T.O. 33B-1-14-89f. Check all measured values against the tolerances specified by the written procedure. All abnormalvalues should be reported as required by the procedure.4.8.2.18 PlatingThicknessReferenceStandards.Reference standards for plating thickness measurements must have the same electrical conductivity, magneticpermeability, and geometry as the part. These requirements apply to both the base material and the plating. Electricalconductivity and magnetic permeability for the base material are usually obtained by using the same alloy and temperfor the standards as used in the part. Particular care should be taken in processing the materials to ensure that similarproperties are obtained. The surface finishes of the part and standard should also be alike. To obtain the sameelectrical conductivity, magnetic properties, and surface finish for plating on the parts and reference standards; theplating must be performed in baths of similar composition and subject to similar controls. If the plating on the part isstress-relieved prior to thickness measurement, the references should receive the same treatment. Several methods ofdetermining plating thickness on reference standards can be used. One of these is to carefully measure the thicknessprior to plating and again after plating. The difference represents the thickness of the plating (plating is applied to oneside only). A second method is to measure the plating on an adjacent section of the standard by removal of arnetallographic specimen. The total thickness of the plating plus substrate must exceed the effective depth ofpenetration in the part. A total thickness of 2.5 to 3 combined standard depths of penetration is usually consideredsufficiently thick. This thickness may be determined by adding the standard depth of penetration in the plating and thesubstrate at the frequency used. For example, if approximately 0.003-inch thick sliver plating on aluminum is to bemeasured at 200 KHz, the minimum total thickness can be determined as follows:a. The standard depth of penetration of silver at a frequency of 200 KHz is 0.007 inch. Therefore, the0.003-inch of silver in the plating represents 0.4 standard depth of penetration.b. The 2024-T3 aluminum base material must be at least 2.5 - 0.4 = 2.1 standard depths of penetration.If the conductivity and magnetic permeability of a metal are known, the standard depth of penetration can bedetermined.4.8.3 MeasurementofNonconductiveCoatings.4.8.3.1 NonconductiveCoatings.A wide variety of nonconductive coatings are applied to military hardware. Primers, paints, and plastics and sealantsare widely used to protect metals from corrosion. Anodic coatings are used on metals, particularly aluminum, toprevent surface deterioration. Other oxide coatings provide protection against heat or wear. Boron epoxy laminatesincrease stiffness and strength. To control the thickness of such nonconductive coatings or to measure their loss duringservice, eddy current inspection techniques have been employed with a high degree of accuracy.4.8.3.2 BasisforMeasurementofNonconductiveCoatings.The determination of thickness of nonconductive layers ormaterials is a relative measure of the magnetic couplingbetween the probe and the underlying conductive material. In other terms, the thickness of a nonconductor is a directmeasurement of lift-off or the spacing between the probe and the conductor. Because the properties (electricalconductivity, magnetic permeability, and geometry) of the underlying materials affect the signal detected by the probe,they must be constant or their variation minimized by instrument adjustment. Three requirements for measurement ofnonconductive coatings by eddy current techniques are: (1) the nonconductive coating must be in intimate contact witha conductive material; (2) the thickness of the coating must be less than the effective range of the varying magneticfield generated by the probe; and (3) the thickness of the substrate must be at least 2.5 times the standard depth ofpenetration at the frequency employed.
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