X-raying is an established method of nondestructive testing (NDT) in the aerospace industry. Among its many application areas are (remaining) wall thickness measurements, the checking of welded joints, cast parts, or multilayer materials such as CFK (carbon-fiber reinforced plastic). Often, X-ray inspection is still based on conventional film that is developed after exposure so the image can be interpreted by NDT experts. However, digital technology is moving forward quickly now.
It offers a couple of core benefits: For one, computer radiography (CR) does away with the environmentally harmful or even toxic chemicals needed for developing the film. Secondly and likely more important, CR offers a better level of information. This is owed to the enormously wide dynamic range of the image plates that replace conventional film in CR. While film usually offers a dynamic range of more than 700 gray values, images plates together with Dürr NDT systems, for instance, can provide 65,500 storable gray values. These image plates are available in about every standard format that testing experts are familiar with from film technology. Depending on the test area, the image plate material can also be cut to the desired size and shape. In contrast to X-ray film, image plates can be handled and used under ambient light conditions, which can make the life of NDT staff easier.
How does CR work? The first step is nearly identical with film-based X-raying: The image plate is placed behind the sample or weld that needs checking, and the sample or part is exposed to high-energy, short wavelength radiation that is emitted either by an X-ray generator or by a radiation source. The X-rays pass the sample and enter the image plate where the remaining level of ray intensity gets stored.
After exposure, the image plate is placed in a scanner. To read the stored information, the image plate is excited by red laser light, which causes the image plate to emit blue light luminescence. The intensity of this light emission depends on the intensity of X-ray energy. The blue light is collected and computed to produce the actual image. After the image plate information has thus been read out, the plate gets erased and can be reused.
Understanding this process, it is clear that the scanning resolution is a critical hurdle of CR as it defines the quality level of the digital image. Therefore, Dürr NDT, a member of the German Dürr group, has come up with an efficient way of increasing scanning resolution. When it introduced its portable HD-CR 35 NDT image plate scanner, this was a breakthrough as this system offers a laser spot size of just 12.5 µm and hence a spatial resolution of up to 40 µm BSR from image processing level IP1 through to IP6. Combined with the image plates’ dynamic range of 65,000 gray values, the system produces 16 bytes raw data, which reveals small details. The manufacturer claims that this is currently the only 40 µm BSR system worldwide certified according to EN 14784 / ASTME 2446 by the German Federal Institute for Materials Research and Testing (BAM).
In contrast to other systems, Dürr NDT uses a high-precision pentaprism mechanism that rotates at 16 m/s speed and fans out a red laser beam coming from a fixed source. The blue luminescent light emitted by the image plate is collected by a parabolic mirror and directed to a receiver. This mechanism produces a very good signal-to-noise ratio with little interference. As a result, the Dürr photon collecting system (PCS) captures around 96% of the luminescent light. This compares to a ratio that is usually more in the magnitude of 30% with other technologies. “In many cases, this high level of efficiency can be used to work with shorter exposure times,” explained Hans-Ulrich Pöhler, Senior Application Specialist of Dürr NDT.
The company is selling around 3000 image plate scanners per year, according to Nils Kah, head of sales and marketing. Spanish Air Forces use it on their F5 Phantoms and F/A 18A/B Hornets and on Hercules helicopters. Also, the system is presently under evaluation by Eurocopter France.
From a practical point of view, image plate technology has further advantages to offer: “If a conventional X-ray shot is either under- or overexposed, the shot needs to be done again. With image plate technology, you can modify parameters such as contrast and brightness to the level of exposure. That way the shot can still be used,” said Kah. “Even with samples of uneven density, you may just need one exposure. Take a turbine blade. The delicate structure of the blade itself requires a different exposure than its more massive base. With CR technology, you can reveal details in both sections, based on just one exposure.” Another advantage of CR is the ease of storing and transmitting of digital images. “They can be instantly e-mailed to customers for instance,” said Kah.
During the Aerospace 09 show at Munich, Dürr NDT introduced a new type of CR image plate scanner. The (HD)-CR 43 NDT is a tower system developed for high workloads. Its minimum laser spot size of 12.5 µm and 40 µm BSR resolution are identical with the portable system. However, the image plates are contained in a cassette equipped with an RFID tag. A newly developed mechanism within the scanner retrieves the image plate from the robust alloy cassette without physically contacting the plate surface. This has a dramatic effect on cycle times: While image plates can normally be used around 300 to 600 times before the sensitive surface becomes too scratched for further use, the image plate cassettes have been successfully tested up to 25,000 reading cycles, said Kah. The image plates are available in two different grades targeted either at on-stream inspection or high-definition images. An ethernet port (TCP/IP protocol) facilitates storage and dissemination of images. “Certification is under process,” said Kah.