Fatigue Mechanism of Superlong Fatigue Failure of steels in Gigacycle Regime

当尺寸为10 mm的试件疲劳寿命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0.01 nm或10^-12；m/cyc1e)。在疲劳过程的早期阶段，裂纹的扩展速率应该远远小于平均扩展速率，因此不能假设裂纹的扩展是一个周期一个周期地进行。在一些可能的机制中，特别关注了在热处理合金钢SCM435断口(非金属夹杂物)附近新发现的一种特殊的疲劳断口形态，该合金钢在N*10^8以下进行了试验。扫描电子显微镜和原子力显微镜观察到的特殊断口形态可能受夹杂物周围的氢的影响。对于Nf=-10^8的疲劳寿命，Roo&lt；&>面积参数模型对疲劳极限的预测是-10%不保守的，尽管它成功地预测了常规的疲劳极限定义…N=10^7时，N*10^8的疲劳失效可能是由于小裂纹中疲劳裂纹闭合现象的断裂或释放所致。在断裂起始处的非金属夹杂物附近，对于疲劳寿命较长的试件，总是在鱼眼标记内观察到暗区。疲劳寿命较短的Nf=-10^5的试件在鱼眼痕上没有这样的暗区。扫描电子显微镜和原子力显微镜观察表明，马氏体板条组织的暗区具有与通常的疲劳断口截然不同的粗糙表面。考虑到高强度钢对氢环境的高度敏感性和夹杂物周围的高氢含量，可以得出结论：非金属夹杂物引起的高强度钢的极高周疲劳失效是由氢脆和疲劳等环境效应引起的。对铝合金、粉末金属和中碳钢在拉压和反向扭转条件下的类似研究也是如此。从超长疲劳寿命的角度讨论了各种金属的疲劳断裂机理。较少

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