Immersion Testing of Curved Profile Objects

The eddy current method and nondestructive testing means implementing it are used to solve problems related to high-performance testing of the surface of long products, such as rails, under conditions of their industrial production. Their inherent disadvantages and limitations are low sensitivity in relation to certain types of defects, relatively low noise immunity in relation to the electromagnetic background of a metallurgical enterprise and residual magnetization of a test object, difficulties in using curved sections of the profile, the need to place eddy current sensors almost right up to the moving test object, as well as the small depth of penetration of eddy currents into ferromagnetic metals and alloys initiate intensive search for alternative methods and means of detecting surface and subsurface defects in long products. One of such alternatives, potentially removing most of the limitations inherent to eddy current testing, is the use of surface ultrasonic waves.

Rayleigh-type surface ultrasonic waves are widely used in solving flaw detection problems. From a practical point of view, the advantages of Rayleigh waves propagating along the free surface of a solid are primarily related to their localization in the surface layer and, consequently, high sensitivity to surface defects land an amplitude decay with distance slower than for bulk waves. Surface imperfections that occur both in the manufacture and operation of metal products (cracks, hairlines, slivers, prints, etc.) have diverse shape and not only lead to a change in the amplitude and time parameters of the transmitted signal, but also create scattered Rayleigh and bulk longitudinal and transverse waves, making it possible to use both echo-pulse and shadow methods of ultrasonic testing. It is also important that Rayleigh-type waves can propagate along curved (convex and concave) surfaces; this makes it possible to detect inhomogeneities on complex shaped surfaces.