13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Fatigue Life Prediction of Polycrystals under Multiaxial Straining Gustavo M. Castelluccio1,*, David L. McDowell1,2 1 Woodruff School of Mechanical Engineering 2 School of Materials Science and Engineering Georgia Institute of Technology, 771 Ferst Drive, N.W, Atlanta, Georgia 30332, USA * Corresponding author: castellg@gatech.edu Abstract: In the high cycle fatigue, the initiation of fatigue cracks is significantly affected by microstructure, loading conditions, and specimen geometry. However, fatigue life estimation traditionally considers microstructure and geometric effects via semi-empirical methods without explicit consideration of the early stages of crack formation, which tends to dominate the total lives in high cycle fatigue. Such a strategy has been useful for existing materials that have been characterized with extensive fatigue experiments, but is less applicable to the design of fatigue-resistant alloys or modification of existing alloy microstructures to enhance fatigue resistance. This paper employs a framework developed to assess the early stages of crack formation and growth through the microstructure in smooth and notched specimens. The methodology employs finite element simulations that render an unimodal grain-size microstructure and a crystal plasticity-based fatigue model that estimates 3D transgranular fatigue growth on a grain-by-grain basis. The crystal plasticity model parameters were calibrated for Ni-base superalloy RR1000. In these simulations, cracks form in near surface grains with highest slip-based driving force and then propagate through the field of adjacent grains. Keywords Fatigue Indicator Parameter, Microstructurally Small Cracks, Fatigue Life 1 Introduction In spite of its significance in industrial applications, the influence of microstructure on the early stages of fatigue cracks in engineering alloys is still poorly understood. The formation and early growth of fatigue cracks can consume a significant portion of the high cycle fatigue life and is strongly influenced by the size and shape of grains, and the crystallographic orientation. Fatigue models have been able to predict the fatigue life as a function of the microstructure by employing parameters aimed at reflecting the role of microstructure without strong physical connections. These methodologies can assess and perhaps compare materials, but they are not fully appropriate to design of fatigue-resistant engineering alloys. During the past decade computational simulations have been increasingly used for designing materials. These models simulate microstructure-sensitive mechanical responses with the aim of reducing experimental effort. Castelluccio [1][2] developed a computational methodology for predicting the number of fatigue cycles required to crack an individual grain with highest driving force. The algorithm employs finite element simulations and a crystal plasticity framework to compute nonlocal fatigue indicator parameters (FIPs) which are correlated with the cycles required to crack a grain. This methodology has been successfully employed to assess the effect of bimodal
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