Kinetics of fatigue crack growth in ferrosilicon in a vacuum and in air

Abstract

1. Two stages of fatigue crack growth were observed in ferrosilicon, alloys in air and in a vacuum. The first is one of slow crack growth characterized by a low value of acceleration, while in the second stage acceleration abruptly increases. 2. The rate of crack growth is less in a vacuum than in air: 10 times less in the first stage, and 10–4.0 times less in the second stage in proportion to crack growth. 3. Fatigue crack growth in both stages is accompanied by the formation on the fracture surface of brittle cleavage faces and fatigue striae, as well as sections of tough fracture. 4. With loading both in air and in a vacuum, the spacing of striae over the entire length of the crack is practically constant. 5. The macroscopic rate of crack growth in the stage of slow growth is less than the rate measured by the spacing of striae, being less by a factor of 3 in air and by a factor of 30 in the vacuum. This can be explained by stoppages of the fatigue crack and, in particular, by the fact that a striation is not formed after each cycle. 6. The decrease in the rate of fatigue crack growth in a vacuum compared to the rate in air is apparently due to the facilitating of stress relaxation at the crack tip.