ET05.pdf

5 mm 042 in radial distance for three probes a all

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diameter 10.5 mm (0.42 in.) radial distance, for three probes: (a) all three fields; (b) air core coil probe field; (c) cylindrical ferrite core probe field; (d) cup core probe field. Normalized flux density B Y 1.0 0.75 0.5 0.25 0 –0.25 Cup core Ferrite core Air core –75.0 –56.25 –37.5 –18.75 0 18.75 37.5 56.25 75.0 (–3.0) (–2.2) (–1.5) (–0.7) (0.7) (1.5) (2.2) (3.0) Radial distance, mm (in.) Normalized flux density B Y 1.0 0.75 0.5 0.25 0 –0.25 –75.0 –56.25 –37.5 –18.75 0 18.75 37.5 56.25 75.0 (–3.0) (–2.2) (–1.5) (–0.7) (0.7) (1.5) (2.2) (3.0) Radial distance, mm (in.) Normalized flux density B Y 1.0 0.75 0.5 0.25 0 –0.25 –75.0 –56.25 –37.5 –18.75 0 18.75 37.5 56.25 75.0 (–3.0) (–2.2) (–1.5) (–0.7) (0.7) (1.5) (2.2) (3.0) Radial distance, mm (in.) Normalized flux density B Y 1.0 0.75 0.5 0.25 0 –0.25 Radial distance, mm (in.) (a) (b) (c) (d) –75.0 –56.25 –37.5 –18.75 0 18.75 37.5 56.25 75.0 (–3.0) (–2.2) (–1.5) (–0.7) (0.7) (1.5) (2.2) (3.0) M OVIE . Shielded probe.
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145 Probes for Electromagnetic Testing F IGURE 19. Liftoff calculated over nonmagnetic material (Unified Numbering System N06600): (a) simple air core coil; (b) ferrite core probe; (c) cup core probe. Impedance Z ) 7.2 6.6 6.0 5.3 4.7 0 6.0 12.0 18.0 24.0 (0.24) (0.48) (0.72) (0.96) Liftoff, mm (in.) Impedance Z ) 28 24 21 18 15 0 6.0 12.0 18.0 24.0 (0.24) (0.48) (0.72) (0.96) Liftoff, mm (in.) Impedance Z ) 13 11 10 9 8 0 3.0 6.0 9.0 12.0 (0.12) (0.24) (0.36) (0.48) Liftoff, mm (in.) (a) (b) (c) F IGURE 20. Comparison of normalized liftoff curves for three probes over nonmagnetic material. (Z Z min )·( Z max Z min ) –1 1.00 0.75 0.50 0.25 0 0 3.0 6.0 9.0 12.0 (0.12) (0.24) (0.36) (0.48) Liftoff, mm (in.) Air core Ferrite core Cup core F IGURE 21. Comparison of liftoff curves for three probes over nonmagnetic material. Impedance Z ) 28 22 16 11 5 0 3.0 6.0 9.0 12.0 (0.12) (0.24) (0.36) (0.48) Liftoff, mm (in.) Air core Ferrite core Cup core F IGURE 22. Comparison of liftoff curves for three probes over magnetic (carbon steel) material. Impedance Z ) 44 35 26 16 7 0 3.0 6.0 9.0 12.0 (0.12) (0.24) (0.36) (0.48) Liftoff, mm (in.) Air core Ferrite core Cup core
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from unwanted regions. Cup core probes, for example, are actually shielded probes. Thus, there is an increased field gradient under the probe and larger sensitivity is obtained. A very simple shielded probe could be built by covering the coil (with or without a ferrite core) with a sleeve of high permeability, low conductivity material such as ferrite. 16 In active shielding, an active field is generated by means of a coil or system of coils to cancel part of the original field in specific areas. This technique has been used extensively for shielding large structures 17 because magnetic shielding is expensive and bulky for such structures. This technique has not been extensively explored for eddy current tests, perhaps because of the small size of the probes and the ease with which magnetic or eddy current shielding can be achieved. Eddy current shielding uses the skin effect to prevent the magnetic field from extending to its normal limits. At higher frequencies, all that is needed is to enclose the eddy current probe in a conducting shell (usually copper), leaving open the part of the probe that comes in contact with the test material. This has a drastic effect on the field distribution and on the probe impedance. Because a relatively large portion of the magnetic field energy is absorbed in the shield (depending on the shield material and its proximity to the coil), the field is considerably attenuated. Here lies the
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  • Fall '19
  • Wind, The Land, Magnetic Field, Eddy Current Probes, electromagnetic testing

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