Innovative Material Testing Based on Small-Scale Specimens and Application for Turbomachinery Components

Material characterization is usually based on standard specimen testing. In contrast, small-scale specimens require less testing material and offer additional advantages like investigation of size effects with impact on material properties (specimen diameter equivalent to wall thickness of buckets and liners) and testing of samples of virgin or service-exposed turbomachinery components. The paper highlights the small-scale specimen test setup and its application for LCF- and HCF-testing. Innovative small-scale specimen testing with close-to-component structural features enables direction dependent and spatially resolved determination of material characteristics. Cast and forged gas turbine nickel-based alloys and a typical steam turbine steel are the materials of interest in this study. Substrate temperature levels ranges from room temperature up to 1000 °C. Test system, radiation heating, instrumentation and two types of small-scale specimen geometries according to German standard DIN 50100 will be presented. Benefits and limitations of application of small-scale specimens will be discussed. Finally the authors report about application of small-scale specimen testing for remaining life time determination of service exposed gas turbine buckets. Small-scale specimens have been directly extracted from a gas turbine. Bucket root material (conventionally cast nickel-base alloy IN738) and literature data serve as reference base for initial state of the material. Tensile and LCF tests have been carried out at a representative material temperature for service conditions of 850 °C. Published material data of IN738 are available at this temperature. On that basis, fatigue life consumption has been estimated. The presented procedure for remaining life time prediction can also be applied to other turbomachinery components.