T.O. 33B-1-14-284.3.3.5.4 ThicknessVariations.When the part thickness is less than the effective depth of penetration of the test coil at the inspection frequencyemployed, the impedance curve departs from the conductivity curve as shown in (Figure 4-23) Typically, there is anincrease in the resistive component of the impedance with thinner parts, as compared to parts that have thicknessesequal to or greater than the effective depth of penetration (see paragraph 4.2.3.3.2). As the thickness of the partsincrease and approach more closely the effective limit of penetration, the curve tends to spiral as it approaches the endpoint (T=1) on the conductivity curve, where T equals the ratio of the specimen thickness to the effective depth ofpenetration in that specimen.Figure 4-23. Impedance Diagram Showing the Effect of Specimen Thickness.4.3.3.5.5 ConductiveLayers.The impedance curve for thin conductive layers on a substrate of different conductivity is also represented as adeviation in the impedance curve for conductivity. The impedance for the layered material departs from theconductivity curve at the value corresponding to the substrate conductivity and forms a loop that rejoins theconductivity curve at the conductivity of the metal in the outer layer. Increasing thickness of the outer layer
Integrated Publishing, Inc. - A (SDVOSB) Service Disabled Veteran Owned Small Business
