Static Strength Assessment of Turbine Blades in High-Capacity Power Units

The article considers assessment methods and criteria of damage inflicted to turbine blades under the effect of static loads in carrying out 3D structural analyses of modern foreign and domestically produced high-capacity power units. Factors that should be considered in performing strength and lifetime analyses of the rotor blades of high-capacity turbines when subjected to short- and long-term static loading are pointed out. The article also describes 3D techniques for carrying out elastoplastic assessment of short-term static strength using a procedure for determining the limit rotation speed to blade fracture, airfoil residual displacements and strains, shank ultimate strength and displacement, root tearing-off, shear, flexural strength, etc. The article presents mutually complementary techniques for determining the bearing capacity as well as global and local long-term strength with using cumulative strain predictions by creep curves. Criteria used in different lifetime assessment methods are described, including those applied at different design stages and in using thermal protection coatings. Cases are considered in which creep strains are determined in the absence of data on creep curves by carrying out elastoplastic analyses by isochronous curves and lifetime analysis using the Larson–Miller curves. The need to take multiaxiality into account in estimating local creep in places of stress concentration is shown, and the applicability limits and criteria of such assessment that make it possible to increase the predicted lifetime by up to two times are described. Examples of tensile and compressive stress relaxation in estimating cumulative creep strain are given. Matters of creep interaction with other types of damage, including high-cycle and low-cycle (thermal cycling) fatigue, and various turbine loading kinds are considered.