CaltechTHESIS
  A Caltech Library Service

Fracture of materials undergoing solid-solid phase transformation

Citation

Penmecha, Bharat Prasad (2013) Fracture of materials undergoing solid-solid phase transformation. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechTHESIS:05302013-233635296

Abstract

A large number of technologically important materials undergo solid-solid phase transformations. Examples range from ferroelectrics (transducers and memory devices), zirconia (Thermal Barrier Coatings) to nickel superalloys and (lithium) iron phosphate (Li-ion batteries). These transformations involve a change in the crystal structure either through diffusion of species or local rearrangement of atoms. This change of crystal structure leads to a macroscopic change of shape or volume or both and results in internal stresses during the transformation. In certain situations this stress field gives rise to cracks (tin, iron phosphate etc.) which continue to propagate as the transformation front traverses the material. In other materials the transformation modifies the stress field around cracks and effects crack growth behavior (zirconia, ferroelectrics). These observations serve as our motivation to study cracks in solids undergoing phase transformations. Understanding these effects will help in improving the mechanical reliability of the devices employing these materials.

In this thesis we present work on two problems concerning the interplay between cracks and phase transformations. First, we consider the directional growth of a set of parallel edge cracks due to a solid-solid transformation. We conclude from our analysis that phase transformations can lead to formation of parallel edge cracks when the transformation strain satisfies certain conditions and the resulting cracks grow all the way till their tips cross over the phase boundary. Moreover the cracks continue to grow as the phase boundary traverses into the interior of the body at a uniform spacing without any instabilities. There exists an optimal value for the spacing between the cracks. We ascertain these conclusion by performing numerical simulations using finite elements.

Second, we model the effect of the semiconducting nature and dopants on cracks in ferroelectric perovskite materials, particularly barium titanate. Traditional approaches to model fracture in these materials have treated them as insulators. In reality, they are wide bandgap semiconductors with oxygen vacancies and trace impurities acting as dopants. We incorporate the space charge arising due the semiconducting effect and dopant ionization in a phase field model for the ferroelectric. We derive the governing equations by invoking the dissipation inequality over a ferroelectric domain containing a crack. This approach also yields the driving force acting on the crack. Our phase field simulations of polarization domain evolution around a crack show the accumulation of electronic charge on the crack surface making it more permeable than was previously believed so, as seen in recent experiments. We also discuss the effect the space charge has on domain formation and the crack driving force.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:fracture, phase transformation, finite elements, phase field model
Degree Grantor:California Institute of Technology
Division:Engineering and Applied Science
Major Option:Mechanical Engineering
Awards:Charles D. Babcock award for contributions to teaching from GALCIT
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Bhattacharya, Kaushik
Thesis Committee:
  • Ortiz, Michael (chair)
  • Ravichandran, Guruswami
  • Kochmann, Dennis M.
  • Bhattacharya, Kaushik
Defense Date:20 May 2013
Non-Caltech Author Email:prasad.bharat (AT) gmail.com
Funders:
Funding AgencyGrant Number
Army Research LabUNSPECIFIED
Keck Institute for Space StudiesUNSPECIFIED
Record Number:CaltechTHESIS:05302013-233635296
Persistent URL:http://resolver.caltech.edu/CaltechTHESIS:05302013-233635296
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:7785
Collection:CaltechTHESIS
Deposited By: Bharat Penmecha
Deposited On:07 Jun 2013 16:36
Last Modified:12 May 2015 18:43

Thesis Files

[img]
Preview
PDF - Final Version
See Usage Policy.

14Mb

Repository Staff Only: item control page