Entropy-based modeling of fatigue crack growth in nickel-based superalloy GH4169 at elevated temperatures using temperature-dependent viscoplasticity

An entropy-based temperature-dependent fatigue crack growth (FCG) framework is developed for the nickel-based superalloy GH4169 and validated through combined experiments and numerical simulations. Thirteen sets of fatigue crack growth tests were conducted at 300–650 °C, and crack propagation was monitored using the direct current potential drop (DCPD) method in accordance with ASTM E647. A node-release finite element model coupled with a temperature-dependent Chaboche viscoplastic constitutive model is employed. Crack-tip entropy production is evaluated from the multiaxial temperature-dependent viscoplastic field. Analysis of near-tip entropy production with fixed crack length shows that the per-cycle increment is approximately constant, which enables the definition of steady cyclic entropy production (SCEP). The fracture process zone for Mode I cracks is defined based on the normalized near-tip normal stress distribution. To characterize spatially non-uniform entropy production within the fracture process zone (FPZ), an effective steady cyclic entropy production metric and a temperature-dependent crack-growth entropy threshold are introduced. By defining the crack growth rate as the average rate of entropy-based damage accumulation leading to failure within a fracture process zone of characteristic length ℓ*, an entropy-damage formulation is established and yields a temperature-dependent fatigue crack growth model. Validation results show mean prediction errors below 10%, and nearly all predicted crack growth rates fall within a double scatter band relative to the experimental data over the investigated temperature range.