Finding hidden cracks in large structures, such as bridges and airplanes, can be the difference between life and death, if the flaw leads to a catastrophic collapse. Scientists at Bristol University in Britain say they have discovered a new way of looking inside crucial metal parts to identify cracks before they fail, using sound imaging.
On August 1, 2007, an eight-lane steel bridge over the Mississippi River in Minneapolis, Minnesota, collapsed during rush hour, killing 13 people and injuring 145.
An investigation concluded that the main cause of the collapse were undersized elements of the bridge that simply gave way under the heavy load.
Metal fatigue like that starts with microscopic cracks that form at points of concentrated stress.
A group of scientists at Bristol University, led by senior lecturer Anthony Croxford, discovered that sending hundreds of different ultrasonic waves into a structure, and then listening and analyzing their echoes can be used to detect the tiniest cracks.
“It lets you see smaller cracks, closed cracks, cracks that, so when I say closed cracks, imagine if you have a crack in a piece of metal, it could be a bit open like that, it could have a gap in between it. If you have a gap in between it, you get reflections off the edge of it, but you don't really know how big it is," said Croxford.
Croxford says unlike purely linear systems, which create echoes of the same frequency sent into the material, his ‘phased array’ of sound-sending units returns harmonics - echoes of different frequencies.
“The nonlinear approach means that you can actually hear something from them, you're listening to different effects, rather than listening for just that echo from the crack," he said.
Using an array of attached sensors, Croxford is testing a seemingly perfect part of a wing from an Airbus A320. A linear system would not be able to discover cracks forming around the rivet holes, because a hole would create one big echo.
“By using this novel approach we can now pick up a crack close to a hole, which is directly relevant, to say, aerospace applications, where they're worried about cracks growing from rivet holes, things like that," said Croxford.
Croxford says the phased array system uses only one piece of equipment to get both a nonlinear and linear image.
The technology could allow inspectors to get more accurate assessments of damage in materials such as crucial aircraft parts, so they can be replaced before they fail.