A method of eddy-current flaw detection of bars and tubes based on the use of a combined eddy-current transducer with excitation of spatial magnetic-field components at different frequencies

Abstract

Flaw detectors (FDs) with transmission differential and attachable rotary eddy current transducers (ECTs) are predominantly used for nondestructive testing of extended metal articles. Such FDs are best suited to heavy testing conditions directly in workflows in the production of tubes, rolled stock, and wires and are characterized by a wide range of diameters of inspected items, a high testing rate, and the possi bility of testing at large transverse displacements of a tested article and detecting various flaws: faulty fusions, cracks, hair cracks, laps, and nonmetallic inclusions.Flaw detectors with transmission transducers have such disadvantages as a decrease in the absolute sen sitivity to flaws with an increase in the diameter of the tested object, a substantial change in the signal amplitude from a flaw during transverse displacements of the object, and insufficient reliability of detec tion of extended flaws with a small gradient of properties in the longitudinal direction (notches, scratches, and cracks with rounded bottoms and insignificant depth variations). The first two drawbacks of FDs with transmission ECTs can be minimized by using a transmission multisectional ECT [1][2][3]. In this case, the localization of the testing zone of each measuring channel and the possibility of correcting the sensitivity depending on the azimuth and radial displacement of a tested article are provided. The third drawback of FDs with transmission ECTs, which is due to the necessity of using differential transducers with a small relative base because of difficult testing conditions, largely limits their field of application.Eddy current FDs with attachable rotary transducers and rotor type devices ensure the maximum sen sitivity (for eddy current testing facilities) to surface longitudinal flaws, regardless of the fact whether their depth changes abruptly or smoothly in the longitudinal direction, and, according to the physical essence of testing, have no essential limitation with respect to the maximum diameter of tested articles. In order to detect flaws of different types, it is most efficient to use combined transducers with transmission and attachable rotary windings. This solution was used in a BД 41П/2 FD [4].FDs with attachable rotary transducers have the following disadvantages: increased requirements for the curvature and quality of the surfaces of inspected articles and for mechanical devices for centering and mechanical stabilization of a tested article and, in some cases, the impossibility of providing a gap between the transducer and tested article that is acceptable for shop conditions.In order to detect extended flaws with small gradients of the properties in the longitudinal direction in the presence of significant transverse displacements of a tested article, FDs in which attachable multisec tional EDTs with excitation of longitudinal eddy currents of different frequencies are used are the most suitable instruments for such conditions. As was shown in [5], the advantages of such an engineering solu tion...