Geometrically Explicit Finite Element Modeling Of Aa7075-T651 Microstructure With Fatigue Cracks

摘要

This dissertation is divided into three chapters, where each is an independent paper intended to be submitted as a refereed journal article. The main thrust of the research project overarching all three papers is to develop a high fidelity, geometrically explicit approach to finite element modeling fatigue at the microstructural length scale. Each paper is a study within this thrust, and the following is a sweeping overview of each study. More detailed abstracts for each paper are given at the beginning of each chapter. The paper in the first chapter is the fourth in a series of papers focused on implementing, calibrating, and validating criteria for simulating microstructurally small fatigue crack (MSFC) evolution, with high strain conditions in aluminum alloy (AA)7075-T651 as the proof-test application. MSFC evolution is divided into three stages: incubation, nucleation, and propagation. The specific focus of this paper is on the last stage, MSFC propagation, which is microstructure-governed fatigue crack growth through grains and/or along grain boundaries. Three simulated field metrics, crack tip displacement, crack-induced plastic slip localization, and maximum tangential stress ahead of the crack, previously investigated for prediction of nucleation, are investigated in this paper to determine their dependence on microstructural heterogeneities after nucleation. A total of 21 simulations are performed on a simplified baseline model of an AA7075-T651 microstructural region containing an MSFC. All three metrics are determined to be significantly dependent on the local microstructure immediately subsequent to nucleation. The particle spawning the crack and the orientation(s) of the grain(s) immediately surrounding the nucleated MSFC most influence the MSFC metrics. The paper in the second chapter focuses on the implementation of a computational framework that accurately and probabilistically models fatigue crack propagation at the microstructural scale, once again with high strain conditions in AA7075-T651 as the proof-test application. Toolsets are presented that generate and discretize statistically accurate microstructure geometry models and explicitly simulate the evolution of microstructurally small fatigue cracks. The concept is demonstrated through two model simulations and feasibility of the approach is critically evaluated. The paper in the third chapter is the fifth in the same series of papers described above for the first chapter. The focus of this paper is again on the last MSFC evolution stage, MSFC propagation. High resolution, micro-scale images of three propagating MSFC's are analyzed to determine dependencies of MSFC propagation on microstructural heterogeneities. Additionally, the three MSFC metrics studied in the first chapter - maximum tangential stresses, plastic slip localization, and crack displacements local to the crack front - are simulated in a finite element model that replicates an observed MSFC and the surrounding microstructure. The detailed observations and simulation reveal that MSFC propagation in AA7075T651 is highly dependent on the local microstructure, and MSFC behavior due to these dependencies can be predicted by the computed field metrics.