Fraunhofer Institute for Mechanics of Materials IWMTurbine blades in gas turbines are manufactured using the investment casting process. This often results in tiny casting porosity, which can lead to cracks under high loads and temperatures. Existing design methods only evaluate this conservatively, which shortens maintenance intervals and increases costs. Fraunhofer IWM is developing a probabilistic service life model that realistically simulates the interaction between creep and fatigue as well as casting defects.
Flexible Turbomachines for Climate-Neutral and Resilient Energy Systems
Ongoing research project
Project description
Market needs and challenges
Gas turbines must be operated with increasing flexibility to compensate for the volatile feed-in of renewable energy - resulting in more frequent start-stop cycles and higher alternating loads on turbine blades. These blades are manufactured using the investment casting process, in which casting porosity is virtually unavoidable. Existing design methods conservatively treat such defects as cracks and neglect the phase leading up to crack initiation - this results in unnecessarily short maintenance intervals and high costs.
Project approach
Fraunhofer IWM is developing a probabilistic service life model for nickel-based cast alloys (René 80 CC) that realistically simulates the interaction of creep and low-cycle fatigue at casting defects. The model is based on the damage parameter DFC, which was developed at the institute. To this end, real and artificial defects are extracted from turbine blades, characterized in extensive creep, fatigue, and creep-fatigue tests at high temperatures, and analyzed microstructurally.
Contribution to addressing challenges
By explicitly accounting for defect size, morphology, and grain statistics, a significantly more realistic and less conservative evaluation concept is created compared to previous approaches. This enables more accurate service life predictions, potentially longer maintenance intervals, and lower costs for gas turbine operators. The developed model is also transferable to other components and manufacturing processes - such as additively manufactured components.
Fraunhofer IWM’s work packages in the project:
First, Fraunhofer IWM will extract and fabricate specimens with natural and artificial casting defects from actual turbine blades, aided by CT scans (WP 1.3.1). Essentially, approximately 180 samples will be tested in creep, LCF, and combined creep-fatigue tests at two high-temperature levels (WP 1.3.2–1.3.4), accompanied by fractographic and EBSD microstructural analyses to elucidate the damage mechanisms. Based on this, Fraunhofer IWM is calibrating and expanding its existing DFC damage model to account for the influence of casting defects and grain orientation, and is using this to develop a probabilistic assessment concept for creep-fatigue interaction, which will ultimately be applied to a real component via FEM analysis as an example. The final results include a validated, probabilistic life-span model, a comprehensive database of experimental data, and a creep-fatigue interaction diagram for René 80 CC.
Based on the project results, Fraunhofer IWM can offer industrial companies the following research and development services:
Probabilistic service life assessment of casting defects
A more realistic, less conservative assessment of component service life, taking into account defect size and morphology.
Creep-fatigue testing on defective specimens
Experimental characterization of materials under combined creep and fatigue loading at high temperatures.
Application of the DFC damage model to new materials
Application of the established evaluation concept to company-specific alloys for the accelerated qualification of new materials.
Consulting on the integration of probabilistic evaluation concepts into design toolchains
Support for incorporating more realistic life-cycle models into existing design processes; reduces safety margins and increases the cost-effectiveness of component design.
