Uncertainty Quantification of Microstructural Properties due to Experimental Variations

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/143074/1/1.J055689.pd

Stochastic Design Optimization of Microstructures with Utilization of a Linear Solver

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/143035/1/6.2017-1939.pd

Uncertainty Quantification of Microstructural Properties due to Experimental Variations

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/143052/1/6.2017-0815.pd

Fiber Path Optimization of Symmetric Laminates with Cutouts for Thermal Buckling

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/143111/1/1.C033866.pd

Simulation Model and Analysis of a Data Warehouse

Data warehouses are large complex systems with many interacting nonlinear components. It is an amalgamation of many different systems and integration of such diverse elements is its primary concern. It is often difficult for a data warehouse manager to predict the performance of the system especially when demand pattern for the required data keeps changing. We have developed a simulation model using ARENA simulation package that will simulate the behavior and performance of a data warehouse system environment based on its overall design. Given such a model, a data warehouse manager can walk through various what-if scenarios and can pinpoint the areas of weaknesses in the system. This visibility could result in improved operational performance in a data warehouse

Investigation of Alkali metal embrittlement of Aluminum Lithium alloys using first principles calculations and dislocation theory

Segregation of alkaline earth metals at grain boundaries is investigated as a cause of grain boundary embrittlement in Aluminum alloys. Auger spectroscopy shows that grain boundary segregation of Na occurs in an Al­Li alloys. In addition, Aluminum extracted from bauxite bring in alkali impurities. First-principles simulation allows us to understand the energetics of Na segregation at grain boundaries and model the decreased grain boundary cohesive energy. Using this data within the concomitant dislocation theory based on our recent work, we study the effect of Na segregation on static and fatigue fracture of Al-Li alloys. Using DFT calculations, we describe how the presence of alkali impurities could possibly enhance hydrogen embrittlement

Microstructural crack path prediction using graph theory

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Predicting fatigue crack initiation in metals using dislocation dynamics simulations

The presented work aims at deepening the understanding of the initiation of fatigue cracks in metals. The work is based on an argument of Mura and Nakasone [1] where an energy criterion is used to predict the initiation of a fatigue crack from a slip band. The discussion is based on the evolution of dislocation networks, as these are the prime cause for permanent deformation in metals. Using high performance computing, 3D dislocation dynamics simulations [2] are performed over several cycles to study the growth of the dislocation arrangement. Then the evolution of energy in the system is determined, including all relevant terms such as: energy of the elastic field of the dislocations and their interaction, core energies, dissipation, energy stored in the continuum and external work. A hypothetical crack is placed in the region of the largest dislocation density and it is checked, if the energies stored in part of the dislocation network should be exchanged with the surface energy of a crack to lower the overall energy state of the system. The size of the crack is based on the number of dislocations that were previously formed in the network, as the motion of these dislocations towards the hypothetically formed free surface would form the actual crack. The presented work requires only minimal input in form of elastic constants and dislocation mobilities (for example from molecular dynamics). Results are presented for different materials (Cu and Mg), grain sizes and loading rates. [1] T. Mura, Y. Nakasone, A Theory of fatigue crack initiation in solids, Journal of Applied Mechanics, 57 (1990) [2] A. Arsenlis, W. Cai, M. Tang, M. Rhee, T. Oppelstrup, G. Hommes, T.G. Pierce, V.V. Bulatov, Enabling strain hardening simulations with dislocation dynamics, Modelling and Simulation in Materials Science and Engineering, 15 (2007

Study of Mechanical and Thermal Behavior of Polymeric Ablator Using MD

The thermal protection materials used for spacecraft heat shields are subjected to various thermal and mechanical loads during an atmospheric entry which can threaten the structural integrity of the system. This paper describes the development of a molecular dynamics approach to understand the mechanical and thermal behavior of high temperature polymers. One such material PICA has successfully flown on the Stardust spacecraft and is the TPS material chosen for the Mars Science Laboratory (MSL) and SpaceX Dragon spacecraft. Although such polymers have good structural properties at moderate temperature,they became structurally weak at extreme region of temperature and loads. In order to thoroughly understand the response of materials under extreme mechanical and thermal loads it is necessary to investigate atomistic mechanisms of deformation and pyrolysis. MD Simulations are presented to compute the thermal expansion coefficients, stress-strain response,to determine the pyrolysis gas composition entering the char layer from the virgin material, and to identify the main reaction pathways for the interaction between the pyrolysis gases at temperature varying from 500-1500K