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Title:Modeling dynamic impact of layered structures
Author(s):Chan, Jason
Advisor(s):Sofronis, Petros
Department / Program:Mechanical Sci & Engineering
Discipline:Mechanical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Degree:M.S.
Genre:Thesis
Subject(s):dynamic impact
layered structures
nanolayered structures
multilayered structures
Abstract:Multilayered metals have been shown to display unusual strength as compared to bulk materials because interfaces act as barriers to slip transmission. Results from atomistic studies of mechanical properties of material interfaces can be combined with continuum mechanics approaches to simulate experiments involving dynamic impact of layered structures. Recent technological advances have allowed for the use of high-powered lasers to create strain rates above 10^7 1/s. However, experimental data of impact tests at these ultra-high rates is still sparse, so the behavior of materials at these high strain rates is not well-understood. The goal of this study is to understand the mechanical response of multilayered structures at high strain rates through using constitutive models for the characterization of the layer bulk material and interface response to strain. Two dislocation-based constitutive models are proposed and implemented into dynamic simulations using finite element analysis. The Gilman model characterizes plastic flow as a function of mobile dislocation density and accounts for dislocation generation and movement. The characteristics of this model in the context of dynamic impact are investigated, and a modification is made to the model to increase its range of applicability and its usefulness in monitoring dislocation density. The Estrin two-parameter model characterizes plastic flow as a function of mobile dislocation density and forest dislocation density and includes constants accounting for dislocation generation, annihilation, movement, and trapping. While this model was designed for creep applications, its thorough characterization of dislocation-based phenomena may make it viable for characterizing material behavior at high strain rates. A study is performed on the Estrin model constants in order to be able to calibrate the model with experimental data. Lastly, a method of modeling material interfaces through finite element analysis is presented. Dynamic impact simulations are performed on multilayered structures in order to investigate the effect of interfaces on deformation processes and dislocation interactions. The interface is treated as a cohesive region using cohesive elements governed by a traction-separation law. A characteristic length is ascribed to the interface, and a greater length is shown to correspond to increased dislocation buildup close to the interface. The effect of bulk layer thickness on dynamic response is investigated. Structures with thinner layers are shown to have greater dislocation buildup close to the site of impact.
Issue Date:2012-05-22
URI:http://hdl.handle.net/2142/31094
Rights Information:Copyright 2012 Jason Chan
Date Available in IDEALS:2012-05-22
Date Deposited:2012-05


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