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 July 2007
Spotlight

Tackling Structural Failures

GETTING FROM POINT A TO POINT B IN ANY LARGE VEHICLE—BUT PARTICULARLY AIRPLANES—HAS AN EXTRA LEVEL OF SAFETY THANKS TO ASTM INTERNATIONAL STANDARD E 399, TEST METHOD FOR LINEAR-ELASTIC PLANE-STRAIN FRACTURE TOUGHNESS KIC OF METALLIC MATERIALS.

E 399 was developed by ASTM International Committee E24 on Fracture, a committee that merged with Committee E09 on Fatigue to form Committee E08 on Fatigue and Fracture in 1994. E 399 is currently under the jurisdiction of Subcommittee E08.07 on Fracture Mechanics. The test method covers the determination of fracture toughness of metallic materials.

Although E 399 was first approved in 1970, its origins go back much further than that. “The impetus behind the development of this standard is the same as why Committee E24 originally formed,” says Stephen Hopkins, a longtime ASTM member who was part of the team that introduced fracture mechanics into the Pratt and Whitney aircraft jet engine design during his career at that company. “That was because pre-existing cracks were causing large structures to fail.”

Another E08 member, Gary Bray, section head, mechanical metallurgy and corrosion, Alcoa Technical Center, says that there are many examples of these structural failures due to fracture toughness issues. Among those most commonly noted are the following:

• Liberty ships of World War II– These ships were of all welded construction. Over 1,000 of the 4,694 ships constructed experienced fracture of the hull.
• The Comet– Introduced in Britain in 1952, the Comet was the world’s first passenger jet airliner. However, in 1953 and 1954, three Comet jets crashed.
• U.S. Air Force F-111– One of these pivot-wing fighter jets failed after only 105 flight hours in December 1969.

In each of these cases, failure occurred at stresses well below the static design stress of the materials involved and resulted from small flaws or fatigue cracks.

Hopkins says that, while researchers all over the world during the 1950s, ‘60s and ‘70s were developing engineering models to evaluate the resistance of materials to unstable crack advancement, what was missing was a laboratory method to measure a material property that could be used to evaluate the material’s resistance to unstable crack advancement and be used in the engineering models.

Committee E24, which was formed in 1958 as a special ASTM Committee on Fracture Testing, presented a symposium on fracture toughness testing at the June 1964 ASTM meeting in Chicago. This symposium and the collection of papers that resulted from it (published by ASTM as STP 381, Fracture Toughness Testing and Its Applications) proved to be highly influential and provided the basis for Test Method E 399.

“E 399 is used to evaluate initial designs using anticipated nondestructive inspection limits and current damage in a structure using observed crack sizes that have developed within the structure during service,” says Hopkins. “Also, the standard is used to confirm that a specific heat of material is acceptable.”

Hopkins believes that E 399 has had a significant effect on a number of industries. “Every major industry that uses engineering materials to build equipment and structures such as aircraft, ships, railroads, power plants, bridges and large buildings makes use of E 399,” says Hopkins. “Engineering structures are now retired safely from service because of observed damage and not because they have simply been in service for a 1-design lifetime. These structures are allowed to stay in service longer and are only retired when a specific crack size is observed during routine inspections.”

“E 399 provides the historical fracture mechanics foundation for the current E08 committee, with roots deep in the origins of fracture mechanics,” says Mark James, chair of the E08 task group responsible for E 399 and who works in product design and analysis at Alcoa Technical Center. “E08 committee members have a tremendous respect for that history and foundation. But all ASTM committees are also required to keep the standards under their care consistent with today’s science and technology. ASTM enables standards such as E 399 to be living documents by providing tremendous power to individual voters through the consensus process. The end result, as E 399 proves, is a very high quality product that protects people and that meets the needs of a broad audience.”