Theoretical and Experimental Studies of the Remote Field Eddy Current Effect

Abstract

Conventional eddy current (EC) nondestructive testing methods, based on the principle of electromagnetic induction have been used over the years for defect detection. A low frequency alternating current is fed to the excitation coil which induces eddy currents in the material. These eddy currents generate a magnetic field, inducing a secondary voltage in the sensor coil. The change in the impedance of the sensor coil is used to indicate the presence of material inhomogeneities (Figure 1). Figure 2 shows a conventional eddy current probe and a remote field eddy current (RFEC) probe. The difference between the two is that in the conventional EC probe the exciter and the sensor are the same coil, normally operating between 1KHz and 10 MHz, while in the RFEC probe the exciter and the sensor coils are several pipe diameters apart with a frequency of operation from 40 to 160 Hz. The RFEC probe does not measure the change in the impedance of the sensor coil, rather the steady state A.C phase angle difference between the exciter and the sensor is monitored (Figure 3). Skin effect characteristics limit the eddy currents to the surface of the material, restricting the conventional EC testing methods to the detection of near surface inhomogeneities. The RFEC probe in contrast overcomes the above limitation due to its apparent sensitivity to both inner and outer diameter (ID and OD) defects [1]. Recently numerical models have been developed to develop a better understanding of the physics of the phenomenon [2–8].