Crack length can be estimated or predicted from a

This preview shows page 11 - 13 out of 15 pages.

Crack length can be estimated or predicted from a reference standard hole size only by correlations established with experience. New correlations are required when significant changes occur in test 279 Reference Standards for Electromagnetic Testing P ART 4. Techniques of Reference Standard Fabrication
Image of page 11

Subscribe to view the full document.

condition or component geometry. Test system repeatability with the drilled hole reference standard is excellent when test conditions and component geometry are held constant. Care should be taken during drilling on thin materials to avoid distortions of the test object and the hole. Notches Notches are the first choice for producing simulated crack reference standards. The shape of the notch can generally be controlled during fabrication and the shape of the expected crack can be accurately simulated in the reference standard. Several techniques for producing simulated crack reference standards are discussed below. Electric Discharge Machining Electric discharge machining (EDM) is a metal removal technique suitable for a wide range of materials. The technique uses a controlled electrical discharge between the reference standard material and a preshaped electrode. The electrode shape determines the shape of the notch, a simulated crack. Electrodes are generally cut from thin foil, graphite, brass or copper tungsten and notch widths of only a few hundredths of a millimeter (a few thousandths of an inch) are commonly produced. Figure 6 shows an apparatus used to position and move the electrode relative to the reference standard. One caution should be noted when using electric discharge machining on alloys containing significant amounts of the elements iron, nickel and cobalt. The permeability of such alloys (including the nonmagnetic stainless steels and some of the high temperature nickel base alloys) can be modified by the electric discharge machining process. When high power, high speed electric discharge machine cutting is used, a layer of recast material is produced at the base and on the walls of the notch and this recast material can have greatly altered magnetic permeability. For small notches intended to simulate small cracks, the eddy current response can be dominated or greatly affected by the magnetic recast material. Slower cutting speeds and fluid flow during electric discharge machining will reduce or eliminate this problem. Electrode feed rates less than 0.013 mm (0.0005 in.) per minute are common. The primary advantage of electric discharge machining is accuracy. It is possible to have electrodes with widths as small as 0.05 mm (0.002 in.) and to produce slots with very short surface lengths, as small as 0.13 mm (0.005 in.). Errors between the desired dimensions of a notch and the resulting dimensions after the electric discharge machining process are often less than 0.013 mm (0.0005 in.). Because of these small dimensions, electric discharge machining can produce simulated discontinuities whose electromagnetic test indications closely resemble those of actual cracks. By designing the electrode according to the shape of the discontinuity, many different widths and length-to-depth ratios are possible. The electrode may also be placed
Image of page 12
Image of page 13
  • Fall '19
  • Nondestructive testing, electromagnetic testing

{[ 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