X-ray testing for large composites

  • 19-Feb-2015 08:31 EST
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Fisheye view of the Nikon Metrology 225/450kV x-ray scanner at the University of Southampton’s µ-VIS center. (Sharif Ahmed, University of Southampton)

Anyone who has a CT (computed tomography) scan at the hospital or medical clinic knows that fitting into the tight x-ray scanning bed is a problem for large or tall patients. The same goes for many large or bulky composite aircraft components that need x-ray inspection for tiny defects on factory floors. Long parts like wings and helicopter blades, and unwieldy ones such as fuselage assemblies, simply don’t fit into most existing x-ray NDT (non-destructive testing) scanners.

A U.K.-government supported consortium of researchers at the University of Southampton and University College London (UCL), together with British aerospace companies and x-ray equipment makers, is working to remedy that situation by developing two new x-ray imaging technologies that will be able to accommodate overlarge or ungainly parts. Meanwhile, the U.S. Defense Advanced Research Projects Agency (DARPA) wants to replace x-rays with neutron beams for composites NDT.

“To capture the 3D-imaging data you need for image reconstruction, you have to rotate a part in the scanner, or move the scanner around it to image it from all directions,” said Thomas Blumensath, Deputy Director at the University of Southampton’s µ-VIS Centre for Computed Tomography. “Many aerospace components are quite flat and extended, and you can’t move them all the way through the scanner. Others are just too bulky to do so.”

“These new systems will enhance our ability to find small production defects—micrometer-size voids and inclusions—in large multi-ply carbon-fiber composite parts, such as those which are increasingly used in modern aircraft,” he said. “These capabilities will ultimately help in the production and maintenance processes, and will assist in the development of more environmentally friendly airplanes, as well as enhanced overall aircraft safety.”

Seeing defects better

The three-year, cost-shared R&D effort, which is being led by the British defense technology firm QinetiQ, has been operating for six months, with a grant of £2.1 million from the U.K. government’s technology strategy board Innovate UK, and another £600,000 in private funds from the partners. The ProjectCAN consortium supports two teams that are working on two different imaging approaches: laminar tomography and backscatter x-ray.

Researchers at Southampton’s imaging center and Derby-based Nikon Metrology UK will collaborate on scanning and visualizing the insides of large, flat components using an x-ray method called computed laminography or CL.

“We will be developing an alternative technique that applies CL techniques to overcome the limitations of conventional CT for large, flat components,” Blumensath said. CL systems typically use linear translation or limited-angle rotation to scan components.

“We’ll will use a laminography system and a computer algorithm to accumulate scan data and reconstruct it into a 3D volume-image,” he said. The scanner will move parallel to the flat surfaces. “The hardware and software will be developed to allow laminographic imaging within the Nikon Metrology 225/450kV x-ray scanner that we have here in Southampton.”

Meanwhile, researchers at Axi-Tek Ltd. of Nottingham and UCL will develop a new backscatter x-ray inspection technique to study large-area composite structures such as wing sections, engine cowlings, and fuselage components. Backscatter-based, or “one-side,” x-ray systems, which have been used for inspecting luggage at airline security gates, have the x-ray source and the detector on the same side. They typically utilize a constant-potential x-ray source and a rotating collimator to form a flying spot that passes over the surface of the object under observation.

During the past few years, UCL researchers have worked with x-ray system maker Rapiscan to develop a backscatter x-ray system for cargo and vehicle security screening as part of a U.K. government-supported R&D project. Specialists there have also been developing x-ray phase contrast imaging techniques that detect changes in the phase of x-rays as they traverse a material. This method relies on materials “phase-shifting” the in-coming rays rather than absorbing them, which can enable better detection of features that conventional x-ray systems usually cannot see.

Blumensath said that after the project is complete, a proof-of-concept system will have been designed and built. “Afterward, the technology will be developed into a marketable product in order to be made available to aircraft manufacturers,” he said.

Compact neutron scanner

Getting a better view inside dense objects, like seeing corrosion in metal aircraft wings or manufacturing defects in composite ones, is just one of the applications managers at DARPA are hoping to develop in a new program called Intense and Compact Neutron Sources (ICONS).

Although x-ray imaging has proven highly useful in industrial applications, it is limited in what it can detect, DARPA stated. While x-ray radiography can highlight heavier chemical elements such as metals very well, it is not that good at imaging lighter elements such as hydrogen and carbon. That’s why x-ray radiography machines are generally “blind” to elements with low atomic numbers.

By contrast, neutron radiography, which uses beams of neutrons to image objects, is very good at visualizing lighter elements that appear in composite materials, in some cases even identifying a substance’s atomic makeup. But existing neutron sources are big and not nearly as portable as x-ray machines. Neutron units typically extend up to tens of meters in length and require powerful energy sources to generate the neutron beams.

“We’re looking for innovative designs and construction methods to shrink a neutron accelerator from 10 meters or longer down to 1 meter or less, similar to the size of portable x-ray tubes today,” said Vincent Tang, DARPA Program Manager. “Creating a high-yield, directional neutron source in a very compact package is a significant challenge. But a successful ICONS program would provide an imaging tool with significant applications, able to deliver very detailed, accurate internal imaging of objects in any setting.”

Tang said that ICONS could enable non-destructive evaluation with greater fidelity than x-rays, revealing hidden corrosion or small flaws in aircraft wings, for example.

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