Turbines in aircraft turbojet engines are being subject to increasingly higher temperatures to improve fuel efficiency. High turbine efficiency requires the ability of turbine blades to withstand gas temperature of the order of 1350 to 1550 °C. In polycrystalline materials, these increased temperatures would cause creep strains along grain boundaries that would be unacceptable. Even single-crystal materials must be pushed to their limits to insure that engine performance is maximized. Airfoils in modern gas turbine aircraft use a systems approach for cooling to achieve required component life. There are three basic components to these systems: a cast nickel single-crystal superalloy in combination with thermal barrier coatings, and a sophisticated cooling scheme consisting of intricately designed channels and holes through the core and surface of the airfoil.
The excellent high-temperature creep and fatigue resistance of these superalloys is a result of a combination of solidsolution strengthening, absence of deleterious grain boundaries, and a high volume fraction of precipitates that act as barriers to dislocation motion. However, fatigue crack initiation also depends on the microscopic defects, which can be categorized as intrinsic defects and deviant material defects.
Most thermal mechanical fatigue (TMF) cracks in airfoils start from the cooling holes. Thus, a new thermal fatigue experimental technique is needed to measure the structural life of the specimen containing through holes similar to the ones that are drilled in cooled airfoils.
A newly developed TMF test procedure was used on specimens with laser drilled holes. The cooling holes’ effect on intrinsic serviceable fatigue crack growth and on corresponding TMF life compared to baseline cast nickel single crystal data was investigated. The new method would allow explicit measurements of the effects of crystal secondary orientation, hole geometry, skew angles, and laser drilling effects on TMF crack initiation and propagation.
These TMF test results can be directly used to evaluate structural life of the cooled airfoils as well as provide necessary information on the applicability of smooth specimen TMF data to the assessment of real service components with small features causing local stress concentration.
The new experimental method includes: a) a notched test method and procedure for TMF crack growth; b) successful demonstration of induction thermograpghy for capturing crack growth versus cycle count and subsequent analysis of that data; and c) fast cycle thermomechanical fatigue testing using active cooling, allowing 30-second heat-up and 30-second cooling under sinusoidal command and feedback response.
Using this new technique, it was shown that the life of TMF specimens with notched holes exhibit a 4-times debit compared to smooth gage section specimens under the same loading conditions. In addition, the effect of the hole secondary crystallographic orientation on crack initiation and propagation was investigated. All tests demonstrated that the cracks start crystallographically along the crystallographic plane and later change to mixed-mode fracture. Fractographical analysis using both optical and SEM microscopes revealed that major crack propagation takes place at the low-temperature portion of the cycle; however, there is noticeable damage accumulation during the high-temperature compressive load portion of the cycle. Crack propagation under TMF loading conditions is considerably faster than corresponding isothermal LCF crack growth tested at the temperature and similar loading. Such a significant change in number of cycles to failure must be accounted for in any damage tolerant design system.
This work was done by R. K. Kersey and A. Staroselsky of Pratt & Whitney, and D.C. Dudzinski and M. Genest of the National Research Council Canada, Institute for Aerospace Research, for the Air Force Research Laboratory. AFRL-0216
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Thermal Mechanical Fatigue Crack Growth Testing
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