Predictions of the initial non- steady- state crack growth behavior in the creepjfatigue of the nickel- base superalloy AP1

Abstract

Using the fracture mechanics parametersK and C* to analyze cyclic crack growth test results carried out on the nickel-base superalloy API, 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 crack-ing behavior at 700 °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 frequen-cies. 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 sec-ondary regime of crack growth, the effects of frequency are described in a cumulative damage model de-veloped for creepJfatigue 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 frequen-cies. For the higher frequencies, fatigue dominates and creep transient effects are not observed experi-mentally.