Effects of vacuum environment on the fatigue crack growth properties of high strength steel

-Behavior of small crack by ΔK-increasing test-

Abstract

In recent years, it has been revealed that in high strength steel, fracture occurs from non-metallic inclusions inside of the material in the very high cycle region. However, because of the difficulty in directly observing the internal fatigue process, its mechanism is largely unknown. In this study, we try to clarify the internal fatigue process on the basis that the environment around an internal crack is like a vacuum environment. Also, it has been reported that the effect of vacuum environment is more prominent in the initial stages of crack propagation. This suggests that targeting the crack propagation process of small cracks, like that with the same size as a small defect, would shed light on the controlling factors in the internal fatigue process. Thus, we conducted ΔK-increasing tests in air and in vacuum using specimens with a small artificial defect on its surface. As a result, fatigue crack propagation rates in vacuum were lower than that in air. Also, when compared with the results of long cracks, the crack propagation rates in both environments were much higher. However, after eliminating the effect of crack closure for the results of long crack, the crack propagation rates in both environments matched.