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Machine for study of high cycle fatigue wins R&D 100 award

San Antonio, Texas -- August 25, 1999 -- A unique instrument designed and built at Southwest Research Institute® (SwRI®) to study high cycle fatigue has been named one of the 100 most significant technical accomplishments of 1999 by R&D Magazine.

The SwRI-designed High-Cycle Fatigue Machine (HCFM), developed using internal research funds, combines a high-frequency machine with a scanning electron microscope (SEM) to help engineers understand metal fatigue, a potentially catastrophic phenomenon that can affect high performance aircraft engines.

High-cycle fatigue (HCF), so-called because it appears after millions of repetitive cycles of use, can affect even high-strength metals such as the titanium alloys used in the gas turbines that power aircraft. These machines must endure repeated, long-term exposure to high temperatures and static and dynamic stresses caused by rotation speeds of 7,000 to 10,000 revolutions per minute.

Under these conditions, blade and disk materials must be able to withstand a number of stress cycles. HCF cracks can initiate and grow at stress levels that are low in relation to the material's yield stress. And, because of the very large number of cycles involved, laboratory study of HCF failure phenomena must be carried out at high frequencies to characterize the behavior of cracks within reasonable time limits.

The High-Cycle Fatigue Machine applies stress to a test specimen by two means: a steady stress is applied by hydraulic pressure that simulates the centrifugal forces in the engine; at the same time, high-frequency dynamic stresses are applied by piezoceramic plates that cause the machine as well as the specimen to resonate between 1,000 to 1,700 cycles per second.

The resonance conditions amplify the loads generated by the piezoceramic plates and are an important design feature of the machine. Static loads of up to 6,000 pounds, and dynamic loads of 1,200 pounds, can be applied to a specimen. These very large loads mean that specimens with cross-sectional areas comparable to an actual turbine blade can be studied.

The HCFM was designed to be small and light enough to fit within an SEM so that the depth of field and high resolution of that instrument could be exploited to investigate the initiation and growth of fatigue cracks that can occur under HCF conditions. Knowledge of how these cracks develop and interact with the microstructure of the material provides useful information for the manufacture, use, and inspection of a variety of advanced engine materials.

One current application of the HCFM is for a major program sponsored by the U.S. Air Force Research Laboratory, Wright-Patterson AFB, Ohio. A national team comprised of the University of Dayton Research Institute, Purdue University, U.S. engine manufacturers, and SwRI are working together to develop an improved design methodology to alleviate high-cycle fatigue problems in military jet aircraft. The five-year research effort, which began in early 1997, is applicable to a wide range of jet engines in use by the U.S. Air Force.

Dr. Stephen J. Hudak Jr., an Institute scientist and program director in SwRI's Mechanical and Materials Research Division and SwRI project manager for the engine program, says that three key concerns of the jet engine study are being evaluated at SwRI - fretting fatigue, where the blade attaches to the disk, foreign object damage (FOD), and interactions between fatigue cycles of different magnitudes.

"All of these phenomena occur in a frequency regime where we are now able to test because of the HCFM," he said. "We can put 10 million cycles on a test sample in about an hour and a half using the HCFM. Conventional test methods would take nearly a week."

Phase 2 of the program began recently, and the HCFM is expected to play a role there as well. "We're extending a derivative of the HCFM to allow an overall temperature of 1900 degrees F. This will allow us to simulate the hot section components of the aircraft engines."

SwRI's award-winning High-Cycle Fatigue Machine is the third in a series of innovative and successful instruments designed and built in support of a broad historical program for fundamental research on fatigue and fracture. It complements a cyclic loading machine, built in 1978, that operates at tensile loads up to 1,000 pounds, and a high-temperature machine, built in 1985, that can operate at the same loads and at temperatures up to 850 degrees C.

For more information about SwRI's IR&D 100 award, contact Deborah Deffenbaugh, Communications Department, Southwest Research Institute, P.O. Drawer 28510, San Antonio, Texas, 78228-0510, Phone (210) 522-2046, Fax (210) 522-3547.

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