In general the test coil is characterized by two

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In general, the test coil is characterized by two electrical impedance quantities: (1) the inductive reactance X L (where the frequency of the alternating current field is in hertz and coil self-inductance L is in henry) and (2) the ohmic resistance R . It is common practice to plot reactance X L as the ordinate and resistance R as the abscissa in the impedance plane. In this way, the test coil impedance Z is represented by a point P formed by two perpendicular components X L and R on the impedance plane. In the absence of a test object, the empty test coil has a characteristic impedance with coordinates X L0 and R 0 shown on the impedance plane by the coil in air, point P 0 of Fig. 2. If the probe is placed on the test object, the original field of the coil in air is modified by the superimposed field of the eddy currents. This field modification has exactly the same effect as would be obtained if the characteristics of the test coil itself had been changed. The influence of the test object can be Z X R = + L 2 2 Z = + j X R L E I R 2 = E I X 1 = L 323 Electromagnetic Techniques for Material Identification P ART 2. Eddy Current Impedance Plane Analysis F IGURE 1. Voltage plane and impedance plane diagrams: (a) voltage plane; (b) resistance inductance circuit; (c) impedance plane. (a) (b) (c) E 1 IX L 0 90 degrees E 2 I X L E R X L 0 Z a R Legend a = phase angle E = voltage I = current R = ohmic resistance X L = inductive reactive of coil with self inductance L Z = impedance phasors IR
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described by a variation in the test coil characteristics. The apparent impedance P 0 of the coil in air is displaced to P 1 (corresponding to new values of X L and R ) under the influence of the test object (Fig. 2). The magnitude and direction of the displacement of the apparent impedance from P 0 to P 1 under the influence of the test object are functions of the properties of the test object and the characteristics of the instrumentation. Significant properties of the test object include: (1) electrical conductivity σ , (2) dimensions of the test object, (3) magnetic permeability μ, (4) standard depth of penetration δ and (5) presence of discontinuities such as cracks. Significant instrument characteristics include (1) frequency f of the alternating current field in the test coil, (2) size and shape of the test coil and (3) distance of the test coil from the test object (liftoff). Liftoff and Edge Effects on Impedance Plane The spacing between the probe and the test object surface is referred to as the liftoff. Figure 3 shows the impedance plane response that occurs when the spacing is increased between the probe and the test object surface. The upper portion of the impedance plane is the magnetic domain, where responses occur from ferromagnetic materials. The lower portion is the domain where responses are obtained from nonmagnetic materials.
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  • Fall '19
  • Magnetism, Magnetic Field, Electrical conductivity

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