Effects of Frequency and Environment on Fatigue Crack Growth in A286 at 1100 F

Previous low-cycle fatigue tests on A286, which covered a frequency range of 5 to 0.1 cpm, have shown a pronounced frequency dependence when the tests were run in air. In contrast, tests run in a vacuum did not show such a frequency effect. This led to the conclusion that, in this frequency range, environmental effects were responsible for the frequency dependence. Air crack propagation tests have also shown a strong frequency dependence. At frequencies below 0.02 cpm the air crack propagation tests showed a stronger frequency dependence than was observed at higher frequencies and resulted in pure time dependent, cycle independent failure. In order to explain this behavior and to see if it could be observed in low frequency vacuum tests, measurements of the crack propagation rate at 1100 F were made in a 10-8 torr vacuum. These vacuum crack propagation results substantiated the assertion that at 1100 F, air produces a strong influence on the fatigue life or crack propagation rate. Additionally, these tests have shown that below 0.02 cpm the pure time dependent failure noted in air persisted in a vacuum. The vacuum results could be interpreted on the basis of a linear superposition model, where at low frequencies the behavior was that of a purely time dependent failure; at high frequencies, purely cycle dependent; and at intermediate frequencies, that of a linear superposition of these phenomena. In air this linear superposition model was not applicable because of the additional environmental interaction.