AE01.pdf

With a basic system the test object is placed within

This preview shows page 14 - 16 out of 34 pages.

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)
Image of page 14

Subscribe to view the full document.

monitoring may be in different forms.
Image of page 15
Image of page 16
  • Fall '19
  • Nondestructive testing, Acoustic Emission

{[ snackBarMessage ]}

What students are saying

  • Left Quote Icon

    As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

    Student Picture

    Kiran Temple University Fox School of Business ‘17, Course Hero Intern

  • Left Quote Icon

    I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

    Student Picture

    Dana University of Pennsylvania ‘17, Course Hero Intern

  • Left Quote Icon

    The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

    Student Picture

    Jill Tulane University ‘16, Course Hero Intern