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

点击翻译按钮获取中文摘要

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