The operation of NDT instruments based on the impedance method

Abstract The operation of commercially available testing instruments based on the impedance method of non-destructive inspection has been investigated. The sensitivity of the method is greatest when the layer above the defect is thin and the base structure is relatively stiff. The minimum size of defect which can be detected with the commercial instruments is of the same order as that predicted from a fundamental investigation of the technique. The transducer used in the commercial instruments has been investigated and the relationship between the transducer outputs and the structural impedance has been demonstrated. These outputs are highly non-linear functions of impedance which can make physical interpretation of the test results difficult.