10<sup>11</sup> Gigacycle Fatigue Properties of High-strength Steel

Fatigue tests were conducted up to 1011 cycles on high-strength steel to clarify a fatigue limit. The fatigue limit of the high-strength steel was not confirmed by gigacycle fatigue tests up to 1010 cycles, while our previous study suggested that the fatigue limit was probably confirmed by those up to 1011 cycles. However, the 1011 cycles fatigue testing was challenging since it took 2 months even by using ultrasonic fatigue testing at 20 kHz. In this study, 3 specimens were tested beyond 1010 cycles. Although a test on a specimen was terminated at around 5 × 1010 cycles, 2 specimens reached 1011 cycles without failure. In other word, no specimen failed above 1010 cycles. These results demonstrated the fatigue limit on high-strength steel in a gigacycle region. The fractured specimens below 1010 cycles revealed internal fractures originating from oxide-type inclusions. When the specimens failed in long-life regions, clear ODAs (Optically Dark Areas) were observed on the fracture surfaces at around the internal fracture origin, while the ODAs were obscure in case of failure in short-life regions. The runout specimens up to 1011 cycles were forcibly fatigue-fractured at higher stress amplitudes in the short-life regions. As the result, the ODA was observed on the forcibly fatigue-fractured surface. This meant that small internal cracks existed in the runout specimens since the ODA was a trace of small internal crack growth. Namely, non-propagating cracks were the mechanism of the appearance of the fatigue limit.