Fig. 10: Fractographs of fatigued surfaces (a-d) for J2N, (e-h) for the G10 in air
(Ductile striation mechanism (DSM ) with secondary crack (SC ))
The metal crumbs were formed in between branched and re-joined crack fronts. It can also be observed that several small metal crumbs were formed as the crack propagated in air and the occurrence increased with increase in ∆K . In other words, the tendency for crack branching and re-joining tends to increase with increase in the driving force for crack propagation. The length of the branched crack is considerably long, and the extent of bifurcation angle is substantially large. Fig. 11(c) is the magnification of the locations 1 to 8 marked with the yellow boxes. It was observed that the branched crack fronts generally followed the thin ribbon αHA phase as shown in the magnified locations. This tends to suggest thatαHA is the path of least resistance. An important feature of the crack growth is that generally the tip of the propagating crack was always sharp as can be seen from the tip of the branched crack front as shown in the magnified micrographs in Fig. 11(c).