Modelling and predictions of time-dependent local stress distributions around cracks under dwell loading in a nickel-based superalloy at high temperatures

Finite element (FE) analysis modelling of crack configurations was used to provide insights into the role of time-dependent deformation on dwell fatigue crack growth (DFCG) in turbine disc alloys. In particular, the potential influence of time-dependent deformation on fatigue crack growth rates observed during dwell holding periods and the potential crack growth retardation phenomena that can occur for overload-dwell cycles were investigated. The FE model evaluated the mechanical stress state evolution in a two-dimensional cracked geometry subjected to standard dwell and overload-dwell stress waveforms for a given microstructural condition (based on its γ’ particle distribution). Crack-opening stress distributions as a function of dwell time and distance ahead of the crack-tip were interrogated, with the aim to investigate the potential influence of local crack-tip stresses on crack growth during dwell holding periods and the potential for crack growth retardation following overload cycles. Stress distribution profiles predicted by dwell-only simulations showed how quickly local crack-tip stresses relax due to time-dependent plasticity. Characteristic effects of overloads of dwell crack growth resistance observed in experiments were also reasonably well captured. A numerically-derived incubation time was successfully applied in predicting the general trends of achieving more pronounced retardation effects with larger overload factors and extended periods of overloading prior to periods of dwell. FE modelling predicted a significant difference in behaviour between an overload for a given overload factor with a hold time at peak load of 1 s and 10 s. This study emphasises the importance of studying local crack-tip stresses in regards to characterising DFCG behaviour of turbine disc alloys.