Fatigue crack growth rate in axial, torsional and multiaxial mode in 316L austenitic steel

Abstract

Austenitic stainless steel 316L was tested at room temperature in three different cycling modes – axial tension/compression, pure torsional and multiaxial in phase mode with the same equivalent strain amplitude. Two types of hollow cylindrical specimens were used in this study. The first type of specimen with smooth surface was used for studying the mechanism of initiation of fatigue crack and for determination of fatigue life. The second type of specimen with artificial hole was used for measuring of crack growth rate. The mechanism of crack initiation was observed by scanning electron microscopy and by focused ion beam technique. Detailed observations of microstructure were done by transmission electron microscopy. The mechanism of fatigue crack initiation is following – persistent slip bands are formed and fatigue cracks initiate from them. Crack growth rate was determined from micrographs taken by light microscope during cycling. It was found that the cycling in axial mode leads to the fastest crack growth rate and the shortest fatigue lives. On the contrary, cycling in torsional mode leads to the slowest crack growth rate and the longest fatigue lives. One of probable causes of such behavior can be found in phase transformation from austenite to α´ martensite which is much more intense in the case of pure torsional loading than in other modes.