Tham khảo tài liệu 'volume 17 - nondestructive evaluation and quality control part 6', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 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 mm in. and rotated 360 through each hole to be inspected. The bushing made of a copper alloy had a thickness of about mm 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 .
