Thermomechanical fatigue crack formation in nickel-base superalloys at notches
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Hot sections of gas engine turbines require specialized materials to withstand extreme conditions present during engine operation. Nickel-base superalloys are typically used as blades and disks in the high pressure turbine section because they possess excellent fatigue strength, creep strength and corrosion resistance at elevated temperatures. Components undergo thermomechanical fatigue conditions as a result of transient engine operation. Sharp geometric features, such as cooling holes in blades or fir-tree connections in disks, act as local stress raisers. The material surrounding these features are potential sites of localized inelastic deformation and crack formation. To reduce customer costs associated with unnecessary overhauls or engine down-time, gas turbine manufacturers require accurate prediction methods to determine component endurances. The influence of stress concentration severity on thermomechanical fatigue crack formation is of particular importance as cracks often initiate in these hot spots. Circumferentially notched specimens were utilized to perform thermomechanical fatigue experiments on blade material CM247LC DS and disk material PM IN100. A parametric study on CM247LC DS was performed utilizing four notched specimens. Experimental results were coupled with finite element simulations utilizing continuum based constitutive models. The effects of applied boundary conditions on crack initiation life was studied in both alloys by performing experiments under remotely applied force and displacement boundary conditions. Finite element results were utilized to develop a life prediction method for notched components under thermomechanical fatigue conditions.