Fig.14 Instantaneous fracture area
As shown in Fig.12, the surface of the origin area is rough, and the striation is generally divergent, showing the micro morphology of cleavage. The red circle is the crack origin location.
The surface of the expansion area is bright (Fig. 13), showing  river pattern, which is quasi cleavage micro morphology with obvious stripe fatigue striae.
The surface of the instantaneous fracture area is rough (Fig. 14), where the crack finally breaks rapidly, with many tear marks and a small amount of dimples, showing the micro morphology of quasi cleavage.
It can be concluded that the fracture is fatigue fracture.