134 electromagnetic testing f igure 2 coil cross

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134 Electromagnetic Testing F IGURE 2. Coil cross section and dimensions used for analytical calculations. b c r m a = b × c Legend a = cross sectional area b = radial thickness, or length c = axial thickness r m = coil mean radius
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The introduction of conducting and magnetic cores within the probe creates two major difficulties as far as the design is concerned: (1) the need to calculate losses within the core and any conducting bodies within the probe’s field and (2) the frequency dependence of the probe response. The second of these difficulties is relatively easy to handle by calculating the coil parameters at a fixed frequency (or at a few frequencies) within the expected range of application. The calculation of losses within the conducting bodies is far more complicated but is absolutely necessary to estimate the probe impedance. Similarly, calculation of the probe reactance becomes complicated unless the magnetic path is very simple. Analytical tools for designing such probes are discussed elsewhere. Numerical Design Numerical tools to aid in the search for better designs and the tighter requirements imposed on systems offer some unique opportunities. The chapter on modeling describes some of the more common numerical approaches that can be used to design eddy current coil probes. Numerical techniques offer certain advantages over other design techniques. The probe response is calculated from the true physical description of material interactions with the electromagnetic field. The inclusion of discontinuities, material properties and coil parameters are therefore an integral part of the model. Very few assumptions are made. In many cases, however, some assumptions may be useful in reducing the effort and cost involved in the application of the model. Linearity, two dimensionality and axisymmetric formulations are examples of such assumptions. In addition, the probe response is a complete simulation of the test performed. This is extremely important because it reveals the probe characteristics in a way very similar to the real test and leads to a better design. Unconventional probe shapes can be modeled whereas analytical techniques can only handle a limited number of coil shapes, such as round coils with rectangular cross sections. Finally, probes can be optimized to detect specific types of discontinuities. Because numerical techniques can model complex discontinuities (subsurface discontinuities), the probe response can be optimized for any type of discontinuity and test condition. To illustrate some of the foregoing arguments, the design of a reflection probe specifically intended for measurement of crack depth, coating thickness and corrosion effects is presented below, using a finite element eddy current model. Two types of probes are considered: (1) a simple absolute, surface probe and (2) a reflection probe consisting of a driving coil and a pickup coil. This latter probe will be called a double-coil probe to distinguish it from the single-coil probe.
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  • Fall '19
  • Wind, The Land, Magnetic Field, Eddy Current Probes, electromagnetic testing

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