Initiation and growth of short fatigue cracks in austenitic Sanicro 25 steel

Austenitic heat-resistant stainless steel Sanicro 25, often used in energy production installations, was cyclically strained at room temperature, and initiation and growth of short fatigue cracks have been studied. Cylindrical specimens with pre-existing shallow notch were cyclically strained under constant total strain amplitudes, while the surface relief, crack initiation, and short crack growth were observed, in-situ, using optical and electron microscopes. Persistent slip markings were found to be the principal locations where natural fatigue cracks initiated. The character of the crack paths of short cracks was studied using SEM and EBSD techniques. The results exhibit prevailing transgranular crack growth, ie, along the persistent slip markings within the grains. The growth of the largest cracks in the central part of the shallow notch was measured for different applied strain amplitudes. Because the coalescence of initiated cracks plays a significant role during crack growth, the use of the equivalent crack concept gives adequate results in the description of the crack growth rate. The crack growth law of short cracks was established and compared with the Manson-Coffin law of the material.