With a basic system the test object is placed within

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With a basic system, the test object is placed within or next to an electric coil in which high frequency alternating current is flowing. This excitation current establishes an electromagnetic field around the coil. This primary field causes eddy current to flow in the test object because of electromagnetic induction. Inversely, the eddy currents affected by all characteristics (conductivity, permeability, thickness, discontinuities and geometry) of the test object create a secondary magnetic field that opposes the primary field. This interaction affects the coil voltage and can be displayed in various ways. Eddy currents flow in closed loops in the test object. Their two most important characteristics, amplitude and phase, are influenced by the arrangement and characteristics of the instrumentation and test object. For example, during the test of a tube, the eddy currents flow symmetrically in the tube when discontinuities are not present. However, when a crack is present, then the eddy current flow is impeded and changed in direction, causing significant changes in the associated electromagnetic field. Applications. An important industrial use of eddy current testing is on heat exchanger tubing. For example, eddy current testing is often specified for thin wall tubing in pressurized water reactors, steam generators, turbine condensers and air conditioning heat exchangers. Eddy current testing is also used in aircraft maintenance. The following are some of the typical material characteristics that can be evaluated by eddy current testing: cracks, inclusions, dents and holes; grain size and hardness; coating and material thickness; dimensions and geometry; composition, conductivity or permeability; and alloy composition. Radiographic Testing Principles. Radiographic testing (Fig. 8) is based on the test object’s differential absorption of penetrating radiation — either electromagnetic radiation of very short wavelength or particulate radiation (X-rays, gamma rays and neutrons). Different portions of an object absorb different amounts of penetrating radiation because of differences in density and variations in thickness of the test object or differences in absorption characteristics caused by variation in composition. These variations in the absorption of the penetrating radiation can be monitored by detecting the unabsorbed radiation that passes through the object. This 9 Introduction to Acoustic Emission Testing F IGURE 7. Electromagnetic testing: (a) representative setup for an eddy current test; (b) inservice detection of discontinuities. Coil in eddy current probe Primary electromagnetic field Direction of primary current Eddy current strength decreases with increasing depth Direction of eddy currents Conducting specimen Induced field (a) (b)
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monitoring may be in different forms.
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  • Fall '19
  • Nondestructive testing, Acoustic Emission

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