Abstract
Abstract Background Nickel-based superalloys are typically used as blades and discs in the hot section of gas turbine engines, which are subjected to cyclic loading at high temperature during service. Understanding fatigue crack deformation and growth in these alloys at high temperature is crucial for ensuring structural integrity of gas turbines. Methods Experimental studies of crack growth were carried out for a three-point bending specimen subjected to fatigue at 725°C. In order to remove the influence of oxidation which can be considerable at elevated temperature, crack growth was particularly tested in a vacuum environment with a focus on dwell effects. For simulation, the material behaviour was described by a cyclic viscoplastic model with nonlinear kinematic and isotropic hardening rules, calibrated against test data. In combination with the extended finite element method (XFEM), the viscoplasticity model was further applied to predict crack growth under dwell fatigue. The crack was assumed to grow when the accumulated plastic strain ahead of the crack tip reached a critical value which was back calculated from crack growth test data in vacuum. Results Computational analyses of a stationary crack showed the progressive accumulation of strain near the crack tip under fatigue, which justified the strain accumulation criterion used in XFEM prediction of fatigue crack growth. During simulation, the crack length was recorded against the number of loading cycles, and the results were in good agreement with the experimental data. It was also shown, both experimentally and numerically, that an increase of dwell period leads to an increase of crack growth rate due to the increased creep deformation near the crack tip, but this effect is marginal when compared to the dwell effects under fatigue-oxidation conditions. Conclusion The strain accumulation criterion was successful in predicting both the path and the rate of crack growth under dwell fatigue. This work proved the capability of XFEM, in conjunction with advanced cyclic viscoplasticity model, for predicting crack growth in nickel alloys at elevated temperature, which has significant implication to gas turbine industries in terms of “damage tolerance” assessment of critical turbine discs and blades.
Highlights
Nickel-based superalloys are typically used as blades and discs in the hot section of gas turbine engines, which are subjected to cyclic loading at high temperature during service
The typical application of these alloys are in turbine blades and discs in the hot section of gas turbine engines, which are subjected to cyclic loading at high temperature during their service life
Intergranular cracking modes were observed at longer dwell times (Figure 7b) indicating that creep assisted fatigue does occur in vacuum, but this is swamped by oxidation effects in air which accelerate the crack growth rates considerably via intergranular cracking mechanism (Jiang et al 2014)
Summary
Nickel-based superalloys are typically used as blades and discs in the hot section of gas turbine engines, which are subjected to cyclic loading at high temperature during service. Understanding the fatigue damage behaviour, associated with crack initiation and propagation, of nickel-based superalloys at high temperature is crucial for structural integrity assessment of gas turbines based on the “damage-tolerance” approach. The mechanical behaviour of this type of material involves time consuming and costly tests under cyclic load with superimposed hold time at maximum or minimum load level, representative of typical service loading conditions. The material in such circumstances undergoes a combination of creep-fatigue deformation. The methodology is largely empirical and does not consider the physical mechanism of crack tip deformation, especially the cyclic plasticity, which is believed to control crack growth behaviour in metallic alloys
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