Fatigue crack growth for ferritic steel under negative stress ratio

Yamaguchi, Yoshihito ; Hasegawa, Kunio; Li, Y. 

Crack closure during fatigue crack growth is an important phenomenon for predicting fatigue crack growth amount. Much experimental data shows that fatigue cracks close at not only negative loads but also positive loads during a constant amplitude loading cycle. The Appendix A-4300 in the ASME Code Section XI provides two equations of fatigue crack growth rates expressed by stress intensity factor range for ferritic steels under negative stress ratio. One is the equation taking into account crack closure and the other does not consider the crack closure. The boundary of crack closure is classified by the magnitude of applied stress intensity factor range. The objective of this paper is to investigate the influence of the magnitude of stress intensity factor range on crack closure. Fatigue tests have been performed on ferritic steel in air environment at room and high temperatures. Crack closures were obtained as a parameter of stress ratio. It was found that crack closure occurs more small applied stress intensity factor range than the definition given by the Appendix A-4300.