The development of eddy-current testing techniques for tube inspection

The paper outlines the need for a high speed method of examining tubes for defects and discusses to what extent eddy-current techniques meet this need. A short history is given of the introduction of eddy current testing used for the examination of solid and hollow cylinders. Eddy current testing principles are explained, together with methods of measuring the effects of variations in a test specimen, with the aid of block circuit diagrams and complex impedance curves. The merits of a.c. and d.c. saturation techniques are dealt with and practical testing equipment which employs phase and modulation analysis is described. The application of phase sensitive devices and modulation analysis techniques is explained and the limitations assessed. The uses of internal probes and surface probes are briefly described.