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When a coaxial line is cut at one end and is

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When a coaxial line is cut at one end and is terminated by a metal plate, it is said to be short circuited. As a result, the electric field at the plate (the short circuit) is totally reflected with a phase shift of 180 degrees. When a crack is introduced in the metal plate, nearly all of the incident signal is reflected. However, the phase of the reflected signal depends on the frequency of operation, coaxial aperture dimensions, the dimensions of the crack and its location in the open ended coaxial aperture. The length of the crack exposed to the probing aperture λ 0 = c f β π λ = 2 λ λ ε = 0 r 57 Principles of Electromagnetic Testing F IGURE 15. Measurement apparatus for transmission technique of microwave testing. 15 Sweep oscillator Level in Radio frequency out Isolator Leveling loop Detector Directional coupler Network analyzer Test Reference X,Y recorder Test object Transmitting horn Receiving horn Directional coupler Isolator Frequency meter X Y F IGURE 16. Open ended coaxial aperture for microwave reflection technique for evaluation of surface cracks in metals: (a) side view; (b) axial view. 16 (a) (b) Microwave connector Outer conductor Dielectric material Inner conductor Outer conductor Dielectric material Inner conductor
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changes as a function of the scanning distance, as can be seen in Fig. 17. One may use a vector network analyzer and measure the change in the magnitude and phase of the reflection coefficient. However, a relatively small, simple and inexpensive reflectometer or phase detector may be designed and constructed using discrete microwave components. In this way, a direct current voltage proportional to the magnitude or phase of the reflection coefficient can be measured and recorded to indicate the presence and the properties of a crack. 58 Electromagnetic Testing F IGURE 17. Field distributions and relative crack geometry at coaxial probe aperture, illustrating arrangement of transverse electromagnetic mode. 16 Magnetic field Scanning direction Local crack length Crack width Electric field
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1. Nondestructive Testing Handbook, second edition: Vol. 6, Magnetic Particle Testing. Columbus, OH: American Society for Nondestructive Testing (1989). 2. Udpa, L. Imaging of Electromagnetic NDE Phenomena. Ph.D. dissertation. Fort Collins, CO: Colorado State University (1986). 3. Udpa, S.S. Finite Element Modeling of Residual Magnetic Phenomenon. M.S. thesis. Fort Collins, CO: Colorado State University (1981). 4. Lord, W. and D.J. Oswald. “Leakage Field Methods of Defect Detection.” International Journal of NDT. Vol. 4. Kidlington, United Kingdom: Elsevier Science Limited (1972): p 249-274. 5. Förster, F. “On the Way from the ‘Know-How’ to the ‘Know-Why’ in the Magnetic Leakage Field Method of Nondestructive Testing (Part One).” Materials Evaluation. Vol. 43, No. 9.
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  • Fall '19
  • Magnetic Field, electromagnetic testing

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