Investigations of Surface Instabilities in Soft Materials and Phase Transformations in Additive Manufacturing

Zhang, Qi

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Year:: 2019
Contributor:: Zhang, Qi (creator); Blume, Janet (Advisor); Kim, Kyung-Suk (Reader); Oancea, Victor (Reader); Brown University. Engineering: Mechanics of Solids (sponsor)
Genre:: theses
Subject:: surface instability; wrinkle; crease; Finite element method; bifurcation; Koiter's method; Compressible neo-Hookean bilayer; Blatz-Ko material; Additive Manufacturing; AM Process Simulation; Powder Bed Fabrication; Microstructure; Selective Laser Melting; metallurgical phase transformation; post-buckling analysis
Extent:: , None p.
DOI: https://doi.org/10.26300/z7sj-xj22

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Abstract:: In this thesis I investigate two separate issues. The first was a class of surface instabilities in compressible soft materials. The second issue was the development of a generic finite element modeling framework for metallurgical phase transformations in additive manufacturing (AM) process. In the past a few decades, wrinkling in the bilayer system of incompressible neo-Hookean material has been studied extensively. In this thesis, the effect of substrate compressibility on surface wrinkling in soft materials is analyzed through theoretical and computational analysis. The generalized Blatz–Ko model, which may reduce to commonly used incompressible and compressible materials such as neo-Hookean, Mooney–Rivlin and Blatz–Ko models, is employed. The wrinkling behavior of a thin film bonded to a soft compressible substrate is analyzed. We found the existence of a mode of wrinkling in slabs of finite thickness, which is characterized by sinusoidal displacements in both vertical and horizontal directions. Second-order analysis using Koiter’s method shows that bi-sinusoidal wrinkling is stable in a subclass of the materials considered. Furthermore, FE simulations are performed to verify the critical condition for wrinkling and investigate the post-wrinkling behaviors. FE analyses demonstrate that the critical wrinkling strain agrees with the preceding derivations. A Fourier transformation analysis shows that the wrinkling deformations that emerge in the FE analyses follow the theoretically predicted sinusoidal functions and frequencies. The imperfection sensitivity of the wrinkling is also studied and the critical strains for creases in generalized Blatz-Ko material are determined. In metal AM applications, assessing the amount of unfused powder, melt pool volumes, and metallurgical phase transformations is often of interest. In this thesis, I develop a generic framework in ABAQUS for modeling of metallurgical phase transformations in AM process. Phenomenological plasticity models based on the predicted microstructural evolutions are used to predict yield strength, ultimate strength and ductility limit. Using the framework, FE simulations were performed to validate experimental predictions for Ti-6Al-4V alloys, including temperature measurements, EBSD/XRD microstructural examinations, and tensile tests on dogbone specimens. The framework has produced reliable predictions of volume fractions, grain morphology, and mechanical properties in printing process and heat treatment process.
Notes:: Thesis (Ph. D.)--Brown University, 2019

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Zhang, Qi, "Investigations of Surface Instabilities in Soft Materials and Phase Transformations in Additive Manufacturing" (2019). Mechanics of Solids Theses and Dissertations. Brown Digital Repository. Brown University Library. https://doi.org/10.26300/z7sj-xj22

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Mechanics of Solids Theses and Dissertations

Theses and Dissertations for the Mechanics of Solids department.
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