On August 1, 2007, a steel bridge in Minneapolis, Minnesota, collapsed（倒塌）, sending people and vehicles into the Mississippi River. Thirteen people died and more than 140 others were injured.
The bridge collapse took place during rush hour -- when a lot of cars and trucks were on the road. Investigators found that undersized（尺寸不足的） metal parts of the bridge simply could not support the heavy load.
Finding hidden cracks and other weak areas in large structures can be the difference between life and death. Researchers in Britain say they have discovered a new way to identify cracks inside metal parts before they fail. Their method involves using sound imaging（声成像技术）.
Anthony Croxford leads the team of scientists at Bristol University. They begin their research by sending hundreds of different ultrasonic waves （超声波）into a structure. Then, they listen and study echoes of the sound waves to identify the smallest cracks.
Anthony Croxford says this method is unlike a purely linear system（纯线性系统）. A linear system creates echoes, or similar versions, of the sound wave sent into the material. In other words, you would hear the same frequency（频率） coming back from the object. However, his system of sending out sound waves returns echoes of different frequencies.
Mr. Croxford tested the system on part of a wing from a passenger airplane, an Airbus A320 aircraft. "By using this new approach we can now pick up a crack close to a hole."
He says the system uses only one piece of equipment to get both a linear and nonlinear image.
The technology could help safety inspectors get more detailed estimates of damage in materials like aircraft parts and bridge supports. The damaged materials could then be replaced before they fail.
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