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21/05/2010
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Digital Image Correlation – a technique whereby movements and distortions are detected by comparing digital photographs, is about to get airborne.
 Dr Nick McCormick of NPL explained at the laboratory's recent open day that whereas currently, structural movements and distortions could be monitored by taking two successive digital photographs using a camera on a tripod, the plan was to additionally do this from a small unmanned helicopter.
The technique has been successfully used with 6Mpixel and 39Mpixel cameras to detect movements down to 1/100 pixel - imperceptible to the human eye.
The method has been applied to the measurement of residual stress from incremental hole drilling in small structures in partnership with Airbus, AWE, Stresscraft, British Energy and LaVision. It has also been applied to a full sized bridge on a trunk road, looking for possible developing cracks, polymer composites, thermal expansion of electronic components, nuclear graphite, air bags and damage development in silk screens.
Dr McCormick says that the pixel blocks need to be random and unique with a range of contrast and intensity levels, and it works with painted surfaces as well as bare, provided they have texture, and both ambient lighting and flash illumination. He noted: "One of the big issues is to be able to exactly re-position camera equipment. That is the key to a lot of a these things. For very large images, I have written my own software, which runs on the NPL grid on desk top machines."
Competing techniques include laser shearography and speckle interferometry - both of which can be more cumbersome and costly.
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Author
Tom Shelley
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