Probabilistic Gas Turbine Rotor Disk Forging Flaw Crack Nucleation Model Based on Experimental Data and Plasticity-Corrected Stress Intensity Factor

A probabilistic model for quantifying the number of load cycles for crack nucleation at forging flaws in turbine rotor disks has been further developed [1]. This new fracture mechanics-based approach adequately describes the crack nucleation life. The model employs the range of plasticity-corrected stress intensity factor (ΔKJ) as the crack driving force correlating with crack nucleation cycles. Two different approaches for the calculation of ΔKJ are implemented and compared: I) The analytical solution calculates the stress intensity factor (K) and the plastic limit load based on flaw morphologies, types, boundary conditions, and material properties. Here, the failure assessment diagram (FAD) is considered to account for plasticity effects. II) The finite-element method is used to derive ΔKJ from the elastic-plastic J-integral. In the elastic-plastic finite element approach, a mesh convergence study was performed to reduce the effect of the element type and sizes on the numerical solution. As expected, it turns out that the numerical approach improves the accuracy of results due to the limited analytical validity ranges. Subsequently, a fracture mechanics-based crack nucleation model is developed by applying the numerically determined ΔKJ correlating with the experimental crack nucleation cycles. The numerical crack nucleation model shows conservative results compared to the analytical model. Furthermore, a probabilistic framework is proposed for both analytical and numerical crack nucleation models. An approach of integrating the crack nucleation models into the existing crack propagation model under the probabilistic framework is introduced.