高張力鋼の腐食疲労き裂発生寿命の定量的評価

抄録

Materials for construction tend to be used in more harsh environments nowadays. Much effort has been spent already in order to understand the effect of environment on fatigue strength. Especially, the environmental effect on fatigue crack propagation behavior has been clarified in many combinations of materials and environmental conditions. However, each step of the corrosion fatigue process has not been evaluated in detail. In this study, each period from initiation and growth of corrosion pits to initiation and propagation of corrosion fatigue cracks is estimated. Material used was a carbon steel HT60 of 630MPa class tensile strength. A micro-drilled notch was introduced at the surface of test specimen by machining. The notch of 80μm diameter, which imitated corrosion pits was expected to fill the role of concentrated stress sites. Test environments were 3.5 weight percent NaCl solution and 1.33×10^-4Pa air in a chamber of scanning electron microscope (SEM). Fatgue tests were conducted under a load ratio of 0.1 at a frequency of 1.5Hz in NaCl solution and at 20Hz in a vacuum. While under continuous observation by SEM, a crack which reached 20 to 30μm in length was defined as fatigue crack initiation. A regression equation, with an accuracy within a factor of 5, for fatigue crack initiation life was derived for both environments. It became clear that the effect of stress and notch shape on initiation life was smaller in NaCl solution than that in a vacuum. In addition a relationship between stress intensity factor, calculated from shadow figure of notch, and crack initiation life was deduced. In NaCl solution, the initiation life of corrosion pits was estimated to be quite short, and a ratio of fatigue crack initiation life to total failure life was about 35% regardless of stress intensity factors.