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Δ π μ μ σ 1 f r 130 electromagnetic testing

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δ π μ μ σ = 1 0 f r 130 Electromagnetic Testing Movie. Skin effect. Movie. Standard depth of penetration.
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As Eq. 2 shows, the standard depth of penetration depends on conductivity, permeability and frequency but is relatively small for most metals, about 0.2 mm (0.008 in.) for copper at 100 kHz. The skin effect has two important effects on the design of eddy current probes: (1) the probes are more useful for surface testing and (2) lower frequencies may be necessary for subsurface testing. The standard depth of penetration can be increased in the case of ferromagnetic test objects by magnetically saturating them, thereby reducing their relative magnetic permeability μ r . 131 Probes for Electromagnetic Testing
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Eddy current probes are based on relatively simple principles and consist of one or more coils. The shape of the coils, their cross section, size, configuration and sources are all parameters that are chosen by the designer to accomplish a particular purpose. Practical eddy current probes may range from tiny coils less than 0.5 mm (0.02 in.) to over 300 mm (12 in.) in diameter, may be long or short and may have square, round or elliptical cross sections, with magnetic or nonmagnetic cores or shields. Parameters of interest in the design may include (1) coil inductance, (2) coil resistance, (3) field distribution in space, (4) coil response to relevant material property changes, (5) liftoff characteristics and (6) response to a notch, drilled hole or other simulated discontinuities. In addition, the design may be influenced by other constraints intrinsic to the test environment, such as weather or access requirements for a specific shape or size. Some of these requirements may in fact be contradictory. The design process is usually iterative, proceeding by trial and error. There are three basic techniques of probe design. Although these will be considered separately, a combination of the techniques is perhaps the most appropriate approach. The techniques can be classified as follows: (1) experimental or empirical design, (2) analytical design and (3) numerical design. A practical way to design a probe would be to start with analytical expressions (exact or approximate), design a probe based on some set of initial requirements, construct the probe and then evaluate its performance experimentally. If necessary, the process can be repeated until an acceptable design is obtained. Analytical expressions are not accurate except for the simplest probe geometries and numerical tools are often used in practice. The numerical design of probes has several advantages. 1. The probe, with all its components (coils, core and shield) and the surrounding medium are analyzed. The probe characteristics in the actual test environment can be obtained. 2. A more accurate design is obtained before the probe is actually built by numerically experimenting with the probe parameters.
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  • Fall '19
  • Wind, The Land, Magnetic Field, Eddy Current Probes, electromagnetic testing

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