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The sensitivity of the pickup coil can be improved by

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The sensitivity of the pickup coil can be improved by using a ferrite core. Tools for designing pickup coils, as well as predicting their performance, are described elsewhere in this volume. Magnetodiodes The magnetodiode is suitable for sensing leakage fields from discontinuities because of its small size and its high sensitivity. Because the coil probe is usually larger than the magnetodiode, it is less sensitive to longitudinally angled discontinuities than the magnetodiode is. However, the coil probe is better than the magnetodiode for large discontinuities, such as cavities. Hall Effect Detectors Hall effect detector probes are used extensively in industry for measuring magnetic flux leakage fields in units of tesla (or gauss). Hall effect detector probes are described in this volume’s chapter on probes for electromagnetic testing. Giant Magnetoresistive Probes Magnetic field sensitive devices called giant magnetoresistive probes, 8,9 at the most basic level, consist of a nonmagnetic layer sandwiched between two magnetic layers. The apparent resistivity of the structure varies depending on whether the direction of the electron spin is parallel or antiparallel to the moments of the magnetic layers. When the moments associated with the magnetic layers are aligned antiparallel, the electrons with spin in one direction (up) that are not scattered in one layer will be scattered in the other layer. This increases the resistance of the device. This is in contrast to the situation when the magnetic moments associated with the layers are parallel where the electrons that are not scattered in one layer are not scattered in the other layer, either. Giant magnetoresistive probes use a biasing current to push the magnetic layers into an antiparallel moment state and the external field is used to overcome the effect of the bias. The resistance of the device, therefore, decreases with increasing field intensity values. Figure 10 shows a typical response of a giant magnetoresistive probe. Magnetic Tape For the testing of flat surfaces, magnetic tape can be used. The tape is pressed to the surface of the magnetized billet and then scanned by small probes before being erased. This technique is sometimes called magnetography . 236 Electromagnetic Testing F IGURE 9. Pickup coil and signal integrator (magnetic flux leakage) output for rectangular discontinuity. 7 30 20 10 0 –10 –20 –30 Output from search coil (mV) Coil position mm (10 –2 in.) Search coil output 0 2 4 6 8 10 (8) (16) (24) (32) (40) 0.3 (3) 0.2 (2) 0.1 (1) 0 Magnetic flux density B , T (kG) Magnetic flux leakage Discontinuity width F IGURE 10. Resistance versus applied field for 2 μm (8 × 10 –5 in.) wide strip of antiferromagnetically coupled, multilayer test object composed of 14 percent giant magnetoresistive material. 9 –32 –16 0 16 32 (–0.4) (–0.2) (0.2) (0.4) 4.2 4.1 4.0 3.9 3.8 3.7 3.6 Resistance (k ) Applied magnetic field, kA·m –1 (kOe)
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In automated systems, magnetic tape can be fed from a spool. The signals can be read and the tape can be erased and reused.
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  • Fall '19
  • Magnetic Field, Magnetic flux leakage

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