Fatigue crack growth in a nickel-based superalloy at elevated temperature - experimental studies, viscoplasticity modelling and XFEM predictions

Experimental studies and computational modelling of crack deformation and growth in a nickel-based superalloy at elevated temperature have been carried out for a threepoint bending specimen subjected to fatigue loading condition. In order to remove the influence of oxidation which can be considerable at elevated temperature, crack growth was particularly tested in a nominal vacuum/minimal oxidation environment with a focus on dwell effects. For simulation, the material behaviour was described by a cyclic viscoplastic constitutive model with nonlinear kinematic and isotropic hardening rules. Computational analyses of a stationary crack showed the progressive accumulation of plastic strain near the crack tip, which has been subsequently used as a fracture criterion to predict crack growth using the extended finite element method (XFEM). The crack was assumed to grow when the accumulated plastic strain ahead of the crack tip reached a critical value which was calibrated from crack growth test data in vacuum. During the simulation, the crack length was recorded against the number of loading cycles, and the results are in good agreement with the experimental data which proves the model’s capability to predict fatigue crack growth in nickelbased superalloys at high temperature. It is 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

Effects of oxidation on fatigue crack initiation and propagation in an advanced disc alloy

Powder metallurgy Ni-based superalloys are widely used for aeroengine turbine disc application due to their exceptional strength properties at elevated temperatures, good fatigue and creep performance as well as excellent corrosion and oxidation resistance. However, oxygen enhanced fatigue crack initiation and intergranular propagation at elevated temperatures in air is commonly observed in aeroengine turbine disc superalloys under dwell fatigue testing conditions [1-7], and this phenomenon is usually ascribed to either decohesion/reduction in cohesion strength of grain boundary (GB) due to dynamic embrittlement [8, 9] or GB oxide cracking caused by stress assisted grain boundary oxidation (SAGBO) [5, 10-12]. Although the influence of oxygen on fatigue crack initiation and propagation has been intensively studied, the underlying mechanism for the oxygen-assisted fatigue failure process is still not clear due to the complex composition of disc alloy and the interaction between environmental attack and mechanical load. In this study, fatigue tests were conducted on the Low Solvus, High Refractory (LSHR) alloy designed by NASA for turbine disc application, with a particular focus on studying the influence of the formation of GB oxides on fatigue crack initiation and propagation processes