342 electromagnetic testing p art 5 electrical

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342 Electromagnetic Testing P ART 5. Electrical Resistivity Measurements 1 F IGURE 23. Probe arrangement and schematic diagram of resistivity measuring instrument. I V I S S S W Legend I = current ( A ) through sample S = distance (meter) between probes V = potential difference (volt) W = sample thickness (meter) F IGURE 24. Typical resistivity measuring system. Current source Precision amplifier and signal processing Digital readout Thickness calibration Sample
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there is one flat surface and that the overall sample size is large compared to the probe spacing. Figure 24 shows a block diagram of a typical instrument. The current source puts a known current through the sample, the signal processing electronics remove interfering voltages and the sample thickness (or probe spacing) is accounted for by an analog multiplier. The circuit output voltage is read by a digital volt meter. The calibration is adjusted so that the output reading is directly in microohm meter or microohm centimeter. A minor circuit change could make the output read in a conductivity unit. Applications of Resistivity Measurements This technique can be very useful when absolute resistivity readings are required. There may be cases however when test object geometry, material thickness or probe spacing will not be within the constraints of the theory. In these instances for which no theoretical correction factors are available, the application may be solved empirically. Sorting for mixed material is a good example of an application that usually requires only relative readings. Theoretical correction factors for some applications are available from several sources. Resistivity changes arising from the heat treatment (and subsequent precipitation and recrystallization) of various alloys are frequently large enough to use as a determination of whether the heat treatment has led to the desired microstructure. Resistivity measurements for this purpose are particularly useful on steels because such measurements on steel do not suffer any effects from permeability variations as do eddy current tests. Another important advantage of resistivity measurements over other techniques is that a significant volume of the sample is measured (to a depth of about the probe spacing) rather than just the surface. This distinction can be important in view of the effects of surface oxidation, segregation and cold work in making the surface different from the bulk. So far it has been assumed that the material being studied is homogeneous. Heterogeneities such as voids, cracks, slag or flux inclusions are generally nonconductive and will sharply raise the observed resistivity if they are in the vicinity of the voltage probes. It is estimated that voids greater in diameter than 10 percent of the probe spacing S could be detected if they are within a distance S of the surface. Cracks perpendicular to the surface and the line made by the probe points can be most easily detected whereas those perpendicular to the surface but along the
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  • Fall '19
  • Magnetism, Magnetic Field, Electrical conductivity

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