In general, the test coil is characterizedby two electrical impedance quantities:(1) the inductive reactance XL(where thefrequency of the alternating current fieldis in hertz and coil self-inductance Lis inhenry) and (2) the ohmic resistance R.It is common practice to plot reactanceXLas the ordinate and resistance Ras theabscissa in the impedance plane. In thisway, the test coil impedance Zisrepresented by a point Pformed by twoperpendicular components XLand Ronthe impedance plane. In the absence of atest object, the empty test coil has acharacteristic impedance with coordinatesXL0and R0shown on the impedanceplane by the coil in air, point P0of Fig. 2.If the probe is placed on the test object,the original field of the coil in air ismodified by the superimposed field of theeddy currents. This field modification hasexactly the same effect as would beobtained if the characteristics of the testcoil itself had been changed. Theinfluence of the test object can beZXR=+L22Z=+j XRLEI R2=EI X1=L323Electromagnetic Techniques for Material IdentificationPART2.Eddy Current Impedance Plane AnalysisFIGURE1.Voltage plane and impedance plane diagrams:(a) voltage plane; (b) resistance inductance circuit;(c) impedance plane.(a)(b)(c)E1IXL090degreesE2IXLERXL0ZaRLegenda= phase angleE= voltageI= currentR= ohmic resistanceXL= inductive reactive of coil with self inductance LZ= impedance phasorsIR
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described by a variation in the test coilcharacteristics. The apparent impedanceP0of the coil in air is displaced to P1(corresponding to new values of XLand R)under the influence of the test object(Fig. 2).The magnitude and direction of thedisplacement of the apparent impedancefrom P0to P1under the influence of thetest object are functions of the propertiesof the test object and the characteristics ofthe instrumentation. Significantproperties of the test object include:(1) electrical conductivity σ,(2) dimensions of the test object,(3) magnetic permeability μ, (4) standarddepth of penetration δand (5) presence ofdiscontinuities such as cracks. Significantinstrument characteristics include(1) frequency fof the alternating currentfield in the test coil, (2) size and shape ofthe test coil and (3) distance of the testcoil from the test object (liftoff).Liftoff and Edge Effects onImpedance PlaneThe spacing between the probe and thetest object surface is referred to as theliftoff.Figure 3 shows the impedanceplane response that occurs when thespacing is increased between the probeand the test object surface. The upperportion of the impedance plane is themagnetic domain, where responses occurfrom ferromagnetic materials. The lowerportion is the domain where responses areobtained from nonmagnetic materials.
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