Fatigue crack initiation and propagation in an α-iron polycrystal

The cyclic deformation and crack initiation and propagation of α-iron polycrystals have been studied under stress-control with emphasis on the dependence on the stress amplitudes and orientation of grains. The orientations of grains surrounding cracks were determined by the electron backscatter diffraction (EBSD). It was found that cracks did not originate at grain boundaries but along the slip lines intersecting with the free surface at the corner of the specimen. At lower stress amplitude, the crack propagated in a transgranular mode, but at higher stress amplitude the crack propagated in a mixed mode–transgranular mode and intergranular mode. The stress distribution along the loading axis of grains surrounding the crack was calculated using a three-dimensional anisotropic finite element method (FEM). In the present work, it was shown that the crack initiation mainly depends on three factors: higher stress distribution, abundant dislocation sources and less resistance to dislocation motion.