341 electromagnetic techniques for material

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341 Electromagnetic Techniques for Material Identification F IGURE 22. Spread bands obtained when 1000 samples each of two steels of different composition (Unified Numbering System G10150 carbon steel and free machining steel) were comparator bridge tested at identical instrument settings. Free machining steel Carbon steel Permeability (relative scale) Magnetic flux density (relative scale)
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The resistivity of metals has been studied for many years and resistivity values for most materials are readily available. The reason that this parameter has not been widely used for metal identification or sorting, however, is the difficulty in determining the resistivity without special laboratory techniques. Resistivity testing is also called potential drop testing and can be used for discontinuity detection. Conductivity measurements are often made with eddy current devices. These techniques determine the conductivity or resistivity indirectly by measuring its effect on a coil in a high frequency alternating current test circuit. Consequently surface roughness, surface curvature and trace impurities can significantly affect the results. Another drawback is the requirement for a standard sample to compare with the unknown. Instruments are now available that permit measuring resistivity directly. The advantages of direct measurement are that measurements can be made on bulk material in its manufactured state, that no reference standards are required and that resistivity values do not require intermediate calculation. Principles of Resistivity Tests A typical instrument consists of two parts: a four-point probe and an electronics package to supply current, determine the voltage drop and convert it to a resistivity value. The four-point probe has been widely used for studying semiconductor materials; the relationship between probe geometry, voltage drop and sample resistivity has been established for many common cases. Although this relationship cannot be solved in closed form for a sample of arbitrary geometry, two important cases lead to very simple solutions. Using the notation of Fig. 23 I is the current through the sample, S is the distance (in meter) between the probes, V is the voltage detected across the inner probe and W is the thickness (in meter) of the sample. For samples of length and width several times the overall probe spacing, the resistivity ρ for sheets with thickness W < 0.5 S is: (6) and (7) for sheets with W > 3 S . Resistivity ρ is usually expressed in microohm centimeter or microohm meter. Note that for thin sheets resistivity ρ is determined independently of the probe spacing. For thick sheets the determination is independent of thickness. An important implication of Eq. 7 is that the resistivity can be determined for massive samples like ingots or bars, provided only that ρ π = = 2 6 2 SV I S V I . 8 ρ π = ( ) = VW I VW I ln2 4 53 .
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  • Fall '19
  • Magnetism, Magnetic Field, Electrical conductivity

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