Predicting stress and creep life in Inconel 718 blade-disk attachments

Due to several physical and computational difficulties in Finite Element (FE) analysis of contact in fir-tree blade-disk attachments of gas turbines, there has always been a great demand for stress, fatigue and creep life assessment of fir-tree joints without modeling physical contact. However, reliability of these models is not trivial. To meet these challenges, in the current paper, three different analysis methodologies were examined for creep life assessment in a turbine engine disk. This is a significant problem for designers and operators of gas turbines. Two simple models were developed with removal of the blade from the FE domain as well as a detailed 3D model containing a blade and it’s contact details. Numerical results were compared with available experimental stress measurement. Creep material behavior models were also developed and parameters for the Liu-Murakami damage model together with the Norton creep law were determined for Inconel 718 at 620 °C. A creep subroutine was written and creep analyses of these joints were performed for typical service temperature of 620 °C and the maximum time period of 100,000 h. FE results were compared with fracture mode of service induced cracks. For the different load cases ranging from 3000 rpm to 9000 rpm, it is demonstrated that methodology based on simple models are very efficient. The equivalent pressure model always underestimates the value of peak stress whereas the virtual link model provided more realistic results. The results are discussed according to the different failure criteria for the fir-tree joints life assessment. Results presented in the paper are valuable for failure analysis of these critical components. Moreover, the stress distribution obtained from the simple realistic model can be used for the life assessment codes applicable to other types of failure mechanism.