T- and L-Types of Long-Range Guided Waves for Defect Detection

Abstract

Despite technical advancement and wide industrial application, long-range ultrasonic testing (LRUT) has several bottlenecks, which restrict its abilities and complicate the data interpretation. Particularly, these are related to simultaneous presence along with the main used torsional wave (T-wave) of other wave modes, including longitudinal (L-wave) and flexural (F-wave) ones. The latter are considered as unwanted components, but on the other hand could be a source of additional useful information. The purpose of this study was to use small-scaled models of pipes with simulated defects of different kind to demonstrate some possibilities of signals contrasting for a selected wave mode and sensitivity of T- and L-waves in assessment of these defects. Ultrasonic testing was performed by a specially designed laboratory setup, comprising a data acquisition unit with a waveform generator and an amplifier/digitizer circuitry allowing switching in turn between several transducers pairs. An array of magnetostrictive transducers located stepwise along tubes was arranged, where switching between T- and L-wave excitation modalities was done by changing orientation of the static magnetic field. Experiments were done on thin-walled tubes with diameters of 10 and 45 mm, applying ultrasonic tone-burst pulses with carrying frequency of 125 kHz. Contrast of echo responses from defects for switched in turn T- or L-waves was enhanced by signals processing based on time-shifting of signals from several distantly located transducers along the tube to known time delays for the certain type of wave and further multiplication of the signals amplitudes. The demonstrated efficiency of the approach was in discerning of small defects on the background of parasite components and improved contrasting of large defects. Modeling of several types of defects such as pitting, crevice, and stress corrosion was done by means of mechanical tools. The study showed that different responsiveness of T- and L-waves to these defects could be a basis for combined use of both modes to characterize the type of defect, depth of penetration through the tube’s wall, and expansion along the length. Comparison of T-wave and L-wave responses in the same tube filled by air and water showed that liquid filling not only increased attenuation of the both propagating waves, but also caused significant transformation of echo pattern of L-wave having dispersive nature, while T-wave was more stable. Thus, use of L-wave in LRUT should account for pipes filling by liquid contents.