Superimposed effect of high cycle fatigue and low cycle fatigue loadings on small crack propagation around cooling hole in a directionally solidified ni-base superalloy

Recent introduction of renewable energy systems has impelled the land based gas turbine power generation systems to play two different types of roles: one is a role as base load system, the other is a role as power adjusting system. To cope with the latter role, it is necessary to take into account of thermal fatigue failure of the gas turbine components, where transient thermal stress is often a critical issue to be concerned. In this study, non-stationary induced thermal fatigue failure behavior of a superalloy was explored. At first, a new test equipment was developed so that high cycle non-stationary thermal fatigue loading can be superimposed on stationary low cycle thermo-mechanical fatigue (TMF) loading. By means of the testing equipment, the early growth of small cracks around the cooling holes was investigated under the superimposed condition in a directionally solidified Ni-base superalloy, where some cooling holes were artificially introduced for a simulation. The experimental works clearly demonstrated that the crack growth rate under the superimposed condition was significantly accelerated, compared with that under the stationary low cycle TMF loading. This behavior was discussed based on fracture mechanistic view point. © 2019 The Society of Materials Science, Japan.