Shielded probes have a cylinder of material which encircles the coil of the probe. This. serves to constrict the probes
field and, therefore, limits the spread of eddy currents from much 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, etc.
18.104.22.168 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.
22.214.171.124 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.
126.96.36.199.1 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 indications.
Common materials for mechanical guides are plastic (polyethylene, acrylic, and polycarbonate), wood, phenolic
impregnated mask (masonite), 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.
188.8.131.52.2 Bolthole Inspection. Manual bolthole inspection probes usually consist of a split 90 degree probe with the
exposed shaft inserted in an adjustable collar. The shaft is marked in increments and the collar secured at the desired
increment by means of a set screw through the collar. The probe is then rotated 360 degrees around the hole at each
setting until the entire surface of the bore has been inspected. These probes are available in federal or commercial
catalogs or can be locally manufactured. (Refer to TM 55-1500-335-23.)
184.108.40.206.3 Scanning Fillets and Radii. Using appropriate radius probe, scan fillets and radii several times in each