Showing papers on "Time-of-flight diffraction ultrasonics published in 1990"

Images of Flaws in Generator Retaining Rings Using SAFT Reconstruction of TOFD Data

Abstract: Retaining rings are assembled onto each end of electric generator rotors to support the rotor end-turn windings against rotational forces. They are attached to the ends of the slotted portion of the rotor body with an interference fit. Many of the rings presently in service are susceptible to intergranular stress corrosion cracking (IGSCC) in the presence of moisture. IGSCC seems to occur first, and tends to be more severe, at stress concentrations in the rings and adjacent to geometric discontinuities in the mating rotor surface, such as, the edges of the rotor teeth. These same geometric features also tend to cause ultrasonic reflections that are difficult to discriminate from real damage. The capability of SAFT processing of TOFD data to discriminate IGSCC from geometric reflectors was investigated using several retired retaining rings assembled to mandrels simulating the generator rotor.

The Rôle of Ultrasonic Diffraction in NDT

Abstract: Diffraction is now an accepted ultrasonic phenomenon used in the detection, sizing and characterisation of flaws in solids. Its introduction has led to improved accuracy in flaw sizing and greater reliability in flaw location. The source of these improvements was the combination of diffraction with timing techniques allowing flaw dimensions to be directly related to the transit times of ultrasonic pulses. This resulted in the powerful time-of- flight diffraction (TOFD) technique. Other institutions have used different labels for the phenomenon, such as edgewave scattering. Diffraction data has been incorporated in the ALOK technique developed by IZfP. Other significant uses of diffraction are the FET technique developed by de Vadder, which makes use of focussed transducers, and the SLIC techniques developed by Gruber. The most convincing demonstrations of the superiority of diffraction in flaw sizing have been the many recent inspection blind trials such as those organised by DDT, PISC and EPRI. In all cases the techniques which made use of diffraction data - by whatever name - produced consistently accurate data. In flaw location too, the use of diffraction resulted in some unexpected advantages. This is because the variation of the diffracted energy is not a strong function of the inspection geometry. A correctly set up system will thus reliably locate flaws, even if these are in orientations conventionally thought to give rise to difficulty. It may be thought that this fruitful line of development has now reached its limit but the prospects for the future seem to be equally exciting, particularly because the crack tip is the region which plays the most significant role in determining flaw growth. The development of TOFD-based flaw growth monitoring has been achieved and there are prospects for its extension. The characterisation of the flaw tip itself may lead to a better understanding of flaw significance as well as indicating potential problems in flaw sizing. At the same time, it is important to take a balanced view of the sources of error in the technique and neither overstate nor under-rate its potential. Some of the history of these developments is described in the following sections together with an outline of the present situation and an indication of other important advances, such as flaw size monitoring, which are expected to be important in the future.