Current induced into a metal object by an external coil is called eddy current. The use of eddy current technique to characterize the properties of metal objects is based on the relationship between a conductive metal object structure and electromagnetic properties. More specifically, eddy current testing relies on electromagnetic interaction between the coil driven by an alternating electrical current and the metal object under test. In order to measure applied or residual stresses with a conventional eddy current approach, the application of stress must change the electrical conductivity such that a detectable change in the test coil impedance will occur.
Presently, current commercial eddy current equipment is not sensitive enough to detect extremely small changes in conductivity in a treated metal object. Nor is present commercial eddy current equipment sensitive enough to detect changes in conductivity at the near-surface of a treated metal object, i.e., within 0.020 inch.
The present invention provides a method and equipment to detect very small changes in conductivity between a treated metal object and an untreated metal object sample used as a reference.