Eddy current inspection.
Eddy current inspection is used to detect surface cracks, pits, subsurface cracks, corrosion on inner surfaces,
and to determine alloy and heat-treat condition.
Eddy Current Inspection equipment and instruments.
A wide variety of eddy current test equipment is available. The eddy current test instrument performs three
basic functions: generating, receiving, and displaying. The generating portion of the unit provides an alternating
current to the test coil. The receiving section processes the signal from the test coil to the required form and
amplitude for display.
Instrument outputs or displays consist of a variety of visual, audible, storage, or transfer techniques
utilizing meters, video displays, chart recorders, alarms, magnetic tape, computers, and electrical or electronic
relays.
Principles of Operations. Eddy currents are induced in a test article when an alternating current is applied to
a test coil (probe). The alternating current in the coil induces an alternating magnetic field in the article which
causes eddy currents to flow in the article.
Flaws in or thickness changes of the test-piece influence the flow of eddy currents and change the impedance of the
coil accordingly. Instruments display the impedance changes either by impedance plane plots or by needle
deflection.
A wide variety of eddy current coils and probes is available. Coils and probes are not always interchangeable
between various types of instruments and, for best results, should be matched to a specific instrument and
frequency range. Special probe holders can be fabricated to facilitate eddy current inspection of contoured or
shaped parts including part edges.
Eddy current techniques are particularly well-suited for detection of service-induced cracks in the field.
Service-induced cracks in aircraft structures are generally caused by fatigue or stress corrosion.
Both types of cracks initiate at the surface of a part. If this surface is accessible, a high-frequency
eddy current inspection can be performed with a minimum of part preparation and a high degree of sensitivity. If
the surface is less accessible, such as in a subsurface layer of structure, low-frequency eddy current inspection
can usually be performed. Eddy current inspection can usually be performed without removing surface coatings such
as primer, paint, and anodic films.
Eddy current inspection has the greatest application for inspecting small localized areas where possible crack
initiation is suspected rather than for scanning broad areas for randomly-oriented cracks. However, in some
instances it is more economical to scan relatively large areas with eddy current rather than strip surface
coatings, inspect by other methods, and then refinish.
A reference standard is required for the calibration of Eddy Current inspection equipment. A reference standard
is made from the same material as that which is to be tested. A reference standard contains known flaws or cracks
and could include items such as: a flat surface notch, a fastener head. a fastener hole, or a countersink hole
before performing the eddy current corrosion inspection.
Corrosion inspection.
Eddy current inspection is used to detect the loss of metal as a result of corrosion. An estimation of material
loss due to corrosion can be made by comparison with thickness standards. When establishing eddy current
inspection procedures, where no written procedures are available, the following factors must be considered: type of
material to be inspected; accessibility of the inspection area; material or part geometry, the signal-to-noise
ratio, test system; lift-off effects, location and size of flaws to be detected; scanning pattern; scanning speed;
and reference standards. All of these factors are interrelated.
Therefore, a change in one of the factors may require changes in other factors to maintain the same level of
sensitivity and reliability of the eddy current inspection procedure. Written procedures should elaborate on these
factors and place them in their proper order.
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