PREDICTIONS OF THE INITIAL NON-STEADY-STATE CRACK-GROWTH BEHAVIOR IN THE CREEP-FATIGUE OF THE NICKEL-BASE SUPERALLOY AP1

Using the fracture mechanics parameters K and C* to analyze cyclic crack growth test results carried out on the nickel-base superalloy AP1, the effects of test frequency on the initial incubation time followed by transient cracking rates and the steady-state secondary crack growth rates were considered. The cracking behavior at 700-degrees-C for this material exhibits a frequency dependence over a range of 0.001 to 10 Hz. It has been shown that, at high temperatures under steady-state cracking conditions, fatigue processes are most dominant at high frequencies, and conversely, time-dependent creep dominates at low frequencies. The creep cracking rate is described by a model linked to the exhaustion of available ductility in a creep process zone at the crack tip, and the fatigue rate is linked to the Paris Law equation. For the secondary regime of crack growth, the effects of frequency are described in a cumulative damage model developed for creep/fatigue interaction. For the crack incubation and the transient process under initial loading, the model is extended to predict the cracking behavior in the creep regime at low cyclic frequencies. For the higher frequencies, fatigue dominates and creep transient effects are not observed experimentally.