Advanced thermomechanical fatigue crack growth testing for component life validation

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The increasing performance requirements of gas turbines are driving higher operating temperatures in critical components, leading to greater creep and fatigue interactions. There is thus a requirement for thermomechanical fatigue (TMF) test data, including TMF crack growth rates. The test equipment required to meet this challenge is briefly discussed, along with the development of the test methodology. Thus, the accurate measurement and maintenance of thermal performance during TMF are considered. The determination of fatigue crack growth rates by conventional electrical potential difference measurements has been employed, utilizing convenient dwells within the TMF cycles, where both load and temperature are held constant for a brief period. These experimental methods have been demonstrated with trials on the advanced nickel base superalloy RR1000 used for turbine discs. This work has also highlighted significant microstructural effects on the crack growth rates, with the coarser grain size offering a reduced crack growth rate.

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An experimental test facility was developed to perform thermo-mechanical fatigue crack growth experiments. The thermal cycles were generated using hot and cold air flows distributed by a nozzle onto the test specimen. With the equipment it was possible to obtain TMF cycles with cycle times of 120 s with less than 10 °C temperature variation over the measurement length of the sample, cycling between 200 °C and 550 °C. The equipment allows TMF crack propagation tests up to, at least, 600 °C. The crack length and the crack closure level were determined using the potential drop technique. Thermo-mechanical fatigue crack propagation experiments were performed in-phase and out-of-phase with various R-values on samples of Inconel 718. Cylindrical specimens with a small starter crack were cycled in nominal total strain control. The crack propagation rate was determined and the correlation to the effective J-integral range, ΔJeff, corrected for crack closure presented. The fracture surfaces showed a dominance of trans-granular crack propagation with striations, indicating a low degree of time dependency in the procedure.
The fatigue crack growth rate and fracture behaviour of a nickel–base superalloy UDIMET 720 Li was investigated at 700°C in vacuum and air environments using corner crack specimens. The effects of load ratio at a frequency of 0.25 Hz were examined while the effects of loading frequency from 5 Hz to 0.008 Hz were also examined for a constant load ratio. The mode of fracture was intergranular at all load ratios at a frequency of 0.25 Hz in an air environment. Two-parameter models were proposed to describe separately the effects of load ratio and frequency. The model prediction was combined with data from vacuum tests to form a fracture mechanism map showing limited contribution of creep, while oxidation controls the fatigue crack growth rate as the frequency decreases.
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