rapid deflection and return of the needle. Occasionally, intergranular stress corrosion can occur along a plane roughly
parallel to the part surface. When this type of flaw occurs, needle deflection may be relatively slow.
The distance between scans or the scanning increment is determined by the minimum crack size required to be
detected. For detection of small cracks, the distance between scans should not exceed 0.060 inch. The scanning
procedure is repeated after setting the probe coil at each scanning position until the entire length of the hole has been
inspected. When inspecting multiple layers, inspection should be performed in the materials of both layers adjacent to
each interface. When the specific interface position between layers of similar material is not known, its position may
be established by running the probe down past the interface and marking the position of maximum deflection.
Automatic Bolt Hole Scanning.
Automatic bolt hole scanning should be accomplished in accordance with the scanner manufacturer's recommendations
for operation and the applicable T.O. covering the particular test to be performed.
Evaluation Of Indications.
Beneath the rough surface of many bolt holes, numerous indications are obtained from causes other than cracks.
Indications should therefore be examined carefully to establish if indications could be from cracks or if they are
attributable to other causes. Evaluation can be made on the basis of direction of deflection and rate of deflection.
Indications On Meter Instruments.
Meter deflections obtained during scanning of a hole in a part may be compared to the meter response from the
standard reference. The direction of deflection should be the same for both the reference and the standard. The rate of
deflection and return to nominal reading is rapid for both cracks in the inspection part and the standard when scanning
is performed at recommended speed. Indications from out-of-round conditions, wide grooves, and changes in chemical
composition are relatively slow. Caution must be exercised when inspecting for lamellar conditions because the
response is not typical of fatigue cracks. The magnitude of response at various amounts of lift-off adjustment can also
aid in screening out indications from rapid changes in lift-off.
Indications On Storage Oscilloscope Or Strip Chart Recorder.
The use of a strip chart recorder or storage oscilloscope for recording indications during manual scanning of fastener
holes makes evaluation less subjective. Comparison of rate of deflection from indications in the hole and the reference
can be observed at the same time.
Indications From Automatic Scanner.
The controlled rate of scanning obtained with the automatic scanning unit provides additional improvement in ease of
evaluation. Because of the small scanning increment (pitch of scanner screw), any crack of significant size will be
detected during at least three consecutive revolutions of the scanner. This should result in three or more evenly spaced
indications on the strip chart recorder or storage oscilloscope. If crack-like indications are observed, inspect the hole
visually to determine if the indications are due to obvious deformations such as metal tears or gouges. Gouge
indications, while cyclic in nature, are generally easily recognized due to the fact that such indications usually appear
180 degrees opposite in phase or polarity to crack or slot indications. Additionally, a gouge indication will usually not
be as sharply peaked as an indication from a crack or slot. Careful study must be made of such indications to ensure
that they do not mask an indication of a crack at the bottom of the gouge.
Openings, Large Holes, And Cutouts.
Location And Orientation Of Cracks.
An opening or cutout in a stressed aircraft part serves as a stress riser and a potential source of fatigue cracks and/or
stress corrosion cracks. Fatigue cracks initiate at the edges of an opening, hole, or cutout and grow away from the edge
at right angles to the direction of stress. Stress corrosion cracking usually occurs in sections subject to either an applied
or residual tensile stress. The direction of tensile stresses can often be defined by engineering stress analysis or from
the history of previous cracking in the part. This application covers openings for doors and accesses in aircraft skins,
cutouts at part edges, and attachment holes too large for bolt hole probes.