Fatigue Mechanism of Superlong Fatigue Failure of steels in Gigacycle Regime

当10mm尺寸试样的疲劳寿命Nf大于10^8时，平均疲劳裂纹扩展速率远小于晶格间距（- 0.1或0.01nm或10^<-11-12>m/ cycles）。在疲劳过程的早期，裂纹扩展速率应远远小于平均扩展速率，因此我们不能假设裂纹是逐周扩展的。在这项研究中，可能的机制，极高的循环疲劳进行了研究。在一些可能的机制中，特别关注了在热处理合金钢SCM435断裂起源附近新发现的特殊疲劳断裂形貌（非金属夹杂物），对其进行了高达N* 10^8的测试。通过SEM和AFM观察到的特殊形貌被认为是受夹杂物周围氢的影响。对于Nf= -10^8的疲劳寿命，ROO< >区域参数模型的疲劳极限预测具有-10%的不保守性，而对于N=10^7的疲劳寿命，ROO< >区域参数模型的疲劳极限预测较为成功。因此，假定N*10^8的疲劳破坏是由小裂纹中疲劳裂纹闭合现象的断裂或释放机制引起的。对于疲劳寿命长的试样，在断口处非金属夹杂物附近的鱼眼标记内总是观察到一个黑暗区域。而疲劳寿命较短的试件（Nf=-10^5）在鱼眼标记处没有这样的暗区。扫描电镜（SEM）和原子力显微镜（AFM）观察表明，暗区表面粗糙，与马氏体板条组织中常见的疲劳断口有很大不同。考虑到高强钢对氢环境的高敏感性和夹杂物周围的高氢含量，可以得出结论，非金属夹杂物对高强钢的极高周疲劳失效是由氢脆和疲劳耦合等环境效应引起的。对铝合金、粉末金属和中碳钢在拉压和反扭作用下也进行了类似的研究。从超长疲劳寿命的角度探讨了各种金属的疲劳断裂机理。少

When fatigue life Nf of a specimen of 10mm in size is longer than 10^8, the average fatigue crack growth rate is much less than lattice spacing(- 0.1 _ or 0.01nm or 10^<-11-12>m/cyc1e). In the early stage of fatigue process, the crack growth rate should be much less than the average growth rate and accordingly we cannot assume that crack growth occurs cycle by cycle.In this study, possible mechanisms for extremely high cycle fatigue are investigated. Of some possible mechanisms, a special focus was put on a newly found particular fatigue fracture morphology in the vicinity of fracture origin (nonmetallic inclusions) of a heat treated alloy steel, SCM435, which was tested up to N* 10^8 The particular morphology observed by SEM and AFM was presumed to be influenced by the hydrogen around inclusions. The predictions of fatigue limit by the ROO< > area parameter model are -10% unconservative for fatigue life of Nf= -10^8, though it successfully predicts the conventional fatigue limit defin … More ed for N=10^7. Thus, the fatigue failure for N*10^8 is presumed to be caused by a mechanism which induces breaking or releasing of fatigue crack closure phenomenon in small cracks.In the vicinity of a nonmetallic inclusion at fracture origin a dark area was always observed inside a fish-eye mark for specimens with long fatigue life. Specimens with short fatigue life of Nf=-10^5 do not have such dark area in fish-eye mark. SEM and AFM observations revealed that the dark area has a rough surface quite different from usual fatigue fracture surface in martensite lath structure.Considering a high sensititivity of high strength steels to hydrogen environment and high hydrogen content around inclusions, it may be concluded that the extremely high cycle fatigue failure of high strength steels from nonmetallic inclusions is caused by environmental effects such as hydrogen embrittlement coupled with fatigue.Similar studies have been also done on an aluminum cast alloy, powder metals and medium carbon steels under tension-compression and reversed torsion. The fatigue fracture mechanism of various metals has been discussed from the viewpoint of superlong fatigue life. Less