This article is from the 2022 NESC Technical Update.
What is Standard NDE?
A standard NDE flaw size is intended to represent the largest flaw size that may be missed by most qualified inspectors for a specific NDE method. The benefit of tabulating standard NDE flaw sizes is to avoid the requirement that every inspector perform POD demonstrations, which can be resource intensive.
Dr. William Prosser has spent his NASA career looking for flaws. As the NASA Technical Fellow for NDE, he and his team have an arsenal of nondestructive tools like X-ray radiography, ultrasound, and dye penetrant inspection for detecting the cracks and imperfections in the metals and welds of spacecraft. Along with the inspection tools, the NDE and structures communities rely on NASA Standard 5009B, where the NDE detection capability requirements can be found for NASA systems or components that are considered fracture critical.
“The standard specifies how well we can reliably detect various flaw sizes with our different NDE methods,” said Dr. Prosser. But where did the flaw sizes found in 5009B—the ones used to determine whether a spacecraft will survive its load environments—originate? The question had never been rigorously explored until recently, when Dr. Prosser and a team of NDE experts needed to test the results of a new flaw size probability of detection (POD) method and found some inconsistencies they couldn’t explain. What resulted was a deep dive survey of the NASA flaw size origin story that took Dr. Prosser and his team back 50 years to 1973. “It ended up being an archaeological project we never expected.”
What Prompted the Survey
NDE technologies have advanced in the past 5 decades, allowing the detection of smaller and smaller flaw sizes. X-ray technology, for example, has evolved to a fully digital platform, eliminating the need for film. “Digital radiography has different characteristics than film and may provide different levels of reliable flaw detection capability,” explained Dr. Prosser. “But the detectable flaw sizes for radiography in 5009B are still based on film from testing performed five decades ago.” It left his team wondering if spacecraft designers were adding more weight and mass than necessary because only certain minimum flaw sizes were thought to be reliably detected. That led to Dr. Prosser and his team developing new POD methods to ensure designs based on results from the new technologies continued to have the appropriate levels of conservatism.
“It’s something the NESC has wanted to address for a while,” said Dr. Peter Parker, a NASA statistician and member of
Dr. Prosser’s NDE POD team. “We wanted to propose a new POD method that would fill the technology gap. The majority of NASA spaceflight system designs rely on these flaw sizes, and this new method will have broad impact.”
Researching the Flaw Size Timeline
The concept of NASA’s NDE flaw sizes originated in the early 1970’s with the Space Shuttle Program (SSP) and were documented in the Orbiter Fracture Control Plan (OFCP). From there, the flaw sizes made their way into the fracture control requirements for payloads that were used for the Shuttle, then to Marshall Space Flight Center Standard 1249, and ultimately into NASA-STD-5009.
The specific origin of the initial flaw sizes in the OFCP, though, was more anecdotal. History suggested the flaw sizes were linked to a series of POD test programs performed by Shuttle prime contractors, which were combined and jointly analyzed by C.R. Bishop of Rockwell International, who in 1973 published a formative document titled “Nondestructive Evaluation of Fatigue Cracks.”
Because none of the referenced standards provided details or references to the flaw size data sources, the team consulted with people involved in the SSP. They revealed that while some flaw size values were based on the quantitative analysis performed by Bishop, others were based on undocumented engineering judgement or unnamed data sources. More recent analysis showed the rudimentary methods used by Bishop were nonconservative in estimating POD parameters in most cases and that a key mathematical error had been made.
Why Does this Matter Now?
Since NASA has used these flaw sizes without failures for more than a half century, what could a historical survey reveal that would matter decades later? For Dr. Prosser and the NDE and structures communities, the findings were significant.
“We have been using results for a long time that we thought were conservative values for detectable flaw sizes. When those data are viewed with modern analysis, there is a degree of conservatism, but it’s not as high as we originally thought,” said Dr. Prosser. That understanding will ultimately better inform structure design. But just as important, he said, is that “this historical study reminds us how important independent reviews are to ensuring mistakes don’t get passed on.”
These checks and balances are a tenet of the NESC Review Board, which approved in September 2022 the final paper documenting what Dr. Prosser’s team found during the NDE survey. “In addition to better analysis and reviews, we need to make sure that when we write standards, we provide good references and documentation on where values come from,” said Dr. Prosser. “The sky is not falling because of what we found, but it certainly points to how we want to do things going forward.” The survey also confirmed the importance of optimizing how POD testing is done. “Bishop used more than 400 flaws in the original study, which is above and beyond what we would do in a POD test today,” Dr. Prosser said. This is why Bishop’s 1973 report, though nonconservative in hindsight, was such an important legacy document. “They were figuring out how to assess the reliability to find flaws with NDE techniques, which was something no one had done before. They had to invent the methodology to analyze the data.”
Today, generating 400 cracked specimens for a POD study would be cost prohibitive, so optimizing the test has become a key POD objective. With a better understanding of NASA’s flaw size origins, the NDE community can focus on a specific range of flaw sizes that requires fewer crack specimens.
“Cost has been a perceived barrier in conducting a standard NDE study,” added Dr. Parker. “Our proposed POD method is smaller, making it more approachable and something engineers may be more willing to pursue,” he said, by providing guidance on the numbers of specimens, flaws, and inspectors required, leveraging knowledge of NDE methods and more modern NDE analysis approaches in use now.
The next step would be spreading the word, not only on the historical findings but also the new POD method. “The ultimate user of this information is the fracture control community,” said Dr. Prosser. “These numbers get used. When you are designing spacecraft, you have to show that the structure is going to survive without failing. Knowing the loads and cycles expected, you have to assume there is a certain flaw you didn’t find with your inspection, and you have to show it will survive a certain number of lifetimes for conservatism. These flaw sizes are key in doing that kind of analysis.”
This survey was the motivation for development of the first documented methodology to conduct a NASA standard NDE study that will be referenced in NASA-STD-5009C. This methodology would also enable the updating the standard NDE flaw sizes and specification of standard NDE flaw sizes for methods not included in NASA-STD-5009B.