PREDICTING THE ONSET OF HIGH CYCLE FATIGUE DAMAGE - AN ENGINEERING APPLICATION FOR LONG CRACK FATIGUE THRESHOLD DATA

Fatigue crack propagation rates have been measured for two titanium-based aeroengine disc alloys using compact tension test-pieces. The loading block employed simulates two features of the engine flight pattern. A major stress cycle represents the start-stop operation which leads to low cycle fatigue. In-flight vibrations, which may give rise to high cycle fatigue, are represented by superimposed minor cycles of high frequency. The threshold values associated with the minor cycles have been used to predict the onset of minor cycle activity. These predictions are successful for Ti-6Al-4V, while for Ti-5331S they are found to be either accurate or safe. Ti-5331S exhibits the greater fatigue lives by virtue of a marginally greater resistance to the onset of minor cycle crack growth combined with slower major cycle crack growth rates prior to this event.