Transitioning from familiar and trusted technology to new, more advanced technologies isn’t always a choice; It is a necessity.
This article is from the 2015 NESC Technical Update.
For NASA, x-ray film radiography has been the go-to inspection method for decades. Right now, NASA is using this technique to inspect tube welds on the Orion Multi-Purpose Crew Vehicle (MPCV) that will one day take astronauts to destinations beyond our Moon. But film production is declining, and decisions on what technology will take its place must be made before film costs become unaffordable or production ceases all together.
During the Orion Tube Weld Digital Radiography Assessment, the NESC evaluated technologies that may soon replace film radiography, not just for the Orion MPCV, but for the Agency’s nondestructive evaluation (NDE) community as a whole.
Film radiography works by finding hidden flaws in materials using x-ray photons to capture those flaws on film. “It’s very common around the Agency and it’s used for many different things, everything from solid rocket booster aft skirt welds to small tube welds, tanks, seams, and for part certification,” says Eric Burke, Technical Lead for the NESC assessment. “It’s the first technique we use to get a quick look inside, and it was one of the first techniques added to our NASA Standard 5009 (NDE Requirements for Fracture Critical Metallic Components). It’s been around for a long time.”
Most people are familiar with film radiography’s use in the medical community. “It’s exactly the same as a medical x-ray,” Mr. Burke says, with a slightly different application. While a doctor uses the technology to see through the human body, NASA is looking through composites and metals. “The medical world and the NDE world are very parallel when it comes to using ultrasound and x-rays. However, doctors are looking for things that are millimeters in size, while we’re looking at things down to the micron level,” Mr. Burke says.
“And typically, higher energy sources are used in NDE in order to penetrate denser, thicker materials,” adds Dr. William Prosser, NASA Technical Fellow for NDE.
Aside from having to set up a safety zone prior to use, x-ray radiography is simple to operate and gives a clear image of complex parts, even more so than ultrasonic techniques, which can lose focus the farther it penetrates a material. “x-rays go straight through,” notes Mr. Burke.
But the film needed for x-ray radiography is nearing extinction. “We don’t consume as much film as the medical community, but the medical community has moved to digital technology, so film is harder to get,” says Mr. Burke. Kodak, one of the primary manufacturers of x-ray film, has all but stopped production, leaving only one primary supplier left – Fuji. “It’s getting prohibitively expensive to get it,” he says.
Advantages of Digital
The NESC assessment took a closer look at a newer technology called computed radiography (CR). It has been around for a few years, but only recently reached a level to be comparable to, and in many ways better than, film.
CR uses similar equipment to conventional radiography, except that in place of a film to create the image, an imaging plate made of photostimulable phosphor is used. The plate is housed in a special cassette and placed under the body part or object to be examined, and the x-ray exposure is made. Then, instead of taking an exposed film into a darkroom for developing in chemical tanks or an automatic film processor, the plate is run through a special laser scanner, or CR reader, that reads and digitizes the image. CR can then leverage powerful software to enhance the digital images through functions such as contrast, brightness, filtration, and zoom.
The advantages of CR are akin to the benefits digital cameras offer over film photography. “To develop x-ray film requires chemicals,” says Mr. Burke. “With digital, you don’t have to deal with chemicals and their disposal, which is a big deal from an environmental and cost aspect. Digital gets away from all of that.” And film, which must be stored properly and can deteriorate over time, is replaced by plates that can be reused.
In addition, time is required to get the processing done, adds Dr. Prosser. “You shoot film, process it, and if you didn’t get the shot right, you do it again. Today, there’s a big driver to reduce costs and get results much quicker.” With CR, images can simply be erased and taken again.
Finding defects is easier with digital as well. “In the past you would take x-ray film and put it under a back light while using a magnifying glass to find defects,” Mr. Burke states. “Now, with CR, images are automatically digitized into the computer. There are a lot of enhancements you can do to the images, using digital filters within the software, which make it easier to find defects. Then you can print your report and you are ready to go.”
During the assessment, the NESC team used commercially available CR equipment to put the technology to the test. They used tube weld samples of various diameters, thicknesses, and materials, such as stainless steel and titanium. The team looked at good welds, as well as those where lack of fusion and porosity defects were manufactured into the samples. Overall, they evaluated examples of some of the hardest and easiest scans currently being done on the Orion crew module, looking at more than 100 defects. After careful study of the results, what they ultimately found was that for finding the flaws in tube welds, CR was doing a better job than film.
“Until recently, the pixel size on CR detectors wasn’t nearly as fine as the grain size of the film,” says Dr. Prosser. “Historically we could get better results with film because we had better media to record the images. But now CR plates and scanners have gotten much better.”
Opening the Door for Digital
Results from the NESC assessment have opened the door for production use of CR, states Dr. Prosser. Proving the technology will work for the Orion MPCV Program in certain applications will hopefully stimulate more in depth probability of detection studies. “It also opens the door to trying CR on other problems where we typically use film.”
The transition will not be a fast one, though. Baselines for radiography in NASA Standard 5009 are based on film. “But now that we’ve had some success, it will pave the way to doing the work required to get into the NASA standard and applying the technology on a wider scale. It will help us transition from film and bring significant advantages in capabilities and cost savings,” to NASA as well as its commercial crew partners, Dr. Prosser adds. “There’s still more work to be done, but this is an important step. To include CR in the NASA standard will be a big change across the Agency and for every program.”