Elucidation of effect of hydrogen on giga-cycle fatigue mechanism and establishment of improvement method of fatigue strength reliability

近年来，人们特别关注燃料电池（FC）系统的开发和商业化，以解决全球变暖和能源问题。在这种情况下，该研究项目的作用已显着增加，以确保在不久的将来安全使用的FC系统。在这个项目中，氢对高强度钢的千兆周疲劳机制的影响以及氢对FC系统候选材料的疲劳性能的影响进行了研究。主要研究结果如下：（1）通过含氢量控制试样的疲劳试验、二次离子质谱和氚放射自显影等试验，证实了氢在合金疲劳破坏中的交互作用。(2)提出了考虑氢相互作用的千兆周疲劳机理。结果表明，通过控制材料中氢含量、夹杂物尺寸和类型，可以提高材料的疲劳强度。(3)基于面积参数模型、极值统计和光学暗区（ODA）增长曲线，提出了一种千兆周疲劳设计方法。(4)获得了一些关于FC系统候选材料氢效应的可靠疲劳数据。此外，还对氢的劣化机理进行了研究，如氢的滑移局部化和相变对裂纹扩展加速的影响等。综合以上研究结果，得到了以下两个有意义的结论：（1）氢并不引起材料的所谓“脆化”，而是促进位错的运动，导致滑移集中。(II)夹杂物中的氢在千兆周疲劳机制中的作用是即使在较低的应力下也会引起微观滑移集中。

In recent years, a special concern has been raised about the development and commercialization of fuel cell (FC) systems to solve both the global warming and energy problems. Under such circumstance, the role of this research project has been significantly increasing to ensure the safety use of FC systems in the near future. In this project, the effect of hydrogen on giga-cycle fatigue mechanism in high strength steels has been studied as well as the effect of hydrogen on fatigue properties of candidate materials for FC systems. The obtained results are as follows:(1) The evidences of interaction of hydrogen on giga-cycle fatigue failure have been shown by the fatigue tests of hydrogen-content-controlled specimens, the secondary ion mass spectrometry and the tritium autoradiography.(2) The giga-cycle fatigue mechanism taking the hydrogen interaction into consideration has been proposed. It has been shown that the giga-cycle fatigue strength can be improved by controlling hydrogen content in materials, inclusion size and inclusion type.(3) A fatigue design method in giga-cycle regime has been proposed based on the area parameter model, the statistics of extremes and the growth curve of the optically dark area (ODA).(4) A number of reliable fatigue data on the effect of hydrogen has been obtained about the candidate materials for FC systems. In addition, some important findings about the degradation mechanism due to hydrogen have been given, e.g. the slip localization due to hydrogen and the effect of phase transformations on the crack-growth acceleration, etc.Considering all the results in this project, the following two significant conclusions have been obtained:(I) Hydrogen does not cause so-called "embrittlement" of materials, but facilitates the dislocation mobility resulting in the slip concentration.(II) The role of hydrogen trapped by inclusions in giga-cycle fatigue mechanism is to cause the microscopic slip concentration even at the lower stress.