Fatigue crack growth threshold and short crack growth in austenitic materials

The fatigue crack growth (FCG) and the FCG threshold behaviour of long cracks in 316L(N) material have been determined at 550 degrees C. For the FCG behaviour an influence of R-ratio on the slope of the Paris law trend line was found with higher FCG rates in the range of low Delta K values for example R = 0.5. The FCG threshold values decrease with increasing R values. With an improvement to the analytical description from the literature which takes into account crack closure, an excellent description of Delta K-th versus R was found. Regarding short cracks, no experimental data are available for 316L(N) steel. From a review of mechanisms, the behaviour of austenitic steels can be seen to be consistent with other materials, i.e. anomalous fast growth for microstructurally short cracks, where the crack front exhibits one or a few grains. From data on Type 304 steel it can be assumed that this behaviour of microcracks is restricted to crack lengths of a few grain diameters.