An investigation of the fatigue life of typical aircraft gas turbine compressor discs

Abstract

The numerous discs which are incorporated in gas turbine rotors are subjected to high level stresses at full engine speed. Each time the engine is operated and run up to maximum speed one cycle of stress is applied to each disc. In the course of time there exists the possibility of a fatigue failure. The life of a component cannot be predicted accurately from the properties of laboratory specimens of the material because of the more complicated stress pattern and the number of manufacturing variables which affect the fatigue properties.

A special testing machine was built to subject compressor discs to cyclic loading at the blade fastenings whilst the disc remained stationary. Stress distributions for a typical disc were calculated for rotating and non-rotating conditions. Strain gauges were used to measure the stresses under the non-rotating conditions of the testing machine. Comparisons of theoretical and experimental results were made. It was concluded that by applying loads of sufficient magnitude at the rim it was possible to produce stresses at the bore of the same magnitude as those produced under rotating conditions and that the method of loading produced a reasonable simulation of the stress distribution in the central part of the disc. Under non-rotating conditions with these high loads the tangential stresses in the outer part of the disc were about 30 per cent higher and the radial stresses about 60 per cent higher than under rotating conditions.

It was found necessary to make modifications to the discs for the fatigue tests. When loaded through the standard pinhole blade fastening, the discs failed from the pinholes because of the increased nominal stress in the rim and the high local load. By cutting off most of the rim and using grips instead of the standard pin fastening it was possible to obtain failures in the diaphragm of the discs. Although the individual discs failed in different modes with fractures arising from several sources the lives of all the discs tested lay between 34 000 and 130 000 cycles. These lives were satisfactory compared with service requirements. Typical aircraft engines accumulate from 1000 to 4000 flight cycles per year according to service conditions.