Abstract
Internal fatigue crack growth rate was measured by using beach marks resulting from repeated two-step fatigue tests for JIS-SCM440 low-alloy steel. This material showed internal fractures under stress ratio of R = 0, and the inclusion sizes of the internal fracture origins ranged from 14 to 40 μm. Two types of beach marks were observed: One was small beach marks indicating internal cracks at an initial stage of fatigue. The other was large ones indicating the internal cracks at a final stage. The internal crack shape at the initial stage was a half-ellipse and asymmetric, while that at the final stage were a circle and symmetric. The asymmetric internal crack shape requires three-dimensional modeling, and therefore, an extremely slow crack growth rate, smaller than the lattice length, is expected in two-dimensional modeling. The growth rate of the asymmetric internal crack was evaluated by d√area /dN instead of conventional da/dN. As the result, the asymmetric internal crack revealed the extremely slow crack growth rate, in spite of the conventional crack growth rate of the symmetric internal crack. The border of the two-types internal cracks was corresponding to conventional ΔKth. Moreover, the growth rate of the asymmetric internal crack showed good agreements with that estimated by Omata’s method based on the conventional fatigue test results. These results support validity of Tanaka-Akiniwa model for the internal crack growth. This research thus proved that gigacycle fatigue life of high-strength steel could be predicted by crack growth life based on Tanaka-Akiniwa model.
