Microstructure Sensitive Fatigue Crack Nucleation in Titanium Alloys Using Accelerated Crystal Plasticity FE Simulations

Abstract

This chapter investigates microstructure and load sensitive fatigue behavior of Ti-6242 using cyclic crystal plasticity finite element (CPFE) simulations of statistically equivalent image-based microstructures. A wavelet transformation induced multi-time scaling (WATMUS) method (Joseph et al., Comput Methods Appl Mech Eng 199:2177–2194, 2010; Chakraborty et al., Finite Elem Anal Des 47:610–618, 2011; Chakraborty and Ghosh, Int J Numer Methods Eng 93:1425–1454, 2013; Ghosh and Chakraborty, Int J Fatigue 48:231–246, 2013) is used to perform accelerated cyclic CPFE simulations till crack nucleation, otherwise infeasible using conventional time integration schemes. A physically motivated crack nucleation model in terms of crystal plasticity variables (Anahid et al., J Mech Phys Solids 59(10):2157–2176, 2011) is extended in this work to predict nucleation. The dependence of yield strength on the underlying grain orientations and sizes is developed through the introduction of an effective microstructural parameter Plastic Flow Index or PFI. To determine the effects of the microstructure on crack nucleation, a local microstructural variable is defined in terms of the surface area fraction of soft grains surrounding each hard grain or SAFSSG. Simulations with different cyclic load patterns suggest that fatigue crack nucleation in Ti-6242 strongly depends on the dwell cycle hold time at maximum stress.