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AC 43.13-1B 9/8/98 FIGURE 5-5. Typical instrument displays. Service-induced cracks in aircraft structures are generally caused by fatigue or stress corro- sion. Both types of cracks initiate at the sur- face of a part. If this surface is accessible, a high-frequency eddy current inspection can be performed with a minimum of part preparation and a high degree of sensitivity. If the surface is less accessible, such as in a subsurface layer of structure, low-frequency eddy current in- spection can usually be performed. Eddy cur- rent inspection can usually be performed with- out removing surface coatings such as primer, paint, and anodic films. Eddy current inspec- tion has the greatest application for inspecting small localized areas where possible crack ini- tiation is suspected rather than for scanning broad areas for randomly-oriented cracks. However, in some instances it is more eco- nomical to scan relatively large areas with eddy current rather than strip surface coatings, inspect by other methods, and then refinish. 5-28. SURFACE INSPECTION. Eddy current inspection techniques are used to in- spect for surface cracks such as those shown in figure 5-6. FIGURE 5-6. Typical surface cracks. a. Equipment Requirements. The fol- lowing are typical eddy current equipment re- quirements for surface crack inspections. (1) Instruments must meet the liftoff and sensitivity requirements of the applicable NDI procedures. The frequency requirement is generally 100 Hz to 200 kHz. (2) Many types of probes are available such as: flat-surface; spring-loaded; pencil; shielded pencil; right-angle pencil; or fastener hole probes. Page 5-16 Par 5-27PDF Image | AFS-640
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