Caltech Library System

About | Browse | Search | Caltech Student Instructions

Type of Document	Dissertation
Author	Yang, Qiang
Author's Email Address	yang AT aero.caltech.edu
URN	etd-05282004-152537
Persistent URL	http://resolver.caltech.edu/CaltechETD:etd-05282004-152537
Title	Thermomechanical variational principles for dissipative materials with application to strain localization in bulk metallic glasses
Degree	PhD
Option	Aeronautics
Advisory Committee	Advisor Name Title Guruswami Ravichandran Committee Chair Ares J. Rosakis Committee Member James L. Beck Committee Member Michael Ortiz Committee Member William L. Johnson Committee Member
Keywords	shear band variational principle strain-localization bulk mettalic glasses finite deformation variational update
Date of Defense	2004-05-17
Availability	restricted
Abstract This thesis is concerned with variational principles for general coupled thermomechanical problems in dissipative materials including finite elastic and plastic deformation, non-Newtonian viscosity, rate sensitivity, arbitrary flow and hardening rule, as well as heat conduction. It is shown that there exists a potential function such that both the conservation of energy and balance of linear momentum are the Euler-Lagrange equations of its first variation. Inspired from the time-discretized version of the variational formulation, we present a procedure for variational thermomechanical update, which generalizes the isothermal approach under a variational thermodynamic framework. This variational formulation then serves as a basis for temperature change as well as constitutive updates. An important application of the variational formulation is to optimize the shear band thickness in strain localization processes. We show that this optimization takes the form of a configurational-force equilibrium and results in a well-defined band thickness. We further implement displacement discontinuities into a class of strain-localization finite elements. These elements consist of two surfaces, attached to the abutting volume elements, which can separate and slip relative to each other, and thus enable the accurate and efficient simulation of the dynamical formation of stain localization. The variational formulation also leads to a finite-deformation continuum modeling of bulk metallic glasses. It is shown that the strain softening of bulk metallic glasses is due to the increase of free volume (and thus the decrease of viscosity), while temperature rise accelerates the localization of the deformation. The model reproduces the constitutive behavior of Vitreloy 1 bulk metallic glass at various strain rates and temperatures.
Files	Filename       Size       Approximate Download Time (Hours:Minutes:Seconds)     28.8 Modem   56K Modem   ISDN (64 Kb)   ISDN (128 Kb)   Higher-speed Access  Thesis.pdf 2.42 Mb 00:11:12 00:05:45 00:05:02 00:02:31 00:00:12 indicates that a file or directory is accessible from the campus network only and must not be distributed to non-campus persons.