T.O. 33B-1-14.5.5.9DetectionofCracksUnderMetallicCoatings.The detection of cracks under metallic plating and coating is similar to detection of subsurface f laws. Themagnitude of the total response consistently decreases with increasing coating thickness. With meter typeinstrumentation with a constant frequency test system, the thickness of plating or coating through whichcracks can be detected decreases with increasing plating conductivity and/or magnetic permeability. Ingeneral, decreasing frequency permits detection of larger cracks under thicker coatings because of theincreased depth of penetration. Detection of cracks under metallic coatings with phase analysis instrumenta-tion using the impedance plane diagram can be performed with more accuracy and sensitivity than withmeter instruments because phase information can be measured. Recent research has shown that multifre-quency eddy current systems may find application for detecting and measuring cracks under metalliccoatings.4.5.6EffectsofScanningTechniquesonDetection.4.5.6.1InspectionTechnique.Consistent positioning of the probe in relation to edges and interfaces during calibration and scanning shouldbe established to ensure maximum response from f laws with minimum interference from other sources ofindications. If conditions are known to exist which may result in false indications or which could mask trueindications from f laws, these conditions should be noted in the procedure and a means of interpreting orevaluating the false indications provided. In performing eddy current inspection of an area, the distancebetween scans or between measurements must be selected to ensure complete coverage for the minimum sizef law or variation in properties to be detected. In determining maximum distance between scans, considera-tion must be given to the change in magnitude of f low response as the probe coil center position increases indistance from the center of the crack.4.5.6.2ScanningSpeed.The scanning speed employed in eddy current inspection for cracks is related to the type of equipment utilizedand the inspection technique. Slowest scanning speeds are necessary when the inspector is required to read ameter while manually directing the probe in the specified scanning pattern. The rate of scanning is primarilydetermined by the damping of the instrument meter. Meter damping prevents excessive vibration andoscillation of the meter needle and provides errors in instruments readings. The damping effect reduces theFigure 4-46.Effect of Scanning Speed on Meter Deflection from a CrackChange 34-57
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