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325 electromagnetic techniques for material

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325 Electromagnetic Techniques for Material Identification F IGURE 4. Conductivity and liftoff loci on impedance plane. 0 percent IACS (air) Conductivity locus Graphite, 0.1 MS·m –1 (0.2 percent IACS) Titanium, 0.6 MS·m –1 (1.0 percent IACS) Stainless steel, 1.6 MS·m –1 (2.7 percent IACS) Bronze, 5.8 MS·m –1 (10 percent IACS) Aluminum, 20.3 MS·m –1 (35 percent IACS) Copper, 58 MS·m –1 (100 percent IACS) Liftoff loci 100 kHz Inductance X (relative scale) Resistance R (ratio) θ θ Legend IACS = International Annealed Copper Standard θ = curve of separation between liftoff curve and conductivity curve F IGURE 5. Movement of material points by frequency changes: (a) low frequency, 20 kHz; (b) medium frequency, 100 kHz; and (c) high frequency, 1 MHz. (a) Inductance X (relative scale) Air Graphite UNS R56401 UNS S30400 Conductivity Bronze UNS A92024, temper 3 Copper Liftoff θ Resistance R (relative scale) UNS A97075, temper 73 (b) Inductance X (relative scale) Air Graphite UNS R56401 UNS S30400 Conductivity Bronze UNS A92024, temper 3 Copper Liftoff Resistance R (relative scale) UNS A97075, temper 73 (c) Inductance X (relative scale) Air Graphite UNS R56401 UNS S30400 Conductivity Bronze UNS A92024, temper 3 Copper Liftoff Resistance R (relative scale) UNS A97075, temper 73 θ Legend UNS A92024 = Unified Numbering System A92024 heat treatable wrought aluminum alloy UNS A97075 = Unified Numbering System A97075 heat treatable wrought aluminum alloy UNS R56401 = Unified Numbering System R56401 titanium alloy UNS S30400 = Unified Numbering System S30400 austenitic chromium nickel stainless steel
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At low frequencies (Fig. 5a), the separation angle θ between the liftoff curve and the conductivity curve for bronze is quite small. Thus, it becomes more difficult to obtain liftoff suppression. If a higher frequency is chosen (Fig. 5c), the separation angle for bronze is large, allowing liftoff suppression and good sensitivity to conductivity variations. For sorting titanium alloys, a frequency between 500 kHz and 1 MHz would be chosen but, to sort aluminum alloys, a frequency between 20 kHz and 100 kHz would be chosen. Generally for conductivity measurements (alloy sorting, heat treat determination and others) and for surface crack detection, a frequency should be chosen that places the material point just below the knee in the conductivity curve. At this point, a large separation angle exists between the liftoff and conductivity curves. For magnetic materials, the liftoff and magnetic permeability loci curves are virtually superimposed (Fig. 6a) but their respective values increase in opposite directions. Figure 3 shows that the reactance component of the test coil impedance is decreased by the presence of nonmagnetic materials. This reactance reduction occurs because induced currents flow in the conductive and nonmagnetic object and set up a secondary field that partially cancels the primary field of the coil. The opposite is true when a magnetic material such as iron or ferrite is placed within the field of the coil. This happens because the presence of the magnetic field intensity of the primary coil field causes atomic magnetic elements of the
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  • Fall '19
  • Magnetism, Magnetic Field, Electrical conductivity

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