Doctoral Dissertations

Date of Award

12-2014

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Materials Science and Engineering

Major Professor

Yanfei Gao

Committee Members

Hongbin Bei, George Pharr, Timothy Truster

Abstract

This dissertation further extends nanoindentation to study the initiation of plasticity in single crystals in nanoscale stressed volumes, to the instrumented tests for damage evaluation and monitoring, and to the fundamental issues in deformation and failure mechanisms in relationship to the defect evolutions.

In the first project, model Fe-Cr, Fe-Ni and Fe-Cr-Ni alloys that are the basis of many structural steels were synthesized as single crystals and characterized. The compositions investigated were Fe-15Cr, Fe-30Cr, Fe-30Ni and Fe-15Cr-15Ni (atomic percent). Several key mechanical properties were determined which will be useful in further studies of irradiation/deformation-induced defects. Incipient plasticity and slip characteristics were investigated by nanoindentation on (001) and surfaces. Finally, the effects of heterogeneous pop-in mechanisms are discussed in the context of incipient plasticity in the four different alloys.

Moreover, the pop-in event mode and pop-in excursion are investigated. In previous literature, there are two kinds of pop-in mode: a single large displacement burst and multiple successive pop-ins. The size and microstructure effect are discussed for the two modes showing that multiple successive pop-ins is more likely to be appeared in fcc crystal with a smaller indenter tip. Also an analytical model is established to predict size effect in pop-in excursion for one large pop-in mode. The effect of the dislocation nucleation mechanism is discussed for the pop-in excursion.

In the last project, the pinch-torsion test is designed to evaluate Li-ion cell safety. The failure mechanism of pinch-torsion test is examined by numerical simulations and comparisons to experimental observations. Finite element models are developed to evaluate the deformation of the separators under both pure pinch and pinch-torsion loading conditions. It is discovered that the addition of the torsion component significantly increased the maximum principal strain, which is believed to induce the internal short circuit. It is further found that the separator failure is achieved in the early stage of torsion (within a few degree of rotation).

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