Fatigue crack morphology in 304 stainless steel cycled at constant stress amplitude

A study is made to determine which modes of nucleation and propagation are operating during elevated temperature fatigue of 304 stainless steel. Cracks are nucleated at coarse slip bands, and microstructural features such as inclusions and grain boundaries are not important either as sites of initiation or as obstacles to propagation. One of the major effects of stress amplitude is that the number of cracks nucleated increases rapidly in part because increasing the stress amplitude increases the number of grains which contain coarse slip bands, but there is sufficient data to determine if the crack density increases simply in proportion to the slip-band density or if the slip bands also become more intense and more likely to nucleate cracks at the higher stress amplitudes. The increased density of cracks appears to have significant consequences for the crack propagation process because link up of independently nucleated cracks becomes a prominent mode of propagation along the surface at the higher stress amplitudes. The process of crack linkup may not have had a large effect on the fatigue life of the specimens used in this study because the distance between independent nucleation sites on the fracture surface was never less than about a third of the critical crack size even at the highest stress amplitudes. However, this mode of propagation could have a large effect on the crack propagation portion of the fatigue life of large components, particularly when they have large surface area to volume ratios like plates and pipes