Cyclic Entropy Generation Rate to Signify the Fatigue Failure in Elevated-Temperature Crack Propagation of GH4169 Superalloy

Thermodynamic entropy is a natural metric of irreversible material damage in fatigue process. The cyclic entropy generation rate is utilized to characterize the system state of crack propagation specimen, and is investigated as the indicator of fatigue failure in crack propagation of GH4169 superalloy at elevated temperature. The crack propagation process is analyzed within the thermodynamic framework, and a procedure is proposed to calculate the cyclic entropy generation rate. Finite element simulations were conducted to obtained the data of fatigue life, crack length and cyclic entropy generation rate under different temperatures and different stress amplitudes. The results show that the cyclic entropy generation rate at fatigue failure first decreases and then increases with the increase of stress amplitude. The increasing trend of cyclic entropy generation rate with crack length can be divided into two stages. The cyclic entropy generation rate increases in an approximately exponential form during the second stage. The increase in temperature will significantly increase the cyclic entropy generation rate. The fatigue failure is about to occur when the cyclic entropy generation rate exceeds 1 × 10⁴ J/(m³·K·cycle) for GH4169 superalloy, and this criterion is validated by specimens in other shape, such as compact tension specimens.