T.O. 33B-1-14-23secondary magnetic field from the induced eddy currents. This is equivalent to increasing both the inductance andresistance of the coil. In this manner changes in a part that affect either the strength of the magnetic field at the surfaceof the part or the strength and distribution of the eddy currents in the part, change the apparent impedance of the testcoil(s). These variations in current flow, both phase and amplitude, can be detected, amplified, displayed and analyzedas eddy current test results. The amplitude and phase changes in the signals can be related to changes in the partsinspected.4.3.3 ImpedanceDiagrams.4.3.3.1 Purpose.The impedance diagram shows how changes in eddy current test variables change the apparent impedance of a coil.Typical variables displayed are electrical conductivity, relative magnetic permeability, fill-factor or lift-off, partthickness, and test frequency. Impedance diagrams are very useful for determining optimum inspection parameters andunderstanding eddy current results when more than one variable is changing.4.3.3.1.1The vector representation of inductive reactance on the y-axis and resistance on the x-axis of Figure 4-16 is the basis ofthe impedance diagram. Let the point A represent the impedance of a test coil while on a part. If the probe is moved toa place on the part with a flaw, the impedance will change. This new impedance can be represented by the point B, asshown in Figure 4-18. Each change in the impedance will create a new point on the diagram.Figure 4-18. Vector Representation of Impedance4.3.3.2 DevelopmentofanImpedanceDiagram.To make the impedance diagram into a useful tool for understanding eddy current testing, it is necessary tosystematically change a single test parameter such as conductivity, and observe the changes in the impedance. Usingan eddy current instrument with a two dimensional graphical display (such as the NORTEC-19EII, MIZ-12, MIZ-17, orMIZ-20), a surface probe, a piece of ferrite (a nonconductive, ferromagnetic ceramic) and several nonmagnetic metalspecimens representing a range of conductivity’s from low (titanium, Inconel) to high (copper, silver), approximateimpedance diagrams can be developed and demonstrated. The specimens must have clean, flat, and bare surfaces.When the eddy current probe is held away from the part (in the air) and the instrument is nulled, an indication (spot)
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