Study of the Crack Propagation Property in Very-long Life Regime in Vacuum Environment of High Strength Steel

Abstract

In general, high-strength steel is known to occur an internally initiated fatigue fracture called "Fish-eye failure" in the very long life region. In addition, the fatigue life of a fish-eye failure is dominated by the ODA (Optically Dark Area) that forms around the fracture origin. Therefore, it is necessary to elucidate the formative mechanism of ODA in order to predict the fatigue life of fish eye failure. Besides, it has been reported that small surface cracks propagate at extremely low speeds below the lattice spacing in a vacuum simulating the interior of high-strength steel. However, it has not been confirmed that the growth of these small cracks forms ODA. In this study, fatigue tests were conducted from a very short crack to a long crack, which corresponds to internal failure in the very long life region, instead of the fatigue tests with a relatively long crack of around 150 μm that have been conducted so far. The results of intermittent observation of the growth behavior and the relationship between the crack growth rate, stress intensity factor, and maximum stress suggest that small cracks grow and form ODAs.