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

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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.