Abstract
To clarify the experimental background of characteristics of fracture life of the investment-cast polycrystalline Nickel-base superalloy IN738LC (typical gas turbine blade’s materials), in-situ observational tests under creep–fatigue conditions were carried out, and the effects of both the cycle- and the time-dependent mechanisms on the fracture life were investigated. Creep ductility, stress holding time and temperature were combined with the relationship between the inverse value of fracture life and applied load frequency as a promoting factor of the time-dependent mechanism. Then a three-dimensional curved surface representation of fracture life under arbitrary creep–fatigue conditions has been proposed theoretically according to the non-equilibrium science. As the load frequency decreases or stress holding time and temperature increases, the time dependent mechanism begins to play a role in the manner of creep–fatigue interaction. In this region, the characteristics of the Relative Notch Opening Displacement (RNOD) and crack growth behavior change from a cycle-dependent mechanism caused by fatigue to a time-dependent mechanism caused by creep through an unstable equilibrium transition region. In this paper, comparison of the three-dimensional curved surface representation of fracture life which has unique characteristics and the actual damage behavior measured by the Electron Back Scattered Diffraction (EBSD) method was carried out. As a result, misorientation analysis based on the EBSD method has clarified that the unstable equilibrium transition region in the three-dimensional curved surface was the result of creep damage concentration in the vicinity of notches by a load frequency which has the effect of limiting the damage region.
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