Prediction of fatigue crack propagation lives of turbine discs with forging-induced initial cracks

This paper presents a study on residual life assessment of turbine discs in a military aircraft engine that were found to be susceptible to fatigue cracking failure due to forging flaws formed in the original manufacturing process. As these flaws were not considered in the original life assessment, it is important to predict the residual lives of affected turbine discs and to determine the safe inspection intervals in order to prevent possible failures during service. The examination of the cracked disc revealed that the flaw was formed during the hammer forging. A systematic analysis approach was developed to analyse all four turbine discs and to predict the fatigue crack growth (FCG) rates by using advanced finite element (FE) and numerical FCG predictions. The critical locations for these discs were found to be on the aft neck face of disc web. The predicted FCG for the cracked disc correlated reasonably well with the striation counting from the cracked disc. The residual lives for representative discs at the critical location and associated inspection intervals are determined for life management of the affected turbine discs.