T.O. 33B-1-14-29corresponds to a clockwise direction along the loop. The point at which the loop rejoins the curve represents theeffective depth of penetration in the coating.4.3.3.5.6 Cracks,LiftOffandConductivity.The impedance changes due to surface cracks of different depths. The change for cracks will lie between the lift-offand conductivity (s). As the crack depth increases the response moves further from lift-off and closer to decreasingconductivity (s).4.3.4 HeatTreatConditionorHardness.Heat treating (or age hardening) a metal changes its hardness and its electrical conductivity. Just as above, thealuminum alloys have been the most investigated for the hardness/conductivity effect. Again, the impedance change isalong the conductivity curve in the range of 25% IACS to 65% IACS.4.3.4.1 TemperatureChanging the temperature of a part changes its electrical conductivity. All metals become less conductive astemperature rises. This would be seen on the impedance plane as a movement along the conductivity curve toward thezero (air) end of the curve. For aluminum alloys, conductivity decreases about 1% IACS for a 20O F increases intemperature (see Figure 4-24).Figure 4-24. Effect of Temperature IncreaseIf a conductivity meter is being used to check for proper alloy or heat treat condition, the temperature of all parts andcalibration standards must be the same and kept constant. A change in temperature could be interpreted as a change inalloyor hardness,since all three factors may change the conductivityof a metal.4.3.4.2 CrackDetection,NonferromagneticMaterialsAs discussed previously, the amplitude of the response from a surface crack increases as the crack gets deeper. Whenthe crack reaches three standard depths it is interrupting essentially all of the eddy current flow and no increase in
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