<p>Accurate prediction of cycles to crack initiation in critical turbine components is a major consideration in turbomachinery design, especially in components with highly concentrated stress such as turbine blades with cooling holes.&nbsp; Viscoplastic lifing methods are typically used to predict shakedown and cycles to failure, however complicating factors still exist that produce challenges for traditional methods.&nbsp; Therefore newer methods utilizing constitutive modeling with consideration for isotropic and polytropic hardening have been developed to better capture evolution of cyclic behavior of the material.&nbsp; Presence of mean stress and stress concentration factors are some of the complications that can be better accounted for using constitutive models.&nbsp; Present paper evaluates experimental and theoretical life of specimen made from nickel based super alloy with high stress concentration features under cyclic conditions with mean stress.&nbsp; The specimen geometry and loading are designed to mimic trailing edge holes in an actual IGT turbine blade.&nbsp; Experiments are conducted at an elevated temperature at two peak stress values to determine sensitivity to applied load at operating temperature similar to engine.&nbsp; Cycles to crack initiation are analytically evaluated using the well-known Manson-Coffin method with Morrow mean stress correction and two distinct methods for strain range evaluation.&nbsp; &nbsp;First method is the traditional Ramberg Osgood shakedown that has been extensively used in the industry.&nbsp; Second method is constitutive Chaboche based model run in FEM with custom material model.&nbsp; Constants for Chaboche model are determined from Ramberg-Osgood constants with a method that takes into account yield surface evolution and hardening constants, in addition to rate dependent stress relaxation factor that can be used to model dwell time effects.&nbsp; Analytical results are compared with the experimental data, and advantages and disadvantages of both methods are discussed.</p>

Experimental and Analytical Investigation of Cyclic Crack Initiation in Nickel Based Super Alloy With Stress Concentration Features

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2021/volume/84898

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