Practical long range guided wave testing : Applications to pipes and rail

The testing of large structures using conventional ultrasonic bulk wave techniques is slow because the test region is limited to the area immediately surrounding the transducer. Therefore, scanning is required if the whole structure is to be tested. Ultrasonic guided waves potentially provide an attractive solution to this problem because they can be excited at one location on the structure and will propagate many meters, returning echoes indicating the presence of corrosion or other discontinuities. However, guided wave testing is complicated by the presence of many possible wave modes, most of which are dispersive. These guided wave characteristics offer a wealth of opportunities for the extraction of information about the structure, but it is crucial to manage this complexity if the test is to be useable in industrial practice. Guided waves can be used in three regimes, which have been researched extensively by many authors: short range (« 1 m [39 in.], for example leaky lamb wave testing of composite materials and high frequency surface wave scanning), medium range (up to about 5 m [16.4 ft], for example shear horizontal and lamb waves in the 250 kHz to 1 MHz frequency range for plate and tube testing) and long range (up to 100 m [328 ft], for example the testing of pipelines). This paper concentrates on long range testing using frequencies below 100 kHz. The progress from research work to a robust, commercial pipe testing system is described, together with the more recent development of a test for rail. Future directions for guided wave testing are then discussed.