On the transition of fatigue crack growth from stage I to stage II in a corrosive environment

The growth of a fatigue crack nucleated along the slip bands of persistent slip bands (PSBs) or persistent Luders bands (PLBs) occurs over two successive propagation stages, stage I (shear mode) and stage II (normal mode). The transition of fatigue crack growth from stage I to stage II has been investigated in monocrystals and polycrystals of SNCM439 steel as a function of strain amplitude, crystallographic orientation of specimens, slip behavior of planar or wavy slip deformation, and grain boundaries. The cracks in stage I fatigue continue to grow along the primary slip plane. Further growth of cracks activates several slip systems ahead of the cracks by turbulent deformation or by the crossing of grain boundaries. The cracks deviate from the primary slip planes and grow perpendicular to the stress axis. Then, secondary slip plays an important role in the transition of the fatigue crack growth from stage I to stage II. The past data on the effect of environments on crack growth in stage II fatigue region have been reported in previous work. Comparatively little attention has been paid to the effect of hydrogen-related environments on stage I crack propagation and the transition of crack growth from stage I to stage II.