超声波衍射时差法(带翻译)

国外超声波检测衍射时差法（带翻译）

Time-of-flight diffraction (TOFD)

The Time of Flight Diffraction (TOFD) ultrasonic testing method is relatively new and was first developed at Harwell laboratory in the late1977's by Maurice Silk. TOFD testing has been gaining in profile over the last three or four years with much interest focussing on whether or not it can be used to replace more established NDT methods. Recent survey shows that the annual average growth rate (AAGR) of the TOFD market is 10-20% higher than other NDT technique. The TOFD method is gaining and increasing popularity because of its high probability of detection, low false call rate, portability and most important its intrinsic accuracy in flaw sizing, especially in depth. There is another NDT method called radiography testing (RT/ X-ray) usually employed for flaw sizing. It should be noted that RT/X-ray technique shows better accuracy for lateral flaw sizing but it demonstrates insufficient accuracy in depth assessment. As the standards for radiation safety become tighter by the new European law, many NDT companies are trying to substitute X-ray technique by TOFD technique for cost effectiveness and mostly for safety and environment protection reasons.

1 Theory and Principles

The most significant distinction between TOFD and the other UT methods is that it monitors only forward-scattered diffracted energies from the tips of defects rather than reflected ultrasonic energies. Two wide beam angle probes are used in transmitter-receiver mode. Broad beam probes are used so that the entire crack area is flooded with ultrasound and, consequently, the entire volume is inspected using a single scan pass along the inspection line. Because the technique relies on detection of the forward scattered diffracted signals originating at the flaw edges, precise measurement of flaw size, location, and orientation is possible.

Mr. Udo Schlengermann was one of the creators of the TOFD prestandard

ENV583 and was also involved in the development of the British standard

BS7709.The principle of the TOFD techniques according to that report mention in the below:

The TOFD method only evaluates diffracted echoes, which are 20dB less than the reflected echoes.

●Diffracted waves have a different velocity than reflected longitudinal waves.

●Diffraction is stronger for longitudinal waves than for shear waves.

●The standard is the use of longitudinal probes, 50 to 70 degree, small crystal, with

widely spread sound beam to cover the whole defect.

●The two diffracted signals of the crack tip are generated with a 180-degree phase

shift. The distance between the two signals on the time scale is nonlinear.

●TOFD always uses RF signals to display images, (minus = black, plus = white),