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Tapping rails with lasers finds hidden cracks

Trains move about 42 percent of U.S. coal, chemicals, minerals, food and other goods, but our aging rails are developing hard-to-detect internal cracks that often break under the weight of passing trains. The resulting derailments cause injuries, deaths, equipment and payload damage, and spills of toxic cargo. A team of structural engineers led by professor Francesco Lanza di Scalea has developed an ultrasonic defect-detection technique that is better able than currently used technology to find cracks before they break.

Lanza di Scalea's team described in the Aug. 22, 2006 issue of the Journal of Sound and Vibrationa technique that uses laser beam pulses to gently "tap" on steel rails every few inches. Each tap sends ultrasonic waves traveling 1,800 meters per second along the steel rails. Strategically placed microphones on the underside of a moving test vehicle detect telltale reductions in the strength of those signals to pinpoint defects.

Rails that fail account for about one-fourth of the 1,000 annual track-caused train derailments in the U.S., according to the Federal Railroad Administration, the agency charged with enforcing rail safety regulations. Track-related damage from derailments and other incidents doubled from $55 million in 1993 to $111 million in 2000. Longer trains pulling heavier cars at higher speeds create the potential for even greater losses.

Currently used technologies rely on wheels or sleds that travel at roughly 30 mph while sending ultrasonic pulses downward into the track. Cracks, including superficial surface cracks, reflect the pulses and deeper cracks can be missed.

"Some of the worst derailments in this country have occurred on tracks recently inspected by the current generation of technology, which often doesn't detect interior cracks," said Lanza di Scalea. "Our technique has the potential to offer greater defect detection reliability."

Surface cracking does not interfere with the UCSD technology. "The ultrasonic sound we use doesn't come from the top of the rail, but instead travels along the rail," said Lanza di Scalea.

Structural engineering professor Francesco Lanza di Scalea hopes his pulsed-laser technique will more reliably uncover internal rail cracks.
Structural engineering professor Francesco Lanza di Scalea hopes his pulsed-laser technique will more reliably uncover internal rail cracks.
Lanza di Scalea's team in March 2006 successfully detected internal defects and surface cuts with a prototype vehicle at a test track in Gettysburg, PA.
Lanza di Scalea's team in March 2006 successfully detected internal defects and surface cuts with a prototype vehicle at a test track in Gettysburg, PA.