Ω 10 percent ω 30 percent ω 50 percent ω 70

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ω = 10 percent ω = 30 percent ω = 50 percent ω = 70 percent ω = 90 percent ω = 0 percent ω = 20 percent ω = 40 percent ω = 60 percent ω = 80 percent ω = 100 percent Support plate Tube ω Coils No magnetite Full of magnetite F IGURE 42. Modeling of buildup from one side of support plate with portion of gap clean. ω = 10 percent ω = 30 percent ω = 50 percent ω = 70 percent ω = 0 percent ω = 20 percent ω = 40 percent ω = 60 percent Support plate d Tube ω Coils No magnetite Legend d = 0.1 mm (0.004 in.) ω = magnetite ω 1 = 10 percent F IGURE 43. Modeling of denting in tube without presence of magnetite, representative of flushed gap d . d = 0 d = 0.025 mm (0.001 in.) d = 0.05 mm (0.002 in.) d = 0.075 mm (0.003 in.) d = 0.1 mm (0.004 in.) Tube Coils Support plate d
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The geometries modeled above indicate the extent and versatility of the numerical model. There still remains the question of comparison with real, known data. The experimental measurement of a support plate signal in the presence of a clean support plate is only part of the answer. Most of the trajectories in Figs. 38 to 43 cannot be reproduced experimentally because of the difficulty of building experimental setups that reflect test conditions. To partly answer the question of comparisons with known data, the experiment in Fig. 44 was carried out. Here, a 25 mm (1 in.) thick support plate was drilled to provide a 1.5 mm (0.06 in.) gap, which was then packed axially with magnetite powder at about 30 percent (Fig. 44a), 60 percent (Fig. 44b) and 90 percent (Fig. 44c) of the gap length. The resemblance to corresponding numerical trajectories is immediately evident. More convincing evidence of the ability, accuracy and usefulness of numerical modeling is provided in Fig. 45. These impedance plane trajectories were obtained during the chemical flushing of a model boiler. The data are from a tube with a 0.1 mm (0.004 in.) radial dent at various stages of flushing. Figure 45a was taken before flushing began and shows an identifiable dent filled with magnetite. As the flushing progresses, the gap shows various stages of cleaning. Thus, for example, in Fig. 45b, one side of the gap is almost completely clean, as indicated by the large lower lobe. The rest is still packed with magnetite, indicating an uneven flushing from both sides of the support plate. Figure 45c shows a clean gap while the dent in the tube is visible. This particular experiment provides a convincing experimental confirmation of the numerical model and demonstrates its value in interpreting data and in monitoring the flushing process. Conclusions Computer modeling of eddy current testing phenomena has evolved from an experimental state through more sophisticated analytical techniques into general numerical models. The three techniques of modeling each have advantages and shortcomings but the numerical technique possesses the generality needed to model the intricacies of interactions between field and discontinuity.
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  • Fall '19
  • Wind, The Land, Magnetic Field, Dodd, Modeling of Electromagnetic Testing

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