Therefore the problem of quantitatively detecting the

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Therefore, the problem of quantitatively detecting the discontinuities is magnified because it becomes increasingly more difficult to discriminate between the severity of the various heterogeneities. Also, because the degree of magnetization is so great, the surface roughness is easily mistaken for actual discontinuities and results in unwarranted rejection of test objects. Therefore, there exists an upper and lower limit of magnetization to which a test object should be subjected if the magnetic leakage field technique of nondestructive testing is to be most successful. Magnetization of the test object lies on the linear part of the magnetization curve in such a way that the material permeability is maximum. Magnetization should not approach saturation but should have a value of flux density that locates the initial operating point of the material on the steepest part of the initial induction curve. If the degree of magnetization is too low, discontinuities may go unnoticed and, if the magnetization level is too high, a lack of discontinuity discrimination may result in false indications. 50 Electromagnetic Testing F IGURE 5. Billet with subsurface discontinuity, showing resultant leakage field. 4 Leakage field H field Subsurface discontinuity
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Eddy current techniques of nondestructive testing rely on the principles of magnetic induction to interrogate the materials under test. A complete understanding of the underlying physical process can only be gained through Maxwell’s equations. However, the physical basis of the technique can also be understood qualitatively. Eddy current testing is based on the fact that, when a coil excited by an alternating current is brought close to a material, the terminal impedance of the coil changes. 6 The change is associated with the fact that the primary field set up by the eddy current coil induces eddy currents within the electrically conducting specimen. In conformity with Lenz’s law, the direction of the induced eddy currents and consequently the secondary field generated by these currents oppose the change in the primary field (Fig. 6). 7,8 If the test object is nonferromagnetic, the magnetic flux leakage associated with the coil decreases because of the opposing nature of the primary and secondary fields. Because the self-inductance of the coil is defined as flux linkages per ampere , the inductance of the coil decreases. Accompanying the decrease in inductance is an increase in resistance, owing to the fact that the eddy current losses incurred within the specimen have to be met by the source of primary excitation. This loss manifests itself as a change in coil resistance. The presence of a discontinuity or heterogeneity in the test object causes a reduction as well as a redistribution of the eddy currents. Consequently, the changes in the inductance and resistance of the excitation coil are correspondingly less.
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  • Fall '19
  • Magnetic Field, electromagnetic testing

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