超高强度钢十亿周疲劳研究

利用20kHz压电超声疲劳试验技术，对六种抗拉强度高于1500MPa的低合金钢进行测试。结果表明，这些材料并不存在传统规范中所谓的“疲劳极限”，它们在超过10^7，甚至10^9应力循环后仍然继续发生破坏，而且其S－N曲线表现出“阶梯”型特征。研究还发现，这些材料都存在一个分界应力幅值区间（或应力循环数区间，大约在10^6到10^7的平台内），裂纹萌生由试件表面向其内部转换。实验证明了十亿周级超高周疲劳裂纹由试件内部萌生的机理。

GIGACYCLE FATIGUE OF ULTRA-HIGH STRENGTH STEELS

Inclusions are common sites for fatigue crack nucleation and are known to be particularly deleterious in high strength steels. Fatigue cracks are usually understood to initiate from the surface of smooth specimens. However, fatigue crack can be initiated from the interior of many materials in the case of ultra high cycle fatigue. To clarify the crack initiation mechanism in gigacycle fatigue, six high strength low alloy steels with tensile strengths beyond 1 500 MPa are tested at very high numbers of cycles through ultrasonic fatigue test at 20 kHz. The specimens continue to fail over 10 7, even 10 9 stress cycles. The S-N diagram on these steels shows a stepwise shape. Subsurface crack initiation is dominant in the long life range, while the surface fatigue initiation occurs in high peak stress tests and in short life tests. It is revealed that there is a definite stress range (or cycles regime, 10 7 cycles) where the initiation location changes from the surface to the internal defect (inclusion). In this study, subsurface cracking is established as the principal initiation mechanism in gigacycle fatigue.