Equipment is standardized on a test block (reference block) which is constructed of a known material that contains known
good areas, and either simulated or actual defects of known size. The response of the equipment (eddy current machine
and probe) to the good material is set as the starting point by nulling the equipment on the sound area of the block. By
this action, all subsequent readings represent deviations from the null point and have both magnitude and direction.
Careful manipulation of the controls allows the operator to separate the response (deviation from the null point) for lift-off
and flaw (geometric) effects.
Shielded probes have a cylinder of material which encircles the coil of the probe. This serves to constrict the probes field
and, therefore, reduces the spread of eddy currents beyond the probe's diameter. This concentrated electrical field is
most useful for scanning around fasteners, near edges, and into specific small areas. Other types of probes are used for
wide area scans, alloy sorting, conductivity comparisons, coating thickness comparisons, skin thickness comparisons,
Safety Precautions During Eddy Current Inspection . Follow safety precautions and instructions
contained in this manual and the Nondestructive Inspection Methods manual listed in Table 1-1.
Electrical equipment shall not be operated in areas where combustible gases or vapors may be
present, unless the equipment is explosion-proof.
Eddy Current Scanning Techniques. Eddy current inspection is performed by moving the probe over and
as close as possible to the surface of the area of interest. If the coil(s) pass over a defect like a crack, the impedance of
the coil will change and be represented as a movement of the "flying spot." Before beginning the inspection, the operator
will have separated the response from lift-off and from a flaw by using the test block and manipulating the controls.
Therefore, the crack response will be essentially similar to the response from the known defect and different from the
response from lift-off. Microprocessor controlled instruments have the ability to store responses in memory. Such stored
responses are an invaluable teaching aid.
Scanning Around Fasteners, Inserts, and Edges of Parts. Shielded probes are recommended any time
that the pattern the eddy current field is likely to extend out such that it comes in contact with a feature which would mask
the response from a defect. Such features may include edges, fasteners, dissimilar materials attached to the test piece,
etc. An unshielded probe can be used around such features, but the effect of those features must be made constant by
keeping the distance between the probe and the feature constant. Non-conductive mechanical guides (straight edges,
plugs, spacers, etc.) can be used to maintain a constant distance. In fact, the use of non-conductive mechanical guides
is useful for shielded and unshielded probes alike. As operators gain experience, they become quite innovative in
making guides that maintain constant lift-off, angles, and distance from features which may mask flaw indication.
Common materials for mechanical guides are plastic (polyethylene, acrylic, and polycarbonate), wood, phenolic
impregnated material, and resins for casting into shapes (epoxy, polyester, or hot glue). Careful selection of probes and
construction of suitable mechanical guides will make possible inspection of problem areas such as sharp edges, tight
radii, small openings, and areas near potentially masking features.