Modelling of fatigue crack growth in Inconel 718 under hold time conditions - application to a flight spectrum

Gas turbine operating cycles at high temperatures often consist of load reversals mixed with hold times; the latter occurring either as cruise for aero engines or at continuous power output for land based turbines, but also at low frequency loading conditions, e.g. slow “ramp up” of engine thrust. The hold time conditions cause the crack to grow by intergranular fracture due to material damage near the crack tip, thus rapidly increasing the crack growth rate. Since the damaged zone will affect the crack propagation rate due to cyclic loadings as well, the complete load history of a component therefore has to be considered. The crack propagation model presented in this paper is based on the damaged zone concept, and considers the history effect in the form of damaged zone build up during hold times, and subsequent destruction as the crack propagates onwards by rapidly applied load reversals. By incorporating crack closure for handling different R-values, an aero engine component spectrum is evaluated for a surface crack at 550 ◦C. The result shows a good correlation to model simulation, despite the complexity of the load spectrum. Introduction Predicting life of hot turbine components always presents a challenge, where e.g. the interaction between different load modes, i.e. rapid cycling vs. more slowly applied loads or hold times, can have a great influence on the life time expectancy. The phenomenon has been investigated by several authors and it has been found that the hold times result in material damage at the crack tip, causing the crack to grow by intergranular fracture and thus strongly increase the crack growth rate, cf. [1, 2, 3]. Although many others have dealt with the problem before, either by modified Paris law expressions, cf. [4, 5, 6] where a pure time dependent part and a pure cyclic part forms a linear summation model, or by more physically based models, cf. [7, 8] where damage, driven by e.g. oxygen diffusion, is incorporated into the modelling of crack growth, work is still needed in order to understand the interaction and history effects between load modes. It has also been shown that the increase in crack growth rate is caused by an embrittlement of the grain boundaries [9], and that a hold time block followed by an applied block of continuous cycles also affects the latter one by an increased crack growth rate [10], as the crack propagates through a damaged zone caused by the hold time block. Thus, the importance of considering the load history of a component becomes increasingly important as the resulting life can be strongly affected by such interactions. In this paper, a damage mechanism inspired model is used [11, 12], where the damage accumulation in front of the crack tip controls the crack growth rates during hold times and rapid cyclic loadings, thus interpreting the load history dependent interactions between different parts of the load spectrum.