Volume 17 - Nondestructive Evaluation and Quality Control Part 6 doc

Fig 52 Eddy current inspection of cracks located under installed bushings (a) Schematic of typical assembly employing interference-fit bushings in a clevis/lug attachment assembly (b) Reference standard incorporating an electrical discharge machined corner notch (c) Probe coil positioned in bolthole and encircled by bushing (d) CRT display of a crack located under a ferromagnetic bushing Source: Ref 13 A reference standard was made from material of the proper thickness, and the electrical discharge machined corner notch was made at the edge of the appropriate-size hole The bushing was then installed in the reference standard, as shown in Fig 52(b) The proper-size bolthole probe was selected and inserted into the bushed hole, and the operating frequency was selected to allow the eddy current to penetrate through the bushing in order to detect the notch (Fig 52c and d) After calibration, the bolthole probe was inserted into the appropriate bushed hole in the lug or crevis on the aircraft The probe was inserted at increments of about 1.59 mm (0.0625 in.) and rotated 360° through each hole to be inspected The bushing, made of a copper alloy, had a thickness of about 1.5 mm (0.060 in.) and a conductivity between 25 and 30% IACS, which is easily penetrated at a frequency of 1 to 2 kHz Example 14: Detection of Fatigue Cracks in Aircraft Splice Joints Surface and subsurface fatigue cracks usually occur at areas of high stress concentration, such as splice joints between aircraft components or subassemblies High-frequency (100 to 300 kHz) eddy current inspection was performed to detect surface cracks with shielded small-diameter probes A reference standard was made from typical materials, and a small electrical discharge machining notch was placed at the corner of the external surface adjacent to a typical fastener The high-frequency probe was scanned around the periphery of the fastener using a circle template for a guide, as illustrated in Fig 53(a) Fig 53 High-frequency eddy current inspection of surface and subsurface cracks in aircraft splice joints (a) Calibration procedure involves introducing an electrical discharge machining notch in the reference standard to scan the fastener periphery using a circle template to guide the probe (b) CRT trace on an oscilloscope of typical cracks in both skin and spar cap sections shown in (c) Source: Ref 13 When subsurface cracks are to be detected, low-frequency eddy current techniques are employed Basically stated, the thicker the structure to be penetrated, the lower the eddy current operating frequency that is required However, the detectable flaw size usually becomes larger as the frequency is lowered Example 15: Hidden Subsurface Corrosion in Windowbelt Panels There are various areas of the aircraft where subsurface (hidden) corrosion may occur If such corrosion is detected, usually during heavy maintenance teardown, a nondestructive testing method can be developed to inspect these areas in the remainder of the fleet Following is an example of subsurface corrosion detected by low-frequency (