When the horizontal principal stress difference is small, the main fracture surface is parallel to the laminae strike, and the primary fractures will propagate as follows: propagate through the laminae, then deflect significantly at the surface of the laminae, continue along the laminae surface and finally change direction and propagate again through the laminae.
When the horizontal principal stress difference is big, the main fracture surface is perpendicular to the laminae strike and the primary fractures will propagate as follows: run through the laminae, change their propagation direction dramatically on the laminar surface, and then run through the laminae.
These delaminations form a secondary crack system propagating perpendicular to the primary one into the fracture surface plane.
For the specimen aged for 24 h at room temperature, although no reduction in area is observed (Fig. 5 g), the central part of the fracture surface consists of primary and secondary dimples (Fig. 5 h), while the outer part is flat (Fig. 5 i).
The long crack growth threshold was found to be dominated by roughness-induced crack closure, and the fracture surface roughness is controlled by the primary α-grain size.
Fractographic observations of the primary void size, and its close correspondence with the fracture surface of thin specimens, provided evidence that the individual DRA lamina indeed experienced sufficient loss of constraint in the thickness direction.
The microstructure (primary and secondary inclusions) in the fracture process zone and fracture surface are reconstructed with a microtomography technique.
In addition, the fracture surface of the non-immersed specimen consists of primary and secondary dimples in the central part (Fig. 8b) and shear dimples in the outer part (Fig. 8c).
In addition, the fracture surface of the non-immersed specimen consists of the primary and secondary dimples in the central part (Fig. 5 b) and shear dimples in the outer part (Fig. 5 c).
Secondary cracks are observed to be blocked or deflected around primary γ′, carbides and borides, and their occurrence closely relates to the roughness of the fracture surface, FCG rate and grain boundary oxidation.
