Low-cycle fatigue and creep analysis of gas turbine engine components

Abstract

Factors that determine the damage of aircraft engines due to low-cycle fatigue and creep during service are numbers and rate of transient acceleration and deceleration, operating ambient temperature, time at temperature, and amount of time at over temperatures. The purpose of this paper is an attempt to define a method for predicting this engine damage and establishing the critical engine parameters to be monitored in conjunction with either an on-board computer or a suitable recording system that can be used on a central computer at the conclusion of a flight. The scope of the paper is limited to an analytical study of a typical fan engine to show the important engine operating parameters leading to limiting the useful engine life due to 1) low-cycle fatigue in the fan turbine disk and 2) combination of low-cycle fatigue and creep in the high-pressure turbine blades and vanes. No attempt was made to analyze the propagation of cracks, the assumption being that initiation of a crack defines a failure. For the purpose of analysis, a typical aircraft mission profile was chosen. Transient data (spool speeds, gas temperature, and pressure, etc.) required for the low-cycle fatigue study were generated by the simulation of engine thrust transients under various flight conditions. Metal temperatures for the various components were next determined by means of a finite difference network analysis program that included transient and steady-state heat transfer by conduction, convection, radiation, and film cooling. The stress-strain analysis was obtained by various special purpose finite element programs. Leading edge of both the high-pressure turbine blade and inlet vane was found to be a critical element. The analysis also shows that stress concentration due to the presence of cooling holes in the blade, which is a common feature in present high-performance cooled engines, should be examined in detail for low-cycle fatigue failure. Notch effects at fir-tree connections between turbine blades and disks also represent potential low cycle fatigue problem area. Bolt holes were found to be the most critical stress concentration area in the fan turbine disk. Holes of this type are common in multistage turbine disks to allow the tie bolts to penetrate for disk attachment. Various engine parameters that control the low cycle fatigue damages in these locations are identified and discussed. To demonstrate the effect of ambient temperature on the low cycle fatigue life of a component, the high pressure turbine inlet vane was chosen as an example. It is shown that the low-cycle fatigue damage corresponding to a rate of power lever movement is a function of the ambient temperature as well as the fuel control system of the particular engine. Areas which need further research to achieve the desired goals are also discussed.