Abstract
It has been reported that the minimum observed fatigue lifetimes in numerous structural alloys are dominated by the small-crack growth behavior. While environmental effects on fatigue crack growth behavior of Ni-base superalloys are well documented within the literature, the published research is largely limited to long crack behavior due to the difficulty of measuring small cracks in a vacuum chamber. A testing capability incorporating optical measurement of small fatigue cracks under ultra-high vacuum (~10−9 Torr) and at elevated temperatures has been developed. Optical measurement capabilities have been evaluated under laboratory air conditions to determine crack measurement accuracy. A vacuum fatigue test was then completed at 650 °C on a Ni-base superalloy specimen to quantify the effect of vacuum on the propagation life within the small crack regime. The effectiveness of this test capability and considerations for this technique are discussed.
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