Aircraft safety relies on high-quality manufacturing and continuous safety checks and maintenance. Engineers can easily spot small dents in the fuselage or flecks of peeling paint, but what about microscopic cracks? Can they detect sub-surface defects, assess their risk and take appropriate action?
Several non-destructive testing techniques can ‘see inside’ materials, but each has significant drawbacks. For X-ray imaging, users must take special safety precautions and systems are rarely portable; ultrasound scanning often involves smearing the surface of a material in gel; microwave sensors have poor resolution.
A consortium of EU-funded researchers and manufacturers from the aerospace industry formed the DOTNAC project to develop a new type of materials scanner.
Terahertz imaging looked like the best way to combine the benefits of existing systems while removing many of their drawbacks. In the electromagnetic spectrum, terahertz waves range from the far-infrared to the microwave region. They can penetrate most non-metallic materials without any contact, but pose no health risks to system operators. Their short wavelength also helps to produce high-resolution images.
The researchers collaborated to develop two quite different systems. One sends out rapid-but-short pulses of waves; the other produces a continuous wave at either 100 GHz or 300 GHz.
According to DOTNAC coordinator Marijke Vandewal of Royal Military Academy in Belgium, each system involved significant research and technological development, combining the expertise of leading terahertz research groups from across Europe.
“To make the pulsed system portable, for example, we had to find a way to separate the scanning antenna from the bulky pulse generator,” she says. “By removing all mechanical parts, we are first to build an all-optical system. It sends the signal wave straight from the laser generator through optical fibres.”
To build the continuous wave scanner, the project had to push existing radar technology to its limits in order to generate terahertz signal frequencies.
Each prototype scanning device went head-to-head against existing imaging systems as they tested a set of specially made samples.
“The terahertz systems produced very satisfactory results,” says Vandewal. “Depending on the application, their performance was equal to or better than many of the conventional techniques.”
The continuous wave device seems ideal for finding defects in multilayer sandwiches of composites. The pulsed wave device was the only system to detect micro-millimetre defects inside materials – something even X-rays sometimes fail to do because composite materials are often transparent to X-rays.
However, the big advantage of terahertz systems comes from the way they operate: they are safe, require no contact with materials and can be used on mounted components in operational aircraft.
Vandewal suggests that terahertz testing will focus maintenance on prevention rather than repair. “By detecting tiny weaknesses and defects early, the industry can plan maintenance or take measures to avoid deterioration. Automated scanning in manufacturing and in situ testing on aircraft will also reduce production and maintenance times, helping to lower costs and increase the industry’s global competitiveness.”
Since the project finished in August 2013, the partners have received more than several requests from aerospace companies to test material samples using the terahertz technologies. The project partners are currently exploring opportunities to develop a prototype portable system or a laboratory-based terahertz testing service.