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Eddy Current Testing
in the
Petrochemical Industry
Figure 1-3
Figure 1 - The eddy current principle: (a) eddy current coil in air - the coil produces a magnetic field HA and the coil impedance is Zair ; (b) eddy current coil over a conductive material - eddy currents are produces in the material that opposes the magnetic field of the coil, the coil's magnetic field changes from HA to HB and the impedance of the coil changes from Zair to ZB along the liftoff line; (c) eddy current coil over a crack - eddy currents are distorted by the crack, which changes the magnetic field of the eddy currents from H'B to H'C, thereby changing the loading on the coil; (d) the coil impedance changes to Zcrack.
Figure 2 - Eddy current calibration: (a) surface eddy current calibration on electrical discharge machined notches with a surface probe; (b) the phase and amplitude of the signal depends on the notch depth.
Figure 3 - Measuring stainless clad over carbon steel.
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