T.O. 33B-1-1
4-67
4.5.9.2
Impedance Plane Analysis Instruments.
The speed of manual scanning with impedance plane analysis instrumentation does not affect signal response because
the system response time is not limited by the response of a meter movement.
SECTION VI
SPECIFIC APPLICATIONS
4.6
SPECIFIC APPLICATION — FLAW DETECTION.
4.6.1
Fastener Holes Removable Fasteners.
4.6.1.1
Cracks In Fastener Hole Walls.
A common application of eddy current inspection in aircraft structures is the detection of cracks in fastener hole walls.
These cracks are usually generated by fatigue, stress corrosion, or a combination of fatigue and corrosion. The progress
of these cracks is often slow in the initial stage and early detection can prevent possible catastrophic failure.
4.6.1.2
Fatigue Cracks.
Fatigue cracks are usually caused by repeated cyclic loading of a structure at levels of stress less than that required for
visible deformation. Because stress is concentrated at areas of localized weakness, such as holes, fatigue cracks often
initiate at such points. The cracks usually propagate normal to the direction of the maximum applied tensile stress.
4.6.1.3
Stress Corrosion Cracks.
Stress corrosion cracks occur under the combined influence of a tensile stress and a corrosive environment on a
material susceptible to stress corrosion cracking. The tensile stress may result from either an applied stress or a
residual stress. Moisture in the air combined with a sufficiently corrosive environment may create stress corrosion
cracking in some instances.
4.6.1.4
Fatigue And Corrosion Cracks.
Cyclic fatigue in the presence of corrosion cracks can cause rapid growth of cracks.
4.6.1.5
Hole Wall Finish And Dimensions.
The hole wall finish and dimensions strongly influence both the occurrence and the detectability of cracks in fastener
holes. Hole wall damage such as scratches, chatter and grooves created during manufacturing can create additional
stress concentrations at the hole wall and provide preferred sites for crack initiation. Loose fitting bolts caused by
oversize or out-of-round holes allow movement in the area of the hole and allow fatigue action. These same conditions
can influence the reliability of inspection. During inspection, severe damage to the hole wall results in strong eddy
current indications that may not be separable from crack indications. Excessive lift-off from out-of-round conditions
can also mask indications from cracks. All of these conditions can be created during manufacturing processes on the
hole or as a result of fatigue action during service and from bolt removal.
4.6.1.6
Edge Effects.
Many cracks in fastener holes occur at or near the edge of the hole. Adjoining structures, non-uniform countersink and
deburring radii, and damage at the hole edges increase the background noise and decrease the signal-to-noise ratio.
This leads to a general loss of detection of cracks at the edge of holes. Further effects on crack detectability result from
the presence of other metals adjacent to the hole edge. Countersunk surfaces also limit eddy current inspection by
manual techniques adjacent to hole edges.
4.6.1.7
Fastener Hole Inspection Equipment.
A considerable number of fastener holes are still inspected manually using meter type instruments and a standard bolt
hole probe. For some bolt hole inspections, recorders are used in conjunction with the instrument. Increased use is
being made of automatic scanning equipment. This equipment provides a hand held scanning unit which drives a
