Through-transmission equations for remote-field eddy current inspection o small-bore ferromagnetic tubes

The remote-field eddy current (RFEC) method is widely used for nondestructive testing of ferromagnetic tubes such as those found in heat exchangers. An exciter coil generates an electromagnetic field that diffuses through the pipe wall, axially along the outside of the pipe, and back through the pipe wall to a detector coil. Phase and amplitude readings can be interpreted to characterize pipe defects. Skin depth theory is commonly used to calculate through-transmission for a cylindrical wave impinging on a conducting tube. The cylindrical through-transmission equations agree well with RFEC data. The application of the new equations to RFEC defect signal analysis is discussed. A case study of an RFEC scan of a metal loss defect is described. The through-transmission equations were found to hold at the defect. However, the RFEC scan data deviated slightly from the value predicted by the through-transmission equations. The deviation was attributed to a perturbation of the field on the outside of the pipe caused by the defect, an effect not considered in a through-transmission analysis.