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.