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On Determining Continuum Quantities of Non-Equilibrium Processes via Molecular Dynamics Simulations

Fu, Yao (2013) On Determining Continuum Quantities of Non-Equilibrium Processes via Molecular Dynamics Simulations. Doctoral Dissertation, University of Pittsburgh. (Unpublished)

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In this dissertation, a high-fidelity atomistic-to-continuum link for highly non-equilibrium processes has been established by making several modifications to Hardy’s theory. Although Hardy’s thermomechanical quantities were derived analytically to conserve mass, momentum and energy, they have not been rigorously tested and validated numerically in the past.
Hence the first task was to investigate the effectiveness of ensemble averaging in removing thermal fluctuations and compare with conventional time averaging for fcc crystals simulated using both equilibrium and non-equilibrium molecular dynamics (MD) simulations, where the non-equilibrium process was introduced by a shock impact. It has been found that the ensemble averaging has better convergence than time averaging due to the statistical independence of the thermomechanical quantities computed using ensemble averaging.
The second task was to test the validity of Hardy’s theory by checking if it is able to conserve mass, momentum and energy numerically. A few highly non-equilibrium processes were simulated using MD, including Gaussian wave and shock impact propagation in 1D and 3D fcc crystals. Based on the test results, a new normalization rule has been proposed so that the computed thermomechanical quantities can conserve the fundamental properties more accurately. To a large extent, Hardy’s theory has been found to be valid regardless of the width of the localization function, the interatomic potential and crystal structure, and with and without ensemble averaging.
To further test the validity of Hardy’s theory for more complex non-equilibrium processes, where plastic deformation is accomplished through dislocation glide and slip band emission, a crack propagation problem in iron crystal with a pre-created center crack is simulated using MD. The computed Hardy’s thermomechanical quantities can generally conserve mass, momentum and energy. Exceptions have been found around the crack region, where the computed quantities cannot obey the balance of energy but still conserve mass and momentum.
The results from this dissertation will help 1) inspire confidence in employing Hardy’s theory with the proposed modifications to analyze MD simulation results, especially for non-equilibrium thermomechanical processes and 2) pave the way for concurrent atomistic/continuum coupled simulations.


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Item Type: University of Pittsburgh ETD
Status: Unpublished
CreatorsEmailPitt UsernameORCID
Fu, Yaoyaf10@pitt.eduYAF10
ETD Committee:
TitleMemberEmail AddressPitt UsernameORCID
Committee ChairTo, Albertalbertto@pitt.eduALBERTTO
Committee MemberMao, Scottsxm2@pitt.eduSXM2
Committee MemberWeiland, Lisalmw36@pitt.eduLMW36
Committee MemberSlaughter, Williamwss@pitt.eduWSS
Committee MemberJordan, Kenneth D.jordan@pitt.eduJORDAN
Date: 25 September 2013
Date Type: Publication
Defense Date: 19 July 2013
Approval Date: 25 September 2013
Submission Date: 9 July 2013
Access Restriction: No restriction; Release the ETD for access worldwide immediately.
Number of Pages: 113
Institution: University of Pittsburgh
Schools and Programs: Swanson School of Engineering > Mechanical Engineering
Degree: PhD - Doctor of Philosophy
Thesis Type: Doctoral Dissertation
Refereed: Yes
Uncontrolled Keywords: non-equilibrium process; molecular dynamics; Hardy's formulas; thermomechanical quantities; atomistic-to-continuum; ensemble averaging
Date Deposited: 25 Sep 2013 15:39
Last Modified: 15 Nov 2016 14:14

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  • On Determining Continuum Quantities of Non-Equilibrium Processes via Molecular Dynamics Simulations. (deposited 25 Sep 2013 15:39) [Currently Displayed]


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