Fatigue crack growth behaviour and life prediction following a lifetime extension strategy in the low-cycle fatigue regime for FV566 turbine blade steel

Bend bars made from FV566 martensitic stainless steel were extracted from the root of ex-service power plant turbine blades and representative notches introduced. Constant amplitude cyclic loading, and with overload cycles of OLR of 1.56 applied every 150 baseload cycles, were applied to the samples until a crack of 1 mm was observed. A ‘lifetime extension strategy’ was applied, involving grinding away existing short cracks, followed by T0 shot peening on the top surface of the sample. Fatigue cyclic loading was recommenced with initial strain range loading conditions until failure. A lifetime prediction method combining the Coffin-Manson and Paris Law relationships were used to investigate the efficacy of the ‘lifetime extension strategy’ for industrial use. The ‘lifetime extension strategy’ increased the life of U-notched samples tested and a minimum increase in strain range following the application of the ‘lifetime extension strategy’ was estimated. It is expected that the application of a’ lifetime extension strategy’ such as the one tested may provide some additional service life to existing in-service turbine blades.