[ti:Scientists Use Sound Waves to Find Hidden Cracks in Bridges, Airplanes] [ar:Jonathan Evans] [al:Science In The News] [by:www.51voa.com] [00:00.00]On August 1, 2007, a steel bridge in Minneapolis, Minnesota, collapsed, [00:07.40]sending people and vehicles into the Mississippi River. [00:11.50]Thirteen people died and more than 140 others were injured. [00:17.23]The bridge collapse took place during rush hour -- [00:20.94]when a lot of cars and trucks were on the road. [00:24.54]Investigators found that undersized metal parts [00:28.55]of the bridge simply could not support the heavy load. [00:32.44]Finding hidden cracks and other weak areas in large structures [00:37.55]can be the difference between life and death. [00:40.99]Researchers in Britain say they have discovered a new way [00:45.54]to identify cracks inside metal parts before they fail. [00:50.19]Their method involves using sound imaging. [00:53.88]Anthony Croxford leads the team of scientists at Bristol University. [00:59.03]They begin their research [01:01.49]by sending hundreds of different ultrasonic waves into a structure. [01:06.13]Then, Mr. Croxford says, they listen and study echoes [01:11.28]of the sound waves to identify the smallest cracks. [01:15.37]"It lets you see smaller cracks, closed cracks, [01:19.17]so when I say closed cracks, you can magine [01:21.27]if you have a crack in a piece of metal, [01:22.64]it could be a bit open like that, it could have a gap in between it. [01:25.22]If you have a gap in between it, [01:26.63]you get reflections off the edge of it, [01:29.52]but you don't really know how big it is." [01:31.22]Anthony Croxford says this method is unlike a purely linear system. [01:37.17]A linear system creates echoes, or similar versions, [01:41.91]of the sound wave sent into the material. [01:44.81]In other words, you would hear the same frequency coming back from the object. [01:51.10]Mr. Croxford says his ¡®phased array' system [01:55.55]of sending out sound waves returns harmonics -- [01:59.24]echoes of different frequencies. [02:01.54]"The nonlinear approach means that [02:04.29]you can actually hear something from them, [02:05.73]you're listening to different effects, [02:07.04]rather than listening for just that echo from the crack." [02:09.78]Mr. Croxford tested the system on part of a wing [02:13.83]from a passenger airplane, an Airbus A320 aircraft. [02:18.83]A linear system would not be able to identify cracks [02:23.42]forming around the rivet openings. [02:25.52]Openings that large would create one big echo. [02:30.03]"By using this novel approach we can now pick up a crack [02:33.53]close to a hole, which is directly relevant, to say, aerospace applications, [02:36.89]where they're worried about cracks growing from rivet holes, things like that." [02:40.49]He says the phased array system uses only one piece of equipment [02:45.73]to get both a linear and nonlinear image. [02:49.49]The technology could help safety inspectors [02:52.73]get more detailed estimates of damage [02:55.83]in materials like aircraft parts and bridge supports. [02:59.73]The damaged materials could then be replaced before they fail. [03:05.42]I'm Jonathan Evans. [03:07.46]¸ü¶àÌýÁ¦Çë·ÃÎÊ51voa.com