Fatigue Crack Initiation and Growth Behavior in a Notch with Periodic Overloads in the Low-cycle Fatigue Regime of FV566 Ex-service Steam Turbine Blade Material
Abstract
The effect of periodic overloads on crack initiation, growth rate, and fatigue life, within a notch stress field representative of a turbine blade root fixing, has been investigated. Bend bars made from FV566 martensitic stainless steel were extracted from the root of ex-service power plant turbine blades and representative notches introduced. These notched plain bend bars were loaded plastically in the low-cycle fatigue regime and were tested with overloads up to 150% of the cyclic baseload every 150 baseload cycles. A periodic overload of 50% of the cyclic baseload increased the number of cycles to crack initiation and slightly retarded the crack growth rates of both short and long cracks, leading to a slight improvement in fatigue life. The results suggest that small overloads (less than 10%) are not expected to significantly impact fatigue lifetimes or service scheduling of components such as low-pressure steam turbine blades.