Characterisation of short crack growth behaviour in a notch stress field under varying block loading cycles in steam turbine blade material

Confidence in accurate fatigue lifing of turbine blade components in the steam turbine fleet has significant implications for power generating companies. How much the company can safely extend the operating life is a critical input in the planning and commissioning of new power station build and in optimizing the timing of retiring the steam turbine (coal-fired) fleet. The fatigue behavior of these turbine blades is controlled by the initiation and growth of small defects in stress concentration features such as the fir-tree root fixings. Knowing how fast these defects will grow in blades and discs that remain in service under representative service conditions (including overloads) is now a critical requirement. One of the challenges in fatigue lifing these components is the lack of specific fatigue crack growth data obtained from the actual materials (in their current aged state) used in the steam turbine blades and to understand the implications of service relevant overloads on this behaviour. Overload effects have been considered in the critical crack size analysis but not, to date, in the crack propagation phase. A clearer understanding is lacking of the relative/competing effects of limiting the overloads in terms of the benefit to critical crack size evaluations versus the possible reduction in beneficial crack retardation mechanisms at these lower levels of overload.

Crack growth rate evaluations will establish crack initiation and early stages of growth for semi-elliptical surface breaking cracks in a characteristic notch stress field. This data will be gathered using interrupted tests and replica tracking of the crack evolution in the notch root. The data gathered will explicitly evaluate the behavior of cracks growing through a plastically deforming notch stress field. Figure 1c shows a typical surface breaking crack that has initiated from a stringer in a similar tempered martensitic steel. Increased part-loads and the associated stop-start cycles on steam turbines due to the inevitable variable supply of renewables-based power to the grid offer new challenges to fatigue evaluations. We will also explore repair and fatigue mitigation strategies using coatings to repair turbine blade surfaces and evaluate how the coatings affect the initiation and growth of fatigue cracks