Effect of Frequency on Fatigue Crack Growth Response of AISI 4340 Steel in Water Vapor.

摘要

The effect of cyclic load frequency on the room-temperature fatigue crack growth response of AISI 4340 steel (tempered at 205 C) in water vapor at 585 Pa (4.4 torr) has been investigated. The water vapor pressure was chosen to avoid capillary condensation at the crack tip. Crack growth tests were conducted over a frequency range from 0.1 to 10 Hz, and a range of stress intensity factors (K). The maximum K in these tests was below the apparent threshold K for sustained-load crack growth in this environment. Fatigue crack growth rates were found to increase with the reciprocal of frequency (that is, with the period at load), and exhibited transient behavior when the test frequency was altered. By separating the growth rate into an environment independent component (pure fatigue) and an environment dependent component, it was found that the environment dependent component varied linearly with period over the frequency range 0.1 to 10 Hz. The results are correlated with the kinetics for water vapor/metal surface reactions, and with other fatigue crack growth data on high-strength steels tested in aqueous and H2S containing environments. The frequency and transient effects are discussed in terms of possible crack-tip processes. (Author)