Direct comparison of microstructurally small fatigue crack growth modeling: 3D mechanistic modeling compared with subsurface experiments

Microstructurally small fatigue cracks (MSFCs) challenge the predictions of classical linear elastic fracture mechanics (LEFM) due to significant microstructural influence on their growth directions, rates, and crack closure. This study integrates in-situ far-field high-energy diffraction microscopy and micro-computed tomography providing 3D, time-lapsed crack front characterization with direct comparisons to crystal plasticity finite element method (CPFEM) modeling on the exact same voxelized experimental IN718 microstructure. The accumulated plastic strain energy was employed as a metric to connect micromechanical fields to the crack extensions and growth rates. Pointwise comparisons along identical crack fronts at multiple fatigue stages quantify the accuracy of the CPFEM models developed across three different cycle counts in capturing experimental-derived stress fields, grain-scale crack path tortuosity, local crack growth rate variations, and crystallographic versus non-crystallographic transitions. Therefore, this comprehensive experimental dataset offers unparalleled opportunities for conducting a detailed, grain-by-grain comparison and corroboration for MSFC modeling for an additively manufactured IN718 material. This work advances MSFC modeling from a largely phenomenological basis, towards a physics-based, grain-resolved direct comparison between CPFEM and in-situ 3D experiments, from which mechanics-based modeling principles are extracted for dissipative energy driving force and local slip descriptors in order to inform microstructure-sensitive fatigue models.