Fatigue failure from sub-surface defects in a gas turbine disc alloy at elevated temperature.

The influence on fatigue life of sub-surface non-metallic inclusions has been studied in Waspaloy at 650 degrees C under both load and effective strain control at a strain ratio of R=0.1. The specimens were shot peened to represent the component condition and to suppress free surface initiation. When compared with defect-free material in terms of total strain range, the presence of the defects reduced average cyclic lives by an order of magnitude. Use of a strain-life based model incorporating a mean stress effect, e.g., the Smith, Watson and Topper approach, could be used for life prediction, but this would require a large data base and/or large reserve factor on life. Alternatively, the use of the initial defect cluster size and LEFM shows a reasonable correlation in terms of the effective stress intensity range (Delta K-eff) and life, suggesting that confident safe life prediction requires the quantification of defect behaviour in terms of crack development and growth from the defects.