Quantitative Evaluation of Environmental Effects on Irreversibility of Cyclic Plastic Deformation by STM Observation

为了更好地了解环境对疲劳的影响，对不同气体环境下的疲劳裂纹扩展进行了研究。对钛、8090Al-Li合金和Fe-3%Si合金在控制湿度、氮气、氩气和真空的实验室空气条件下进行了恒载荷幅值疲劳裂纹扩展试验。与真空条件下相比，在任何气体环境下都获得了更高的疲劳裂纹扩展速率。对8090 Al-Li合金进行了恒定DELTAK疲劳裂纹扩展试验，研究了水蒸气对疲劳裂纹扩展的影响。在0.01% ~ 40%的范围内系统地改变实验室空气的相对湿度（RH）。随着相对湿度的增大，疲劳裂纹扩展速率增大。在0.01% ~ 30%RH范围内，裂纹扩展速率增加，超过30%RH时裂纹扩展速率不变。在真空条件下未观察到裂纹分层现象。利用扫描隧道显微镜（STM）对Fe-3%Si合金的疲劳表面形貌进行了精确观察和定量评价。为了评估环境对疲劳裂纹萌生和扩展的影响，需要进一步的AFM实验研究。

To get a better understanding of the enviromental influence on fatigue, a study has been undertaken on the fatigue crack growth in various kinds of gaseous environments. Constant load amplitude fatigue crack growth tests were performed in titanium, 8090Al-Li alloys and Fe-3%Si alloys under laboratory air with controlled humidity, nitrogen, argon and vacuum. Increased fatigue crack growth rates were obtained in any gaseous environments compared to that for the vacuum condition. Constant DELTAK fatigue crack growth tests were also performed for the 8090 Al-Li alloys under the controlled laboratory air condition to evaluate the effect of water vapor on the fatigue crack growth. The relative humidity (RH) of the laboratory air were systematically changed in the range of 0.01% to 40%. Fatigue crack growth rates were increased as increase in the relative humidity. However, the increase was observed in the range of 0.01% to 30%RH,and over the 30%RH the crack growth rates were unchanged. No crack delamination was observed in the vacuum condition. Precise observation and quantitative evaluation of the fatigue surface morphology of Fe-3%Si alloys have been made using an STM (Scanning Tunneling Microscope). Further experimental research using AFM is strongly required in order to evaluate the environmental effects on the fatigue crack initiation and growth.