Sounding out the danger on the rails

An ultrasound system capable of detecting cracks in rail welds is undergoing trials at various sites on the UK railways.

The inspection tool, being developed by Imperial College spin-out Guided Ultrasonics using technology from the Research Centre in Nondestructive Evaluation (RCNDE), can be used to detect cracks deep within rail welds, which are difficult to spot using existing techniques.

Conventional ultrasound inspection devices send a signal into the rail head from above, either straight down or at an angle, and if the signal meets a crack in the rail it is reflected back.

But if there are a number of cracks, those lower down will be masked by defects closer to the surface, said Dr. Mike Lowe, reader in mechanical engineering at Imperial College.

‘If the deeper crack is behind another you won’t see it, but it could be a lot bigger. There is concern in the rail industry about multiple crack problems, such as gauge corner cracking. A deeper crack is much more difficult to see, and is what leads to rail breaks, such as the one that caused the Hatfield disaster.’

The situation is even worse at rail welds, as defects can appear at any point within a weld, including the foot, while in the track itself cracks tend to appear only in the rail head. Current inspection systems can only look at the rail head, said Lowe.

Welds made from alumino-thermic material are often made on site, as stretches of track of over 0.5km cannot be easily transported. As a result the quality of the welding work can be affected by conditions such as bad weather, increasing the importance of inspections.

The Guided Ultrasonics system uses a Rayleigh wave, or a wave that travels along the rail, rather than down into it, and as a result is not prevented from reaching deeper cracks by those above.

The system, which has been developed with support from Network Rail and the EPSRC, has been undergoing site trials since last August. These trials are set to continue for another two months before the company applies for full product approval, according to Dr. Brian Pavlakovic, technical director of Guided Ultrasonics.

The system uses low frequency waves, allowing it to inspect stretches of track of up to 50m in either direction at once, although this also means it can only detect larger cracks.

RCNDE researchers are working on a high frequency device, which also uses Rayleigh waves. This will take detailed track measurements within a small stretch of rail, close to the transducer.

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