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9/8/98 AC 43.13-1B (3) A reference standard is required for the calibration of Eddy Current test equipment. A reference standard is made from the same material as that which is to be tested. A refer- ence standard contains known flaws or cracks and could include items such as: a flat surface notch, a fastener head, a fastener hole, or a countersink hole. 5-29. SUBSURFACE INSPECTION. Eddy current inspection techniques are used to inspect for subsurface cracks such as those shown in figure 5-7. The following are typical eddy current equipment requirements for sub- surface crack inspections. a. Use a variable frequency instrument with frequency capability from 100 Hz to 500 MHz. b. The probe used would be a low- frequency; spot, ring, or sliding probe. c. Use a reference standard appropriate for the inspection being performed. 5-30. CORROSION INSPECTION. Eddy current inspection is used to detect the loss of metal as a result of corrosion. An esti- mation of material loss due to corrosion can be made by comparison with thickness standards. Figure 5-8 shows typical structural corrosion that may be detected by the use of eddy current inspection. Remove all surface corrosion before performing the eddy current corrosion inspection. The following are typical eddy current equipment requirements for corrosion inspection. a. Use a variable frequency instrument with frequency capability from 100 Hz to 40 kHz. b. Use a shielded probe with coil diame- ter between 0.15 and 0.5 inch and designed to operate at the lower frequencies. c. A reference standard made from the same alloy, heat treatment, and thickness as the test structure will be required. 5-31. ESTABLISHING EDDY CURRENT INSPECTION PROCEDURES. When es- tablishing eddy current inspection procedures, where no written procedures are available, the following factors must be considered: type of material to be inspected; accessibility of the inspection area; material or part geometry, the signal-to-noise ratio, test system; lift-off ef- fects, location and size of flaws to be detected; scanning pattern; scanning speed; and refer- ence standards. All of these factors are inter- related. Therefore, a change in one of the fac- tors may require changes in other factors to maintain the same level of sensitivity and reli- ability of the eddy current inspection proce- dure. Written procedures should elaborate on these factors and place them in their proper or- der. Par 5-28 Page 5-17PDF Image | AFS-640
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