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Damage assessment in most forms of magnetic flux

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Damage Assessment In most forms of magnetic flux leakage testing, discontinuity dimensions cannot be accurately measured by using the signals they produce. The final signal results from more than one dimension and perhaps from changes in the magnetic properties of the metal surrounding the discontinuity. Signal shapes differ widely, depending on location, dimensions and magnetization level. It is therefore impossible to accurately assess the damage in the test object with existing equipment. Under special circumstances (for example, when surface breaking cracks can be assumed to share the same width and run normal to the material surface), it may be possible to correlate magnetic flux leakage signals and discontinuity depths. This correlation is normally impossible. Commercially available equipment does not reconstruct all the desired discontinuity parameters from magnetic flux leakage signals. For example, the signal shape caused by a surface breaking forging lap is different from that caused by a perpendicular crack but no automated equipment uses this difference to distinguish between these discontinuities. As with many forms of nondestructive testing, the detection of a discontinuity and subsequent followup by either nondestructive or destructive methods pose no serious problems for the inspector. Ultrasonic techniques, especially a combination of shear wave and compression wave techniques, work well for discontinuity assessment after magnetic flux leakage has detected them. In some cases, however, the discontinuity is forever hidden. Such is very often the case for corrosion in downhole and subterranean pipes. 243 Magnetic Flux Leakage Testing
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1. Nondestructive Testing Handbook, second edition: Vol. 6, Magnetic Particle Testing. Columbus, OH: American Society for Nondestructive Testing (1989). 2. Stanley, R.K. Section 21, “Diverted Flux Theory.” Nondestructive Testing Handbook, second edition: Vol. 4, Electromagnetic Testing. Columbus, OH: American Society for Nondestructive Testing (1986): p 607-630. 3. Stanley, R.K. and G.L. Moake. “Inspecting Oil Country Tubular Goods Using Capacitor Discharge Systems.” Materials Evaluation. Vol. 41, No. 7. Columbus, OH: American Society for Nondestructive Testing (1983): p 779-782. 4. Moake, G.L. and R.K. Stanley. “Capacitor Discharge Magnetization of Oil Country Tubular Goods.” Electromagnetic Methods of Nondestructive Testing. New York, NY: Gordon and Breach (1985): p 151-160. 5. NFPA 70, National Electric Code, 2002 edition. Quincy, MA: National Fire Prevention Association (2001). 6. Beissner, R.E., G.A. Matzkanin and C.M. Teller. NDE Applications of Magnetic Leakage Field Methods: A State of the Art Survey. San Antonio, TX: Southwest Research Institute (1980). 7. Owston, C.N. “The Magnetic Leakage Field Technique of NDT.” British Journal of Non-Destructive Testing.
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  • Fall '19
  • Magnetic Field, Magnetic flux leakage

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