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

近年来，燃料电池（FC）系统的发展和商业化已经引起了人们的特别关注，以解决全球变暖和能源问题。在这种情况下，本研究项目的作用已经大大增加，以确保在不久的将来FC系统的安全使用。本项目研究了氢对高强度钢千兆周疲劳机理的影响，以及氢对FC系统候选材料疲劳性能的影响。结果表明：(1)氢含量控制试样的疲劳试验、二次离子质谱分析和氚放射自显影证实了氢对千兆循环疲劳破坏的相互作用。(2)提出了考虑氢相互作用的千兆周疲劳机理。研究表明，控制材料中氢含量、夹杂物尺寸和夹杂物类型可以提高材料的千兆周疲劳强度。(3)提出了一种基于面积参数模型、极值统计和光学暗区（ODA）生长曲线的千兆周疲劳设计方法。(4)获得了氢对FC系统候选材料疲劳效应的一些可靠数据。此外，还对氢的降解机理进行了一些重要的发现，如氢引起的滑移局部化和相变对裂纹扩展加速的影响等。综合本项目的所有结果，得出了以下两个重要的结论：(1)氢不会引起材料的所谓“脆化”，而是促进位错迁移，导致滑移集中。(2)在千兆周疲劳机制中，夹杂体捕获氢的作用是在较低应力下引起微观滑移集中。

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.