SMALL FATIGUE CRACKS IN AN AUSTENITIC STAINLESS-STEEL

The present study concerns nucleation and growth of small surface crac ks during the low-cycle fatigue of a nitrogen-containing austenitic st ainless steel. Metallographic replicas as well as longitudinal section ing were used to record the developing crack pattern on the specimen s urface. The influence of grain size and nitrogen content is considered . Small surface cracks are observed after about 10% of the fatigue lif e. The nucleation of cracks continues until about half of the lifetime , when the crack density saturates. This saturation phenomenon is rela ted to the local unloading effect of growing cracks. The mean crack le ngth increases continuously as a power-law until specimen failure. How ever, small grains and a low nitrogen content amplify the effect of cr ack-grain boundary interactions resulting in an intermediate retardati on in growth. At high nitrogen contents, the crack growth characterist ics are very much related to the slip bands formed. This results in a more simultaneous growth of cracks, a more jagged Feature of the crack s introducing a higher roughness-induced crack closure effect, and, co nsequently, better fatigue properties.