The effect of water vapor on fatigue crack tip stress and strain range distribution and the energy required for crack propagation in low-carbon steel

Abstract

The formation of a subgrain structure by the passage of a fatigue crack in low-carbon steel has been detected by backscattered electron imaging in the SEM. The distribution of subgrain sizes has been measured as a function of cyclic stress intensity and environment, and from this data the cyclic stress and strain range distributions have been determined, as has the energy dissipated during propagation of the fatigue crack. These results are compared to theoretical and other experimental findings. The energy equivalent (per unit area of incremental crack advance) of the environment is found to be 106 times the surface energy of iron. In an inert environment, the maximum cyclic stress range developed at the crack tip is found to be approximately 2.3 times the yield stress, independent of cyclic stress intensity. Water vapor effectively lowers this stress range, and causes it to assume a dependence upon cyclic stress intensity.