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Imaging technique can detect acoustically invisible cracks

University Of Bristol : 08 October, 2014  (Technical Article)
The next generation of aircraft could be thinner and lighter, thanks to the development of a new imaging technique that could detect damage previously invisible to acoustic imaging systems.
It has long been understood that acoustic non-linearity is sensitive to many physical properties, including material microstructure and mechanical damage. The lack of effective imaging has, however, held back the use of this important method.
Currently, engineers are able to produce images of the interior of components using ultrasound, but can only detect large problems such as cracks. This is like detecting only broken bones in a medical environment.
Imaging of acoustic nonlinearity is achieved by exploiting differences in the propagation of fields produced by the parallel and sequential transmission of elements in ultrasonic arrays.
The Ultrasonics and NDT (non-destructive testing) group aims to undertake internationally leading research into ultrasonics with a particular focus on NDT applications. Non-Destructive Evaluation concerns the inspection, characterisation and quantification of the ‘health’ of engineering structures. The team at Bristol focusses on a range of sonic and ultrasonic techniques. Research encompasses the development of new models of wave interactions, the use of arrays to image the interior of components and, in close collaboration with industry, practical application and technology transfer.
Traditional “linear” acoustic imaging techniques use an array of transducers to send sound pulses into a material and then measure the reflections, for example, to pinpoint potentially hazardous cracks in a highway bridge. But some cracks aren’t visible with linear imaging. Hairline fractures or cracks oriented edge-on to the incoming sound pulse might generate very small reflected waves.
But cracks have another important acoustic property—in response to a single input frequency, they can generate sound waves of other frequencies. Cracks generate these other frequencies in part because broken bonds allow them space to vibrate in ways that the intact material cannot. The extra frequencies can also come from regions near a crack where fatigue has altered the material’s microstructure. Detecting this property, known as nonlinearity, would allow more cracks to be identified, but nonlinear acoustic techniques in the past have had difficulty locating a specific source of nonlinearity. Practical nonlinear imaging could also be useful for identifying pathologies in biological tissue and for distinguishing different types of rock in geological research, says Dr Jack Potter, Research Assistant in the Department of Mechanical Engineering of the University of Bristol in the UK, who led the study,.
Potter, working with Bristol colleagues Paul Wilcox and Anthony Croxford, has now demonstrated a simple, nonlinear acoustic imaging technique that uses the standard linear imaging equipment. Their system combines two established linear methods. In the first, “parallel” method, sound pulses are sent from all transducers at once, but with a slight delay applied to each transducer, to focus the waves at a single focal point in the material. The transducers then detect the reflected waves. In the second, “sequential” method, each transducer fires in sequence, and the array detects a separate response from each one. For this method, the reflected waves are combined later, in the data processing stage, when the delays are added to give the equivalent of parallel inputs.
These two methods give identical results when applied to a purely linear material, but not when there are nonlinear effects. The difference in the amplitudes of the responses from these two methods, the team realized, is a measure of the nonlinearity at the focal point. By repeating the process with different focal points, they could build up an image of the nonlinearity in the material.
To test their technique, the team created fatigue cracks in an aluminium block by subjecting it to thousands of compression/release cycles, according to a standard protocol. To image a crack, they used an ultrasound device with 64 transducers to send sound pulses into the block with both the parallel and sequential methods. Rather than detecting the immediately-reflected waves, the researchers waited a millisecond before recording the sound level. By that time, the sound had bounced around enough that the intensity was equal everywhere inside the block but still a direct measure of the material’s response at the focal point. They generated an image of the nonlinearity by subtracting the two signals at every point, and the crack showed up cleanly in the image. But it was barely visible with linear imaging.
As an additional test, the team drilled a 5-mm-wide hole touching a 2.5-mm-long crack, to mimic the damage at a bolt hole in a load-bearing part. They also drilled a second hole away from the crack, for comparison. The linear method revealed the two holes, but the crack was invisible because it was small and overwhelmed by the strong signal from the nearby hole. The nonlinear method showed the location of the crack clearly, with hardly any sign of the holes.
Potter said: “Imaging acoustic nonlinearity not only provides sensitivity to smaller defects than is currently possible but may have the potential to detect damage before macroscopic material changes occur. This would enable intervention before cracks have even begun to form, as well as predicting the remaining life of an engineering structure. Crucially the technique has been achieved using standard inspection equipment, which will allow for the rapid implementation of the technique in numerous applications.”
Such advances in non-destructive evaluation not only increase the safety of engineering structures but can help future design, for example, allowing the next generation of aircraft to be built thinner and lighter. 
Right: Results from linear (top) and nonlinear (bottom) imaging of an aluminium block containing two holes (black circles), with a crack at the left hole. Only the nonlinear image reveals the crack.
JN Potter, AJ Croxford and PD Wilcox, Physical Review Letters Nonlinear ultrasonic phased array imaging
The study was supported by the UK Research Centre in Non-destructive Evaluation (RCNDE).
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