13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Variability of the Fatigue Driving Force within Grains of Polycrystals 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 Experimental studies in the last few decades have exhibited higher fatigue crack growth rates for cracks with size on the order of grains than would be predicted using growth laws based on LEFM. Small crystallographic fatigue cracks are affected by microstructure features that are not captured by traditional homogenous fracture mechanics theories (i.e., LEFM, EPFM). Since far-field driving force parameters cannot capture the intrinsic variability of the local fatigue driving force of small cracks induced by microstructure, alternative measures of the fatigue driving force are sought. This work employs finite element simulations that explicitly render the polycrystalline microstructure to compare nonlocal fatigue indicator parameters (FIPs) averaged over multiple volumes. The model employs a crystal plasticity algorithm in ABAQUS calibrated to study the effect of microstructure on early fatigue life of Ni-base RR1000 superalloy at elevated temperature under constant amplitude loading. The results indicate slight differences in the extreme values of distributions of FIPs for each element, slip plane cross-section (bands) and grain volumes. Furthermore, the grain average FIP better reflects the driving force for cracks on the cross section at the center of the grain while the extremes values of the FIPs averaged along bands tend to be located away from the grain centers. Keywords Fatigue Indicator Parameter, Microstructurally Small Cracks, Fatigue Driving Force 1 Introduction Extensive literature shows that fatigue experiments on metals in the high cycle fatigue (HCF) regime present variability in fatigue life of over a factor of 10. Multiple investigators have demonstrated that the underlying microstructural attributes [1][2] (i.e., grain size effects, elastic and plastic anisotropy, pre-existing defects) are usually responsible for the large variation in fatigue life. Indeed, in the HCF regime, the heterogeneous plastic deformation within favorably oriented and/or highly stressed grains controls the nucleation and early growth of fatigue cracks. Recent finite element approaches that render the microstructure of metallic alloys have estimated the fatigue damage by assessing nonlocal fatigue indicator parameters (FIPs). These parameters typically refer to the value of the FIP averaged over certain mesoscale volumes (e.g., grain volumes). In contrast to local magnitudes within each finite element, these nonlocal FIPS mitigate effects of the mesh sensitivity and represent the physical length scale over which the fatigue damage occurs. Recent work [3][4] pursued definition of the averaging volume in terms of bands
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