Fig.10. Fractography for blade.
From the experimental observation, as shown in Fig. 10(a), it was
clearly seen that fatigue failure modes of fan blades could be reckoned
to be characteristic of fatigue crack initiation and propagation from
the edge of blades root until the blades penetration and rupture. In
order to understand the fatigue damage mechanism of blade roots,
scanning electron microscope (SEM) was used to analyze the fracture
surfaces of the failed blades, so as to provide direct evidence of crack
nucleation and growth in areas that could not be observed visually. Fig.
10(b) showed the obvious fatigue origin and fracture propagation zone
during the fatigue process. Obviously, the fatigue origin started out in
the edge corner of blade root (consistent with the position predicted by
finite element method), and then the fatigue crack propagated radially
outward from the origin of fatigue, and finally completely broke the
blade root, leading to blade failure. Whereas, from the macro level the
blade surface did not seem to be damaged, and the failure mode was
fatigue crack nucleation and growth given rise to the fracture of blade
root. As a consequence, it could be concluded that fatigue failures of
root of blade was the primary reasons to cause the failure of fan blade.