Fatigue crack behavior of incology 800 tested in the bend and the push-pull mode

Abstract

Crack initiation and crack propagation behavior of Incology 800 push-pull and bend fatigue specimens tested at elevated temperatures were investigated. Specimens which were fatigue tested in air at 649, 704 and 760 °C, at a strain rate of 4 × 10 −3 s −1 and at 0.5, 1.0 and 2.0% total strain ranges were considered. Scanning electron microscopy was used to construct composites from which crack growth information was obtained. Push-pull specimens exhibited increasing crack growth rates except at the 0.5% strain range where the crack growth rates were constant just prior to fracture. Bend specimens displayed increasing crack growth rates initially, and decreasing growth rates after some time up to the time of fracture. The number of cycles to initiate a crack of the size of one grain diameter (≈ 0.1 mm) was determined by extrapolating curves of log a vs. N (where a is the crack length and N is the number of cycles). When the fraction of fatigue life to initiate a crack, N 0/ N f, was plotted vs. the plastic strain range it was observed that: for the push-pull mode, N 0/ N f varied only slightly with the plastic strain range, and appeared to be insensitive to the temperatures investigated; for the bend mode, N 0/ N f was strongly dependent on the plastic strain range, but also appeared to be insensitive to the temperature.