Study on the mechanism of internal fracture process and establishment of reliable method for fatigue design in very high cycle fatigue regime

在高强度钢试件的疲劳试验中，出现了一种显著的S-N曲线形状，称为双S-N曲线。S-N曲线由两条不同的S-N曲线组成，两条S-N曲线对应于各自的断裂模式：其中一条S-N曲线出现在高应力幅值水平和低循环次数，并且由于表面裂纹的萌生和扩展而受表面断裂模式的支配。另一条S-N曲线出现在高循环次数时，受内部裂纹萌生和扩展控制。在高周疲劳状态下，由于地下裂纹的萌生和扩展，在断口鱼眼内的断裂原点附近观察到非金属夹杂物的显著特征。与鱼眼内的区域相比，该区域显示出非常粗糙和颗粒状的形态，作者将其命名为“颗粒状明亮面”（GBF）。弄清长寿命疲劳状态下的内部疲劳断裂机理，对建立可靠、安全的机械元件疲劳设计具有重要意义。本研究的主要目的是阐明包裹体附近GBF区域形成的机制。利用三维扫描电镜（SEM）和SPM对裂缝表面进行了详细观察和分析，并利用FRASTA对裂缝过程进行了计算模拟。本研究得到的结果如下：1。与GBF外表面相比，GBF区域的粗糙度非常大，并且与测试材料微观结构中的细小球形碳化物颗粒大小相似。此外，GBF区凸颗粒的大小和分布与被试材料微观结构中碳化物颗粒的大小和分布相对应。EPMA检测了GBF区与GBF外的富碳分布。由此推断，在GBF区域中，粗糙和颗粒状断口的形成与微观组织中的细小碳化物颗粒有关。从FRASTA方法的计算模拟来看，在非常高的周疲劳状态下，多个微裂纹在夹杂附近分散、生长，然后彼此合并。提出了高周疲劳状态下GBF的形成机理为“球形碳化物弥散脱黏模型”。该模型对含碳率不同的钢和旋转弯曲疲劳试验及轴向加载疲劳试验的试样均成立。少

A remarkable shape of S-N curve, named as a duplex S-N curve, was appeared from the fatigue tests using specimens of high-strength steel. The S-N curve consists of two different S-N curves corresponding to the respective fracture mode : One of the two S-N curves appears at a high-stress amplitude level and low number of cycles, and is governed by a surface fracture mode as a result of the surface crack initiation and propagation. The other S-N curve appears at a high number of cycles and is governed by internal crack initiation and propagation. A distinctive feature was observed in the vicinity of a non-metallic inclusion at the fracture origin inside the fish-eye on fracture surfaces resulting from subsurface crack initiation and growth in a high-cycle fatigue regime. This area revealed a very rough and granular morphology in comparison with the area inside the fish-eye and named a "GBF (granular-bright-facet)" by authors. It is very important for the establishment of a reliable and s … More afe fatigue design for mechanical elements to clarify the mechanism of internal fatigue fracture in long-life fatigue regime. The main aim of this study was to clarify the mechanism of the formation of a GBF area in the vicinity of an inclusion. Detailed observations and analysis of a fracture surface by a three-dimensional SEM and SPM, and computational simulation of the fracture process by FRASTA were conducted. Results obtained in this study are as follows.1.The roughness in a GBF area was very large compared with the surface outside the GBF, and of similar size to the fine spherical carbide particles in the microstructure of the tested materials. In addition, the size and distribution of convex particles in the GBF area corresponds to those of carbide particles in the microstructure of the tested materials.2.Rich carbon distribution in the GBF area, compared with that outside the GBF, was detected by EPMA. It was inferred that the formation of the rough and granular fracture surfaces in the GBF area relate to fine carbide particles in the microstructure.3.From the computational simulation using the FRASTA method, multiple microcracks are dispersed in the vicinity of an inclusion, grow, and then coalesce with each other under a very high cycle fatigue regime.4.The mechanism of GBF formation in a very high cycle fatigue regime was proposed as the "Dispersive Decohesion of Spherical Carbide Model". This model was confirmed to come into existence for steel that contains different carbon percentage and for specimen obtained from not only rotary-bending fatigue test but also axial loading fatigue test. Less