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Authors: Iesulauro, Erin
Issue Date: 20-Jul-2006
Abstract: Since the 1950's, researchers have studied fatigue crack propagation utilizing fracture mechanics. Such work has provided advances in calculating stress intensity factors, determining elastic-plastic crack tip parameters, and investigating the effects of crack closure. Predictions of fatigue life have been made using crack growth rate models. Over the years, this work has served to influence structural maintenance and damage tolerance philosophies; however, understanding, predicting, and simulating fatigue crack growth is still based on experimental curve fitting and phenomenological rate ``laws.'' The work discussed in this thesis is a step toward understanding fatigue crack incubation, nucleation and microstructurally small crack growth from a first principles approach. To this end, capabilities have been created and assembled to generate, mesh, analyze, and post-process 3D statistical representations of metallic polycrystals with cohesive grain boundaries. A component-based framework facilitates flexibility, growth, and multiscale modeling. Components are accessed and connected through Web service interfaces. The Polycrystal Generator accesses the components for generating, meshing, and assigning properties and boundary conditions to a 3D polycrystal sample. It also provides an interface to a molecular dynamics component to facilitate loosely coupled multi-scale analyses. Analyses are conducted utilizing a parallel solution software package, PETSc, and in-house finite element library, FemLib. The large samples and resulting data is managed using Microsoft SQL Server 2000, an off-the-shelf relation database. Finally, sample geometries, mesh models, and results are visualized using PView, a real-time visualization tool created using OpenDX, Python, and SQL. The assembled framework is used to conduct a parametric study of 3D statistical polycrystals under monotonic loading. The samples are analyzed with variation introduced in geometry, grain constitutive model and parameter values, cohesive grain boundary parameter values, and boundary conditions. This parametric study gives insight into how each variation influences when and where cracks nucleate. Finally, the results from the parametric study are utilized to conduct simulations under cyclic loading. These analyses give insight into the ability to accurately capture grain boundary decohesion leading to fatigue crack nucleation.
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