Abstract
Effect of microstructure on the crack initiation and early propagation mechanism in the very high cycle fatigue (VHCF) regime was studied in 316L stainless steel (316L SS) by atomic force microscope (AFM) and electron back scattered diffraction (EBSD). The results show that small fatigue cracks initiate from the slip band near the grain boundaries (GBs) or the twin boundaries (TBs). Early crack propagation along or cross the slip band is strongly influenced by the local microstructure such as grain size, orientation, and boundary. Besides, the gathered slip bands (SBs) are presented side by side with the damage grains of the run-out specimen. Finally, it is found that dislocations can either pass through the TBs, or be arrested at the TBs.
Highlights
It has been reported that low-strength steels like 316L stainless steel (316L SS) usually have similar fatigue limits in the very high cycle fatigue (VHCF) regime [29,30]
The fatigue behavior of 316L SS is investigated using symmetrical bending loading by the very high cycle bending fatigue system developed in the laboratory and examining the slip band features on fatigued surfaces of specimens
Fatigue cracks are initiated from multiple crack sources, while at very high cycles, fatigue crack is initiated from a single crack source
Summary
316L SS is widely used as the nuclear engineering structural material, such as the support plate, primary coolant pipe, and main coolant pump in a pressurized reactor, owing to its better welding performance [1], low radiation sensitivity [2], excellent corrosion and oxidation resistance [3], et cetera. Even when the fatigue loading is macro elastic, the inhomogeneity of adjacent grains in the material will lead to permanent (inelastic) microstructure deformation in the local region, and the stress concentration caused by the accumulation of such deformation plays a leading role in the early initiation of fatigue cracks [8,9,10,11,12,13,14]. For polycrystalline materials such as 316L SS that do not contain obvious internal defects, the internal atoms will undergo dislocation slip motion under the action of shear stress, and many slip bands. The mechanism of fatigue crack initiation and propagation of 316L SS at low-stress amplitude (VHCF) needs to be studied in detail
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