Effects of Boundary Conditions on Microstructure-Sensitive Fatigue Crystal Plasticity Analysis

The relative fatigue resistance of different polycrystalline microstructures can be evaluated using fatigue indicator parameters (FIPs) that serve as surrogate measures for the fatigue crack formation driving force. This typically requires simulating many grains/phases to capture sufficient microstructure heterogeneity. Thus, the concept of a representative volume element (RVE) for fatigue-related properties has remained computationally prohibitive and elusive. Alternatively, ensembles of statistical volume elements (SVEs) can be simulated to build up the extreme value fatigue response. A crucial consideration in these types of crystal plasticity finite element method (CPFEM) simulations is the nature of applied boundary conditions. Fatigue crack formation has been experimentally observed to occur either at or near the free surface or throughout the specimen volume, depending on the material microstructure, surface conditions, and the fatigue regime (e.g., low cycle, transition, or high cycle fatigue). The recently developed open-source PRISMS-Fatigue framework (Yaghoobi et al. in NPJ Comput Mater 7:38, 2021) enables the simulation of very large microstructures that may be used to study fatigue RVE characteristics. The available multi-point constraints in PRISMS-Fatigue impose periodic boundary conditions that can appropriately distinguish between the bulk and surface driving forces for fatigue crack formation. We demonstrate the efficacy of these multi-point constraints in microstructure-sensitive CPFEM simulations and compare the extreme value fatigue crack driving force response using various boundary conditions, microstructures, and crystallographic textures. The effects of applied boundary conditions on different mechanical responses such as the macroscopic stress–strain response, local measures of plastic slip, and corresponding FIPs are studied. The results provide guidance for microstructure-sensitive crystal plasticity fatigue studies and demonstrate the advanced capabilities of PRISMS-Fatigue to model large volumes of material microstructure.