Eddy-current test device for detecting surface defects

摘要

In an eddy-current test device consisting of a combination of two types of high-frequency sensing coil systems for selectively detecting cracks or other material defects in the surface of electrically conducting objects moving relatively to the test device, in which an eddy-current field is excited in the object by means of a coil system of the first or second type and changes in the eddy-current field due to material defects are detected by a coil system of the other type in each case as a result of evaluating the induced voltages, the object is to facilitate good separation between different surface defects such as cracks and slits. The test device should also be capable of operating at elevated temperatures (over 1,000 DEG C). The object is achieved by constructing a sensing coil system of the first type in such a way that during excitation in an object (1), an eddy-current field (2) is produced which is geometrically similar to that of a U-shaped core coil system operated with alternating current with its limbs (3) directed towards the surface of the object and that the sensing coil system of the second type (4) is arranged in the symmetry plane of the sensing coil system of the first type and is itself symmetrical with respect to said plane. The invention is applied in the production of semi-finished products, for example for examining material before rolling processes. <IMAGE>