Ensuring Infrastructure Integrity
On Thursday, March 2, 1854, an explosion rocked Hartford, Conn. A new, recently installed steam boiler blew up at the Fales and Gray Car Works. Twenty-one workers at the railroad car manufacturing plant were killed instantly. More than 50 were seriously injured.1
The tragedy was just one of numerous catastrophic failures that occurred as the production and use of boilers increased\; new fuels, like coal and petroleum, powered the engines\; thermodynamic principles weren't thoroughly grasped and nondestructive testing techniques, to ensure the soundness and safety of equipment, were rarely used. Even after the Hartford explosion, there were 1,600 additional boiler explosions throughout the United States between 1889 and 1902.2
After the Hartford explosion, Connecticut state legislators were finally moved to pass a law requiring the annual visual inspection of boilers. That law was a first step toward recognizing NDT as a valuable routine practice that saves lives, money, time and industries' reputations. NDT detects defects early, before, during or after production, or before a failure occurs.
Today, NDT is used for nearly all infrastructure and nearly every industrial product or process.
"NDT is really healthcare for our infrastructure and industrial products and equipment," says Trey Gordon, manager, NDE Program and In-Service Support for Boeing Research and Technology, Seattle, Wash. Mark Carlos, executive vice president, Mistras Group Inc., Princeton Junction, N.J., and current chairman of ASTM Committee E07 on Nondestructive Testing, concurs. "NDT is often likened to health screening because it uses noninvasive methods that do not harm the product and can test in service, while a process continues."
This year, ASTM International Committee E07 on Nondestructive Testing marks its 75th anniversary\; its membership of more than 550 represents 37 countries. The committee oversees some 175 standards. More are being developed as new technologies evolve to help detect and monitor conditions that affect performance, health and public safety.
Prior to World War II, NDT was promoted by the U.S. Department of Defense. In particular, the U.S. Navy wanted to ensure the integrity of cast materials used in its ships. After the war and into the 1950s, NDT expanded to quality control, inspecting raw materials for defects before being put into production. Subsequent concerns about public safety led to NDT in-service inspections of everything from aircraft to automobile components, to bridges, to nuclear power plants, to the equipment that builds them.
ASTM Committee E07 was formed in 1938 as a result of DOD's need for standards governing radiography, an NDT method that uses photons produced by X-rays or gamma rays to inspect materials - primarily metals - for hidden flaws.
Originally, test material was placed in an X-ray machine or between a radioactive source and a film. Since 2005, however, Subcommittee E07.01 on Radiology (X-ray and Gamma) Method has been developing standards for digital technology that is quickly replacing film. The subcommittee was first to develop standards for digital detector array systems. The digital detector array is about the size of a pizza box and operates somewhat like a digital camera, capturing a digital X-ray image and sending it to a computer for viewing and storage. Standards are currently being developed for computed radiography, which employs what resembles a bendable and reusable sheet of white plastic about the size of a standard piece of X-ray film. After the sheet is exposed to radiation aimed at the test object, it's put through a reader, which transmits the digital image to a computer for viewing and storage.
Since both digital systems allow NDT images to be stored as computer files, they eliminate the problem of the deterioration of X-ray film over time.
"Our work is complementary to what Subcommittee E07.02 Reference Radiological Images is doing in digitizing reference radiographs," explains Gordon, who chairs Subcommittee E07.01. (See below, "Going Digital - Reference Radiographs Get a New Format.")
Neutron radiography, a much later variant of radiographic testing, spun off into its own subcommittee as technology evolved. It uses neutrons to penetrate materials that photons cannot, like lead and steel.
Following radiography, liquid penetrant and magnetic particle testing were the next methodologies in the NDT arsenal to require standardization. "These methodologies are crucial to public safety and nearly every industry uses them," says George Hopman, chairman of Subcommittee E07.03 on Liquid Penetrant and Magnetic Particle Methods and a Phoenix, Ariz.-based independent NDT consultant and educator.
Liquid penetrant testing is the most widely applied and least expensive of NDT methods. It's used on metals, glass, ceramics, rubbers and composites that are smooth and nonporous to find surface-breaking defects that occur during welding, forging or casting. Detected defects include hairline cracks, porosity, leaks in new products and fatigue cracks on in-service components.
During the test, an article is cleaned and a penetrant, which contains dye, is applied. After sufficient penetration time and removal of the surface penetrant, a developer helps draw the penetrant out of any flaws, making them visible under ultraviolet or white light.
Magnetic particle testing detects surface and near-surface flaws, but only in materials that have strong magnetic properties, such as iron, nickel, cobalt and some of their alloys. Once a test item has been magnetized, small, fluorescent magnetic particles are applied to the surface. The particles accumulate where there's a break in the material, creating a visible indication of a potential defect that might compromise performance or safety.
Magnetic particle testing was widely publicized by the Chicago, Ill.-based, Magnaflux Corp. in 1936 when it inspected steering arms in cars at the Indianapolis Motor Speedway and found that more than half of the parts tested were cracked. Today, the magnetic particle method is used worldwide by aerospace, nuclear, railroad, automotive, manufacturing and steel fabrication industries.
Earlier this year, Subcommittee E07.03 began developing a new specification for magnetic particle inspection of drill pipes and pipelines to be referenced in American Petroleum Institute codes. Additionally, the committee continues to work on incorporating new technologies, such as using LED lights versus mercury vapor black lights, into existing practice.
Ultrasound testing uses sound waves at high frequencies to detect flaws, measure their size and identify their location. Like radiography, ultrasound is considered a volumetric NDT method because it can look beyond the surface of a material, providing a cross-sectional view of metals, plastics, aerospace composites and even dense materials like wood, concrete and cement, making it a valuable testing method for a wide variety of industries.
"While the physics behind NDT methodology really hasn't changed, the interface with computers has produced much improved results," says Robert Hardison, a former chairman of ASTM Committee E07 who is retired but still an active ASTM member. That's particularly true of ultrasound, where testing used to be heavily reliant on the skill and judgment of inspectors. "Now, with computer interface, there's much more information provided, so the inspector can do an improved job of analyzing the nature of the flaw and its potential seriousness," says Hardison.
Subcommittee chairman Michael Ruddy, quality/technical services manager at National Oil Well Varco, in Houston, Texas, says the standards that he uses most - ASTM E213, Practice for Ultrasonic Testing of Metal Pipe and Tubing, and ASTM E273, Practice for Ultrasonic Testing of the Weld Zone of Welded Pipe and Tubing - are still the quintessential standards in his field. Newer standards, such as E2700, Practice for Contact Ultrasonic Testing of Welds Using Phased Arrays, and E2904, Practice for Characterization and Verification of Phased Array Probes, were developed in response to the use of probes, or transducers, typically in multiples of 16 that produce enhanced resolution and results.
"Standards are typically driven by industry and composites are now used widely in the aerospace industry," notes Ruddy, so his subcommittee is currently working on WK40704, Guide for General Composite UT Standards, as well as collaborating with other NDT subcommittees to develop standards for NASA. In the future, he anticipates a need for more application-specific standards for composites, such as specific aircraft components.
Four additional E07 subcommittees develop standards for other established NDT methodologies, plus emerging ones. Acoustic emission testing uses transducers to listen for cracks that might be growing and to pinpoint their location during manufacturing processes or periodic examinations of tanks that may deteriorate over time.
The electromagnetic method, or eddy current testing, uses electromagnetic induction to detect near-surface cracking, through-wall flaws or thinning in conductive materials. "Any industry that uses condenser tubes is likely to use eddy current testing," says Hardison. That would include the aerospace, automobile, steel, nuclear and other power generation industries.
The leak testing method involves pressurizing a component to determine if there are any through-wall leaks. For instance, a tank containing an explosive material might be pressurized with a nonflammable gas. The detection of an odor coming from the tank would indicate a leak. "Even a small leak in a tank containing an explosive material could be catastrophic," notes Hardison.
Among newer NDT methodologies, standards have been developed for thermal and infrared imaging. "Suppose you're working in the field for the power generation industry. You can test for hot spots in an electric high voltage system, and if a transformer is in danger of failing, you can take it offline," explains Hardison.
Beyond developing standards for NDT methodology, ASTM Committee E07 is charged with ensuring that digital inspection data remain available and accessible despite the advent of new information technologies. For example, to evaluate the performance of long-lived but aging bridges, aircraft and nuclear power plants, inspection data on key components need to be referred to for decades, long after the computer equipment that collected it has become obsolete.
In 2004, ASTM Subcommittee E07.11 on Digital Imaging and Communication in Nondestructive Evaluation (DICONDE) published E2339, Practice for Digital Imaging and Communication in Nondestructive Evaluation (DICONDE). Based on a standard previously developed by manufacturers and users of medical digital imaging equipment that allows digital images produced by different equipment to be shared and viewed in a common format, the ASTM standard allows wide-scale adoption of a common standard for data storage and exchange of NDT data.
"That standard really sets the guidelines that apply to all NDT imaging modalities," says Patrick Howard, Subcommittee E07.11 chairman and principal engineer for digital nondestructive testing at GE Aviation in Cincinnati, Ohio. "All the subsequent standards we've developed extend that standard with guidelines that are specific to a single imaging modality."
The subcommittee's current focus is on interoperability testing, ensuring that different manufacturers can read and access each other's data using the software standards.
"It will take a while before all manufacturers and industry users comply with the standards," says Howard, "but they understand the challenge and they're supporting and driving the implementation of the standards."
To a business like The Boeing Company, the world's largest aerospace company, NDT and the standards that guide it are absolutely essential. Boeing uses nearly all NDT methods: radiography for inspecting metal castings and doing engineering evaluations of new materials and structures\; ultrasound for inspecting the raw materials that go into its products, especially carbon fiber that makes up the fuselage and wings of the Boeing 787 aircraft\; liquid penetrant and magnetic particle testing to check for cracks in plane components, such as landing gear and interior wing structures, during their manufacture\; eddy current for in-service aircraft inspection to evaluate performance and determine how to maintain the integrity of aircraft\; and even, occasionally, thermography for engineering evaluations.
"NDT helps us ensure safety, maintain our products at a cost the airline industry can support, develop new materials and designs for building better performing and more fuel-efficient products, and make sure our suppliers deliver lower cost and higher quality components," says Gordon. He adds, ASTM Committee E07 "allows our company to achieve leverage and advance aerospace technology while benefiting from the knowledge of people who are the best in the industry."
1. Heller, Charles, "In the Beginning …" American Welding Society.
2. Heller, Charles, "In the Beginning …" American Welding Society.Adele Bassett is a freelance writer who has covered everything from youth gangs in Colorado to earthquakes in Connecticut while working for a variety of corporations and publications. She holds a B.A. in English, an M.S. in journalism and an M.B.A.
Some 40 years ago, while standards were being developed for radiography, they were also being created for reference radiographs, or X-rays that are used to judge the acceptability of castings and welds in metals and alloys, including steel, magnesium, bronze, aluminum and titanium. Reference radiographs show different degrees of the severity of a condition - for example, shrinkage in a steel casting - allowing manufacturers, buyers and sellers to get an expected level of quality from a product.
Initially, reference radiographs were sets of X-ray films in cardboard mounts, compiled in three-ring binders. With the introduction of digital detector array systems and computed radiography, it made sense to convert the films to a digital format, which is what Subcommittee E07.02 Reference Radiological Images and Subcommittee E07.93 on Illustration Monitoring have been doing jointly since 2005. Sets of reference radiographs are now available on DVD in TIF (tagged image file format) or DICONDE formats, and in resolutions ranging from 10 to 400 microns, designated by different standard numbers and meant to be viewed side-by-side with test images.
Reference radiographs for aluminum castings, pertaining to the aerospace industry, were the first to be digitized. Boeing funded the project, which started in 2005, while the digitizing was done by the German Federal Institute for Materials Research and Testing in Berlin. Subsequently, the U.S. Air Force covered digitization of steel and titanium reference radiographs beginning in 2006, and United Technologies' Canadian and Sikorsky Divisions funded the digitization of magnesium reference radiographs. ASTM is financing the current digitization of reference radiographs dealing with thicker materials, with Parker Industrial X-Ray Laboratory Corp. in East Hartford, Conn., handling the digitizing.
To date, nearly 95 percent of the images have been digitized though not yet released, according to Thomas Jones, chairman of Subcommittee E07.93 on Illustration Monitoring and senior development engineer with Howmet Research Center, a subsidiary of Alcoa Power and Propulsion, in Whitehall, Mich. He anticipates that all current images will be available in digital format by the end of next year. However, that doesn't mean the two subcommittees' work is winding down. "We'll continue to resolve issues about how to properly use digital images and produce reference radiographs for new materials or applications that our existing images don't address," says Jones.