Assessment of Thermal-Fatigue Resistance of High Temperature Alloys

Extract

Conditions of thermal fatigue encountered in the Aircraft Industry are usually located in engine components subjected to severe temperature and stress cycling. These components include gas turbine blading, nozzle guide vanes, flame tubes and turbine discs fabricated from a number of different materials ranging from Nimonics to sheet iron— and cobalt-base alloys. In recent years considerable experimental work has been carried out to assess material behaviour under thermal fatigue conditions and several means of testing have emerged. Rapid heating and cooling cycles using a “fluidised bed” technique have been extensively applied by Glenny and co-workers, at NGTE, and by material manufacturers to aid alloy development. Other experimenters have used axial straining devices based on expansion and contraction of heated columns followed by air cooling. One aircraft engine manufacturer has developed a thermal fatigue rig incorporating a repeated tensile axial load and temperature cycle. A further means of assessment has been developed by Forrest at NPL, in which the test specimen is strain cycled under reverse-bend conditions at constant or fluctuating temperatures.