of magnetic
fields and circulatory currents. The most widely used technique is the eddy
current.
Eddy currents are
composed of free electrons under the influence of an induced electromagnetic
field that are made to “drift” through metal.
Eddy current is used to detect surface cracks, pits,
subsurface cracks, corrosion on inner surfaces, and to determine alloy and
heat-treat condition.
Eddy current is used in aircraft maintenance to inspect jet
engine turbine shafts and vanes, wing skins, wheels, bolt holes, and spark plug
bores for cracks, heat, or frame damage. Eddy current may also be used in
repair of aluminum aircraft damaged by fire or excessive heat.
Different meter
readings are seen when the same metal is in different hardness states. Readings
in the affected area are compared with identical materials in known unaffected
areas for comparison. A difference in readings indicates a difference in the
hardness state of the affected area.
In aircraft
manufacturing plants, eddy current is used to inspect castings, stampings,
machine parts, forgings, and extrusions. a technician performing an eddy
current inspection on a fan blade.
Basic Principles
When an alternating current (AC) is passed through a coil,
it develops a magnetic field around the coil, which in turn induces a voltage
of opposite polarity in the coil and opposes the flow of original current. If
this coil is placed in such a way that the magnetic field passes through an
electrically conducting specimen, eddy currents are induced into the specimen.
The eddy currents
create their own field that varies the original field’s opposition to the flow
of original current. The specimen’s susceptibility to eddy currents determines
the current flow through the coil.
The magnitude and phase of this counter field is dependent
primarily upon the resistance and permeability of the specimen under
consideration and enables us to make a qualitative determination of various
physical properties of the test material.
The interaction of
the eddy current field with the original field results is a power change that
can be measured by utilizing electronic circuitry similar to a Wheatstone
bridge.