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Using the foregoing rule of thumb, the higher the eddy current frequency, the smaller the standard depth of penetration δ and the narrower the apparent crack width w = 2 δ . The magnetooptic eddy current images in Fig. 53 were made at an eddy current frequency of 50 kHz with a standard depth of penetration δ = 544 μm (0.021 in.) in a wrought aluminum alloy (Unified Numbering System A97075, temper 6, or 7075-T6) but current technology permits images at frequencies up to 200 kHz with a standard depth of penetration δ = 305 μm (0.012 in.) in the same material. Hence, the apparent crack width in aluminum at 200 kHz will be w = 533 μm (0.021 in.) whereas at 50 kHz the apparent crack width will be twice as large, namely, w = 109 μm (0.043 in.). At still lower frequencies in aluminum, the apparent crack width is wider still. At 10 kHz in the wrought aluminum alloy, for example, the standard depth of penetration δ = 117 μm (0.046 in.) and therefore the apparent crack width w = 2.34 mm (0.092 in.). To make magnetooptic eddy current images that resemble actual discontinuities as closely as possible, the highest possible eddy current frequency that still permits detection of the discontinuity should always be used. This is true for both surface and subsurface discontinuities. Discontinuities that are more than one standard depth of penetration below the surface of a material can sometimes be difficult to detect unless the eddy current magnitude is sufficient. Accordingly, high power settings are invariably used when attempting to detect subsurface discontinuities or corrosion at depths of two or more standard depths of penetration from the surface of materials such as aging aluminum airframes. Typically, the highest possible power level 164 Electromagnetic Testing F IGURE 54. Two-layer setup standard in which each layer is 1 mm (0.04 in.) thick aluminum (Unified Numbering System A82024, temper 3) containing electric discharge machined notches and second layer hole. Legend A. Rivet without anomaly. B. First layer electric discharge machined notch of 45 degrees, 2.5 mm (0.10 in.) long. C. Horizontal electric discharge machined notch, 1.8 mm (0.07 in.) long. D. Second layer electric discharge machined notch, 5.0 mm (0.20 in.) long. E. Hole in second layer, 9.55 mm (0.376 in.) in diameter, mimicking corrosion. A B C D E F IGURE 55. Magnetooptic eddy current images made with rotating eddy current excitation and corresponding to artificial discontinuities in Fig. 54: (a) image of discontinuity free rivet, corresponding to notch A, made at 100 kHz; (b) image of notch B, made at 100 kHz; (c) image of notch C, made at 100 kHz; (d) image of notch D, made at 10 kHz; (e) image of hole E, made at 10 kHz. (a) (b) (c) (d) (e)
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is selected and the highest workable eddy current frequency is one that achieves the best possible discontinuity resolution.
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  • Fall '19
  • Wind, The Land, Magnetic Field, Eddy Current Probes, electromagnetic testing

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