Lifing assessment of Gas Turbine blade root affected by Out-of-Tolerances

Climate changes of recent decades forced engineers and scientists to challenge themselves with new solutions and improving existing ones. In this scenario heavy-duty gas-turbines (GTs) development was focused on improving performances, e.g. augmenting power-output and efficiency, and employing them as a support to renewable energy sources (RES). As a consequence, GTs components have to withstand always higher thermomechanical loads. Moreover, nowadays GTs are operated in cyclic conditions much more frequently than in the past. Turbine blades are surely one of the most critical components due to the harsh operating conditions. Hence, the production process of these components is a critical aspect to be considered in design and qualification phase. Nevertheless, machining process limitations must be considered and implemented to ensure a feasible compromise on production tolerances. Supplier qualification process has to be well implemented to guarantee high quality level products. This work deals with out-of-tolerances recorded after machining process of a turbine blade fir-tree attachment during a new supplier qualification. The goal of this study is better handling the impact of out-of-tolerances on components lifing by means of 2D finite element analyses (FEA). The selected gas turbine component for this manuscript is a third stage rotor blade, designed for a F-class engine operation with a long-proven endurance. This situation permits the use of mean material properties during the component lifing evaluation. The study workflow of this activity is reported as follows. First of all, a statistical analysis was carried out to choose the most representative parameters affected by out-of-tolerances. Then, a FE model of a nominal geometry was set and a mesh refinement process has been carried out to achieve mesh-independent results. Secondly, the same FE mesh setting has been applied to geometries modified with out-of-tolerance parameters by a 2D parametric CAD model. The component life assessment has been performed by means of Ansaldo Energia LCF in-house tools. Furthermore, also a stress gradient approach has been applied to better evaluate the notch stress factors in the contact areas and to obtain more reliable fatigue damage results. A results comparison between LCF in-house tools and stress-gradient approach has been performed and reported for the blade. For sake of completeness, also the disc lifing has been evaluated by means of a dedicated LCF in-house tool, more specific for the disc material. In conclusion, a detailed comparison between different out-of-tolerances parameters is reported and discussed. Moreover, these results have permitted to better understand the impact of these out-of-tolerances on components lifing and a final assessment on their employment is efficiently allowed.