13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Effect of Notch Severity on Thermomechanical Fatigue Life of a Directionally-solidified Ni-base Superalloy Patxi Fernandez-Zelaia1 and Richard W. Neu1,2* 1 Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, Georgia, 30332, USA 2 School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia, 30332, USA * Corresponding author: rick.neu@gatech.edu Abstract The aim of this research is to understand the influence of notches under thermomechanical fatigue (TMF) in a directionally-solidified (DS) Ni-base superalloy. Experiments were performed utilizing linear out-of-phase (OP) and in-phase (IP) TMF loadings on longitudinally-oriented smooth and cylindrically-notched specimens. Several notch severities were considered with elastic stress concentrations ranging from 1.3 to 3.0. The local response of the notched specimens was determined using the finite element method with a transversely isotropic viscoplastic constitutive model. Comparing the analysis to experiments, the locations observed for crack nucleation in the notch, which are offset from the notch root in DS alloys, are consistent with the maximum von Mises stress. Various local and nonlocal methods are evaluated to understand the life trends under OP TMF. The results show that a nonlocal invariant area-averaging method is the best approach for collapsing the TMF lives of specimens with different notch severities. Keywords notched specimens, thermomechanical fatigue, Ni-base superalloy, notch analysis 1. Introduction Components in the high pressure turbine section of gas turbine engines undergo complex thermomechanical loads during engine cycle transients. Turbine blades contain stress elevating features such as cooling holes or fir-trees that can have a detrimental effect on component endurance and make endurance prediction difficult. The surrounding material around these stress elevators undergo cyclic inelastic deformation providing a source for fatigue crack nucleation [1, 2]. To improve component level endurance prediction methodologies, the effect of stress elevators on thermomechanical fatigue (TMF) needs to be addressed. Few studies have targeted the problem of determining crack initiation of notched specimens under TMF. Presently, no accepted methods exist for predicting TMF crack initiation life at notches. Studies on the TMF of circumferentially-notched specimens fabricated from a rotor steel, 1CrMoV, found that uniaxial life results could be used to predict the notch endurance given that conditions at the notch root were comparable [3, 4]. Large spatial gradients were shown to reduce the driving force for creep damage and fatigue crack growth away from the notch root in notched specimens. For identical crack initiation criteria, this effect could potentially increase the measured endurance as physically small cracks grow slower in notched specimens. A study on the TMF of notched specimens of Ni-base superalloys by Kupkovits and Neu [5] found that damage mechanisms present in notched specimens are identical to smooth specimens. Notched specimen geometries corresponding to theoretical elastic stress concentration factors of 2 tk and 3 tk were studied. Under 500 950 C C OP TMF conditions loaded in the longitudinal (L) orientation, the reduction in life was identical for both of these notches in contrast to isothermal fatigue where the more severe notch had a greater knockdown factor [6]. The same held true under 500 750 C C OP TMF. Fatigue-environmental damage mechanisms were observed in notched OP TMF 500 950 C C . All notched specimen experiments resulted in crack nucleation at the location of maximum Hills' or von Mises equivalent stress similar to isothermal fatigue studies [6].
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