T.O. 33B-1-14-674.5.9.2 ImpedancePlaneAnalysisInstruments.The speed of manual scanning with impedance plane analysis instrumentation does not affect signal response becausethe system response time is notlimited by the response of a meter movement.SECTION VISPECIFIC APPLICATIONS4.6 SPECIFICAPPLICATION—FLAWDETECTION.4.6.1 FastenerHolesRemovableFasteners.4.6.1.1 CracksInFastenerHoleWalls.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 progressof these cracks is often slow in the initial stage and early detection can prevent possible catastrophic failure.4.6.1.2 FatigueCracks.Fatigue cracks are usually caused by repeated cyclic loading of a structure at levels of stress less than that required forvisible deformation. Because stress is concentrated at areas of localized weakness, such as holes, fatigue cracks ofteninitiate at suchpoints. The cracks usually propagate normal to the direction of the maximum applied tensile stress.4.6.1.3 StressCorrosionCracks.Stress corrosion cracks occur under the combined influence of a tensile stress and a corrosive environment on amaterial susceptible to stress corrosion cracking. The tensile stress may result from either an applied stress or aresidual stress. Moisture in the air combined with a sufficiently corrosive environment may create stress corrosioncracking in some instances.4.6.1.4 FatigueAndCorrosionCracks.Cyclic fatigue in the presence of corrosion cracks can cause rapid growth of cracks.4.6.1.5 HoleWallFinishAndDimensions.The hole wall finish and dimensions strongly influence both the occurrence and the detectability of cracks in fastenerholes. Hole wall damage such as scratches, chatter and grooves created during manufacturing can create additionalstress concentrations at the hole wall and provide preferred sites for crack initiation. Loose fitting bolts caused byoversize or out-of-round holes allow movement in the area of the hole and allow fatigue action. These same conditionscan influence the reliability of inspection. During inspection, severe damage to the hole wall results in strong eddycurrent indications that may not be separable from crack indications. Excessive lift-off from out-of-round conditionscan also mask indications from cracks. All of these conditions can be created during manufacturing processes onthehole or as a result of fatigue action during service and from bolt removal.4.6.1.6 EdgeEffects.Many cracks infastener holes occur at or near the edge of the hole. Adjoining structures, non-uniform countersink anddeburring 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 fromthe presence of other metals adjacent to the hole edge. Countersunk surfaces also limit eddy current inspection bymanual techniques adjacent to hole edges.4.6.1.7 FastenerHoleInspectionEquipment.A considerable number of fastener holes are still inspected manually using meter type instruments and a standard bolthole probe. For some bolt hole inspections, recorders are used in conjunction with the instrument. Increased use isbeing made of automatic scanning equipment. This equipment provides a hand held scanning unit which drives a
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