A new way to find tiny flaws in aircraft parts

http://ec.europa.eu/programmes/horizon2020/en/news/new-way-find-tiny-flaws-aircraft-parts

An EU-funded project has developed the first terahertz scanners for non-destructive testing of aeroplane parts. Outperforming existing technologies, these systems detect small defects on and deep within composite materials – improving safety in the air and helping manufacturers and airline operators optimise maintenance and lower costs.

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.

Two systems

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.

Rigorous tests

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 situtesting 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

Seeing Inside the Nose of an Aircraft

This terahertz measurement system for non-destructive testing measures the thickness of multi-layered plastic films at a rate of 40 times per second. (Credit: Fraunhofer IPM)

My Note: This article comes shortly after Advanced Photonix announcements of their contract award with the IRPC consortium to inspect airplane structures. Please see articles listed.
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http://terahertztechnology.blogspot.com/2012/02/dr-irl-duling-director-of-terahertz.html
http://terahertztechnology.blogspot.com/2012/02/laser-shearography-chosen-as-primary.html
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By Fraunhofer-Gesellschaft
http://www.pddnet.com/news-seeing-inside-the-nose-of-an-aircraft-042612/
The planned arrival time, the request to land or the landing direction – this is the kind of information pilots discuss via radio with ground staff in the control tower.
The nose of the aircraft, the "radar dome", receives incoming radio signals and transmits radio signals sent by the pilot as well. It is made of a fiberglass composite. But if even tiniest imperfections arise during production — if, for instance, little foreign particles, drops of water or air bubbles become enclosed in the resin – over time they can cause fine cracks through which moisture can seep. This causes interference in radio traffic through the aircraft nose, introducing static into the signal.
As part of the Dotnac project, researchers at the Fraunhofer Institute for Physical Measurement Techniques IPM in Kaiserslautern are working with partners in industry and research to develop a new testing system: the system uses terahertz waves to completely scan the aircraft nose, which is several centimeters thick, and immediately identify any flaws.
The frequency of terahertz waves falls between that of microwaves on the one hand and infrared light on the other. They are completely harmless to humans. The waves are generated in a rolling cabinet not unlike those found in many offices: it contains a microwave source and all electronics to control the system and to collect the data.

A frequency mixer multiplies the frequency of the microwave radiation generated into the terahertz range. Researchers have connected the actual measurement module to this container by means of electrical wires. This module emits the terahertz waves toward the radar dome. The material reflects the radiation, and the detector integrated in this module analyzes the reflected terahertz radiation. If there are any air bubbles or little imperfections embedded in the material, they turn up in the reflected signal.
The main challenge facing researchers was to find out which terahertz frequencies they would have to use to bombard the material to achieve the most effective results for the various imperfections. Higher frequencies create better resolution, while lower frequencies have less difficulty penetrating the material. The researchers select from a range of different frequencies depending on the errors the researchers are looking for in the case concerned.
The scientists have already developed a prototype of the testing system. It will be presented at the Control trade fair, May 8-11, in Stuttgart (Hall 1, stand 1502). Around a year from now, the scanner will have advanced to the point that it will scan and analyze aircraft noses automatically. Thus far, simple scanners for level and rotations symmetrical objects are available.
Researchers have come up with another terahertz testing system as well, one that analyzes the thickness of layers – such as are found on aircraft and cars. "Our terahertz measuring system is one of the few robust enough for industrial use," according to Dr. Joachim Jonuscheit, deputy head of department at Fraunhofer IPM.
Just like the system that checks aircraft noses, this one also consists of a rolling cabinet along with a transmitter and a receiver connected to the container by cables five meters long. This system works with very short terahertz pulses. Each pulse is partially reflected off of the interfaces of the layers: the surface of the first layer, the interface between layer one and two, and so on.
The deeper the layer reflecting the pulses, the longer the pulses take to return to the detector. Using the time each pulse takes to make its way back to the detector, built-in software automatically calculates the thickness of the various layers.
The system's great advantage is its robustness. But how did researchers accomplish this?
"First of all, we no longer shoot the laser that excites the system by open beam as typically used in terahertz systems; instead, we feed it through optical fibers. And secondly, we have fixed and arranged the optical elements to make them mechanically robust. We have also improved the manufacturing processes for the semiconductor components – the transmitters and detectors – to make the individual elements more resistant," Jonuscheit explains. At the Control trade fair, the researchers will demonstrate live measurements on multi-layered plastic films of varying thicknesses.