Near-threshold elevated temperature fatigue crack growth rates in nickel-base superalloy IN-100 are sensitive to load history effects that cannot be attributed to crack closure. The effects disappear in vacuum, suggesting operative mechanisms that are environment related. Experiments on compact-tension coupons under low amplitude loading show that programmed step-wise reduction in stress ratio causes noticeably retarded crack growth that cannot be explained from crack closure considerations. The effect rapidly diminishes with increasing stress-intensity range, indicating a near-threshold phenomenon. This effect is altogether absent in vacuum tests at elevated temperature. The experiments also indicate crack growth retardation when fatigue loading is mixed with hold-time. This appears to be caused by incompatible macro and micro crack front orientation associated with differences between the nature of sustained load and fatigue cracking.
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