T.O. 33B-1-14-494.5.1.2 SensitivityAndReliabilityOfCrackDetection.In establishing eddy current procedures for crack detection, the following factors must be considered:a. Location and size of cracks to be detected.b. Type of material to be inspected.c. Accessibility of inspection area.d. Test system capabilities.4.5.1.3 FlawDetection.Service-induced cracks in aircraft structures are generally caused by fatigue or stress corrosion. Both types of cracksinitiate at the surface of a part. If this surface is accessible, either by direct surface contact or by penetration of the eddycurrent field through the material, eddy current inspection can be performed with a minimum of part preparation and ahigh degree of sensitivity. Unlike penetrant inspection, eddy current inspection can usually be performed withoutremoving such surface coatings as primer, paint, and anodic film.4.5.1.4 InspectionMaterial.The material from which the inspection part is fabricated is of primary importance in determining the possibility ofeddy current inspection and the limitations involved. The conductivity and magnetic permeability influence frequencyrequirements, instrument choice, signal-to-noise ratio, and resulting sensitivity and reliability of inspection. Forexample, aluminum, a material of intermediate conductivity, can be inspected for discontinuities up to 1/8" below thesurface with modern instruments capable of operation at 1 KHz frequency. Titanium, a lower conductivity metal,would require higher frequencies in the range of 1 MHz to obtain optimum sensitivity. If surface cracking is to bedetected in ferromagnetic material, a high frequency can be utilized to limit penetration and thus eliminatepermeability problems (i.e. noise) common to ferromagnetic material testing.4.5.1.5 Accessibility.Most of the eddy current equipment presently available for use in the field is of the small, portable, battery poweredtype. This instrumentation permits operation in relatively tight quarters. However, eddy current inspection is onlyfeasible for surface or near surface conditions because of its limited depth of penetration. For this reason, direct accessto the surface to be inspected is usually preferred. Inspection from the opposite surface is limited to materials with lowconductivity and magnetic permeability of 1. Sufficient freedom of movement must be available in the area to beinspected to allow positioning and movement of the probe to detect or measure the specified variable. The inspectionarea must be visible to enable the inspector to determine the position of the probe. Alternatively, a special probe, afixture, or a guide can be used to position and hold probes in the required location. The extent of disassembly requiredfor inspection should be defined in applicable written procedures.4.5.2 TestSystems.4.5.2.1 CrackDetection.The test system for crack detection includes the probe (or probes), the eddy current instrument, any additional recordingor measuring instruments, and calibration standards. For most crack detection applications, general purpose probesmanufactured for the specific test instrument should be employed. A wide variety of eddy current units are fabricatedfor general purpose or specific applications. General purpose instruments are used for flaw detection. For some fieldapplications, a small, lightweight, battery-powered meter output instrument is desirable. More critical inspections mayrequire detailed phase and amplitude information which is most easily obtained with the scope type of eddy currentinstrument. Also the storage scope-type of instrument and magnetic tape recording systems are very useful forautomated, high scan speed testing.4.5.2.2 ProbeSelection.The primary consideration in selecting an eddy current probe is the type of inspection being performed. To detect smallcracks, a probe coil of small diameter with a ferrite core is desirable to concentrate the induced field into a small
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