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New probabilistic damage assessment code for aircraft turbine rotors enhances flying public's safety
Multiyear FAA project will lead to approved FAA certification standard
San Antonio, Texas -- July 7, 1997 -- Southwest Research Institute® (SwRI®) engineers, in a major, multiyear project for the Federal Aviation Administration (FAA), are developing a probabilistic damage tolerance design code to improve the structrual integrity of gas turbine rotor disks used in commercial aircraft.
The probabilistic code, known as DARWIN (Design Assessment of Rotors with Inspection), incorporates the most sophisticated risk assessment methods into design procedures so that risk of failure is minimized and realistic inspection schedules can be implemented.
The development of the code is being managed by SwRI in collaboration with four major turbine manufacturers: Allied Signal, Allison, General Electric, and Pratt and Whitney. When completed, the code will be the basis for an approved FAA certification standard that the engine companies can incorporate in part or in toto into their design systems. The result, when implemented, will have the potential to reduce the uncontained rotor disk failure rate.
Current design practice for these critical engine parts uses a 'safe-life' approach. 'Safe-life' methods assume that any material or manufacturing condition that could affect the life of a part, such as a material flaw, is addressed by the rigorous standard testing procedures carried out in manufacturers' laboratories as well as by conservative estimates of mechanical properties.
Recent experience within the aircraft gas turbine industry, however, has shown that, despite this rigorous approach, material and manufacturing flaws that can reduce structural integrity may remain undetected. As one example, the loss of a DC-10 at Sioux City in 1989, caused by an uncontained disk failure, was eventually traced to just such an undetected defect. The chance of flaws such as these being detected by standard laboratory mechanical testing procedures is miniscule. Consequently, operators of these engines face a dilemma with current design practices: either frequent and expensive inspection and possible replacement of parts, or the risk of catastrophe if a part is left too long in service.
As a result, the FAA requested the Aerospace Industries Association (AIA) to review available design procedures to see what supplemental methods could enhance safety, with the outcome that the Rotor Integrity Subcommittee of AIA recommended the program now under way at SwRI.
"The probabilistic design code that the FAA has asked us to develop does not replace traditional 'safe-life' methods but provides an additional tool to minimize risk," says Dr. Gerald R. Leverant, SwRI program director of Power Generation Materials and manager of the FAA program.
The first phase of the program is focussing on the presence of melt-related defects, known as hard alpha, found in titanium alloys. Hard alpha refers to small zones in the material microstructure which can be introduced at various stages in the melting history of the alloy. The zones often have cracks or voids associated with them and can initiate the low-cycle fatigue cracks that contribute to disk failure.
Future phases of the program will apply the methodology to
other types of titanium flaws and to other widely used rotor metals such as nickel alloys.