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dc.contributor.advisorSidney Yip and Krystyn Van Vliet.en_US
dc.contributor.authorSilva, Emílio César Cavalcante Melo daen_US
dc.contributor.otherMassachusetts Institute of Technology. Dept. of Civil and Environmental Engineering.en_US
dc.date.accessioned2008-09-03T14:59:22Z
dc.date.available2008-09-03T14:59:22Z
dc.date.copyright2007en_US
dc.date.issued2007en_US
dc.identifier.urihttp://hdl.handle.net/1721.1/42221
dc.descriptionThesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 2007.en_US
dc.descriptionIncludes bibliographical references (leaves 95-103).en_US
dc.description.abstractWe elucidate the tensile failure mechanism of amorphous silica and the effects of water on the process, combining: (a) atomic force microscope (AFM) bending tests, (b) molecular dynamics (MD) simulation and (c) molecular orbital (MO) simulation. Bending tests of silica nanowires provide validation for the predictions of the simulations, in which we study the failure of dry silica using MD and define a representative system to be studied with the more chemically accurate MO method. We used the AFM to perform bending tests on silica nanowires of diameter D < 1 [mu]m, which have very high surface-to-volume ratio and no microscopic flaws. No size effects on elastic modulus were observed down to 130 nm. For 500 nm wires, water reduces the strength from 10.5 GPa in air to 6.5 GPa in water, results comparable to those reported for micrometer-scale fibers. By probing the strength of silica at this scale, we bring experiments to the length scales accessible to atomistic simulation. Using classical MD, we found that crystalline silica fails globally by crack nucleation, but amorphous silica displays plastic deformation due to the formation of local defects, which cascade into larger compound defects. We extend to amorphous systems the instability criterion for material failure and use the Lanczos iteration method to isolate unstable modes. Failure of these modes create local defects, which are used to define a simpler representative system. We studied the water effect on these defects using a semi-empirical MO method, showing first that a water dimer is sufficient to lower the strength of a single Si-O-Si bond. Next, we use a representative system to describe the failure mechanism near instability.en_US
dc.description.abstract(cont.) We found that water reduces the tensile strength by both reducing the athermal failure strain and the energy barrier for failure. In summary, we demonstrate experimentally that the tensile strength of amorphous silica is governed by the nanoscale crack initiation event, after which the system fails in a brittle manner. Using a multiscale approach, we describe the nanoscale mechanism through MD simulation and the effect of water through MO simulation, bridging the gap between breaking a single bond and breaking a macroscopic body in tension.en_US
dc.description.statementofresponsibilityby Emílio César Cavalcante Melo da Silva.en_US
dc.format.extent114 leavesen_US
dc.language.isoengen_US
dc.publisherMassachusetts Institute of Technologyen_US
dc.rightsM.I.T. theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission. See provided URL for inquiries about permission.en_US
dc.rights.urihttp://dspace.mit.edu/handle/1721.1/7582en_US
dc.subjectCivil and Environmental Engineering.en_US
dc.titleEffects of water on chemomechanical instabilities in amorphous silica : nanoscale experiments and molecular simulationen_US
dc.typeThesisen_US
dc.description.degreePh.D.en_US
dc.contributor.departmentMassachusetts Institute of Technology. Dept. of Civil and Environmental Engineering.en_US
dc.contributor.departmentMassachusetts Institute of Technology. Department of Civil and Environmental Engineering
dc.identifier.oclc230946709en_US


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