For each of the materials the liftoff curves

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material at 10 kHz. For each of the materials, the liftoff curves resulting from depth changes in flat bottom holes were obtained and compared. The excitation levels were assumed to be small, so nonlinearities and hysteresis effects could be neglected. Liftoff Parameter Figures 5 and 6 summarize the results for various probes with respect to liftoff. The impedance change rate is plotted for the absolute probes and the rate of change of induced voltage is plotted for the double-coil probe in Fig. 5; Z 0 and V 0 represent the probe impedance and induced voltage at zero liftoff. Figure 5a shows the rates of change of impedance and induced voltage caused by liftoff in the nonmagnetic material. The curve for the double-coil probe indicates that it provides a greater output voltage over a greater liftoff range than any of the absolute probes. Figure 5b shows similar calculations for carbon steel test objects. The change in the induced voltage in the double probe also occurs over a wider range of liftoff values than for the absolute probes. To clarify the differences in range and linearity of liftoff calculations between the various probes, the normalized curves shown in Fig. 6 are useful. The change in impedance defined as Z = Z Z 0 is normalized with respect to Z max , defined as Z max = Z Z 0 , where Z is the coil impedance at infinite liftoff. Similarly, the change in induced voltage is normalized with respect to V max . The normalized rates of change are defined as Z · Z –1 max and V · V –1 max . The double coil exhibits a more useful range for liftoff measurement 136 Electromagnetic Testing F IGURE 3. Surface probe over conducting surface: (a) small diameter single-coil probe; (b) large diameter single-coil probe; (c) double-coil probe. (a) (b) (c) F IGURE 4. Surface probes in presence of flat bottom holes: (a) small diameter single-coil probe; (b) large diameter single-coil probe; (c) double-coil probe. (a) (b) (c)
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and better linearity than the absolute probe exhibits. Range and linearity are shown for nonmagnetic material in Fig. 6a and for magnetic material in Fig. 6b. Hole Depth Parameter To simulate a test for evaluating the depth of discontinuities in metal, flat bottom holes of various diameters were used with the probes described above. The probe was located over the hole, flush with the metal surface and centered with the hole. The hole depth was then changed and the signal from the probe was plotted. Figure 7 shows the rate of change of impedance or induced voltage corresponding to the change in depth of a 10 mm (0.4 in.) diameter hole. Similarly, Fig. 8 represents the normalized rates of change for the same test. Again, the sensitivity, useful range and linearity are better in the case of the double-coil probe. Noise Reduction Effects When eddy current probes are used for applications such as crack and corrosion detection, the noise caused by liftoff 137 Probes for Electromagnetic Testing F IGURE 5. Finite element prediction of change in impedance Z and induced voltage V due to liftoff changes:
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  • Fall '19
  • Wind, The Land, Magnetic Field, Eddy Current Probes, electromagnetic testing

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