Fatigue crack initiation in metallic materials in very high cycle fatigue (VHCF) regime is usually characterized by the formation of subsurface crack origin, mostly at subsurface inclusion or other defects [1, 2]. A fine granular area around the crack origin or defect can be observed [3]. In this work, fatigue crack initiations in non-defect matrix in some austenitic steels in very high cycle fatigue regime are studied using one single specimen stepwise-load increasing test with a load-step of 10 MPa and a cycle-step of more than 108 cycles. With this method, fatigue crack initiation origin was observed in the non-defect matrix as shown in Figure 1. A fine granular area has been formed as the crack origin. By microstructure investigation using the electron microscopy techniques such as EBSD and ECCI, a mechanism for fatigue crack initiation in non-defect matrix is proposed.

Figure 1. Fatigue crack initiation origin with an FGA in the non-defect matrix near the specimen surface.

Strain localization can form in the soft grains by dislocation slip during the cyclic loading. This leads to the formation of dislocation sub-cells as low angle grain boundaries or grain fragmentation. Fatigue crack initiation will then occur at triple junction points of the low angle grain boundaries by strain localization or stress concentration that cause localized plasticity exhausting. This work increases the understanding on the fatigue nature in materials.

References

[1] Y. Murakami, Metal fatigue, effect of small defects and nonmetallic inclusion. Elesvier Science Ltd, 0-08-044064-9; 2002.

[2] C. Bathias, P.C. Paris, Gigacycle fatigue in mechanical practice. Editor: Marcel Dekker, ISBN 0-8247-2313-9; 2004.

[3] T. Sakai, H. Harada, N. Oguma, Crack Initiation Mechanism of Bearing Steel in High Cycle Fatigue. In: Fracture of Nano and Engineering Materials and Structures, 2006;1129-1130.