Comparison of Methods for Lifetime Calculations of Highly Loaded Aero-Engine Discs

The requirement for low weight-to-thrust ratios of modern aero-engines has led to high structure loads and design of parts in the range of low cycle fatigue. In order to ensure high reliability and optimized TBO’s (time between overhaul), methods to calculate accurately the durability of parts are of great interest. For a highly loaded compressor disc of a fighter engine the lifetime was calculated using the nominal stress method and the maximum local strain concept. The nominal and maximum local stresses and strains were calculated with a finite-element program which was extended to consider plastic deformation of materials. The reference load cycle, which was the same for both methods, was derived from actual load measurements during flight missions. The comparison of the results shows the region of validity of both methods and suggests the application of a modified nominal stress method. Two proposals for improving the nominal stress method by superseding the Neuber-Hyperbolic rule and modifying the mean stress influence in the Goodman diagram are discussed.