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3 the numerical technique is applicable to situations

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3. The numerical technique is applicable to situations that cannot be analyzed analytically or simulated experimentally (subsurface discontinuities, layered materials and others). The following discussion focuses on analytical and numerical approaches involving an iterative approach in which the test results from a specific design lead to improvements to that design. Some of the avenues available to a designer are outlined below. In particular, a numerical approach to probe design is highlighted. The discussion below uses the finite element technique but the considerations and the treatment of the problem are similar for other numerical techniques. Experimental Design of Eddy Current Probes Probe design literature 4-9 reveals the experimental nature of eddy current research. This approach was dominant in the early days of nondestructive testing. 10 Three questions are associated with any empirical approach. What is the experimental design procedure? How is the outcome of the design evaluated? For the application, is this technique the best approach, merely an acceptable approach or the only feasible approach? Analytical Design of Eddy Current Probes The design of an eddy current probe may proceed either (1) by calculating the coil impedances for a given geometry or (2) by determining the appropriate dimensions for a probe with a predetermined impedance. Not all probe parameters may be designed independently. For example, if a certain probe diameter and reactance at a given frequency are required, it may not be possible to design such a probe or the design may not be acceptable for the test at hand. In the following discussion, the basic relations necessary for probe design are outlined. First, the design of air core coils is presented for single-coil and multiple-coil probes. The discussion on air 132 Electromagnetic Testing P ART 2. Design of Eddy Current Probes
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133 Probes for Electromagnetic Testing core coils is followed by remarks on magnetic (ferritic) core probes and a short section on probe shielding. Calculation of Probe Resistance The impedance Z of any coil consists of a real part R and imaginary part j ω L : (3) where j is (–1), L is inductance (henry) and ω is angular frequency ( ω = 2 π f, where f is frequency in hertz). The real component of impedance Z is the direct current resistance R (ohm). R is calculated from Ohm’s law: (4) where a is the cross sectional area of the wire (square meter), l is the total length (meter) of wire and ρ is the conductor resistivity (ohm meter). Because the diameter of the coil is important in probe design, the equation may be written in terms of the probe mean diameter: (5) where d is mean coil diameter (meter), N is the number of turns in the coil and π dN is the total wire length (meter). In the more general case of conducting materials in the vicinity of the coil, the real part of the impedance also includes the effects of eddy current losses in the conducting bodies.
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  • Fall '19
  • Wind, The Land, Magnetic Field, Eddy Current Probes, electromagnetic testing

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