Life assessment and extension for gas turbine blades

Gas turbine blades operate under severe loading conditions, and for satisfactory service must be designed to withstand multiple damage mechanisms. On the other hand, if the assumptions on the future service are sufficiently conservative or actual service conditions more benign than foreseen, there is potential for blade life extension. This generally does not justify fully omitting intermediate inspections, as intervening damage by e.g. overheating or foreign objects is not excluded, and restoring coatings or other repairs could be needed. Nevertheless, a significant extension to the time of final replacement can be possible. In this work, example cases are used to highlight the technical challenges and results of blade life extension. Such aspects include assessment of in-service stress and temperature histories, interpretation of condition assessment, and materials performance. The example cases refer to ex-service blade samples extracted at the time approximately corresponding to the nominal blade life as defined by the engine supplier. Both land based and aircraft turbines are considered, and in spite of the differences in design and service profiles, similarities can be pointed out. The results for the example cases generally suggest significant potential for life extension, for reasons related to the case-specific service histories.