Visualization of internal small fatigue crack growth

Abstract

Internal small fatigue crack growth caused by gigacycle fatigue of high-strength steel was successfully visualized using beach marks created by a repeated two-step fatigue test. The created beach marks were clear enough to visualize the initial stage of the internal fatigue crack growth. The beach marks indicated that the internal crack shape just after initiation from an inclusion was not a circle but a half-ellipse, suggesting that the early stage of the internal crack growth requires three-dimensional modeling, and therefore, an extremely slow crack propagation rate, smaller than the lattice length, is expected in two-dimensional modeling. Moreover, it was found that the crack propagation life exceeded 1.42×107 cycles. This meant that the minimum crack propagation rate per cycle in two-dimensional modeling was smaller than the lattice length.