A new method to detect surface crack depth in metal using electromagnetic induction and electric potential

Abstract

Depth of surface cracks in materials is the critical knowledge in order to maintain the safety and reliability of various plants and structures. Surface crack depth in metals has been detected by the electric potential method, which utilizes the potential drop along the electric current flow between both sides of the crack perpendicular to the flow. The authors have devised a new method to detect the depth of surface cracks utilizing the electromagnetic induction and electric potential. The method uses a loop of eddy current induced by a small rectangular coil. When there is no crack, the eddy current is induced parallel to the coil at its center and has no perpendicular component. When there is a surface crack perpendicular to the eddy current, small part of the eddy current flows parallel along the crack making a small closed current loop each side of the crack, although most of the eddy current flows perpendicular to and under the surface crack. The parallel eddy current component is generated by the electric potential parallel to the crack and the potential is generated by the crack because there is no electromagnetic induction by the coil along the crack. The electric potential increases with the crack depth because the parallel eddy current component increases with the depth. Thus the depth of a surface crack can be detected by measuring the electric potential parallel to the crack. The authors are going to explain the principle of the new method and some of the results obtained by electromagnetic analysis and experiments.