Film cooling is a common way to reduce the surface temperature of turbine blades. The cooling air is pumped through multiple small holes through the structure. A drawback is that cracks readily initiate at these holes. Conventional fatigue models cannot be applied to the holes in a straightforward manner. Indeed, the high stress gradients reduce the crack growth rates in comparison to smooth specimens. In addition, a statistical effect appears due to the small size of the highly loaded surface. Nickel base superalloys used for blade material have large grain sizes (>1 mm) and the nucleated short cracks mainly grow in a single grain, which increase the scatter. Finally, the surface state of the laser drilled holes significantly differ from that of laboratory smooth specimens due to partial melting of the alloy.
As part of an ongoing project, our group carries out high temperature LCF (Low-Cycle-Fatigue) tests with center hole specimens of a coarse-grained Nickel base Superalloy. Here, not only the large grain size, but also the shape of the starting crack around the notch is assumed to be of high importance for fatigue life and thus, needs to be considered in a fatigue life prediction model. A new combined procedure to detect the shape of the starting crack using the potential drop method and induction thermography will be presented. Moreover, LCF lifetimes of specimen with differing surface finishes in the hole were compared and the results were used for the identification of a fracture mechanics based model for the prediction of fatigue life time.