Multiscale Mechanics of Failure in Extreme Environments

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Date
2014-12-29
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Johns Hopkins University
Abstract
Over the past five decades there has been an intense effort to understand and control the thermomechanical response of materials in extreme environments. A number of technologies and applications critical to our safety and well-being stand to benefit from such understanding, including inertial confinement fusion, nuclear stockpile reliability, defense systems, spacecraft and hypersonic aircraft shielding, as well as vehicular crashworthiness. Materials in such extreme environments often exhibit complex, somewhat non-intuitive behavior that is difficult to predict with empirical or phenomenological models. As such, there has been an increasing effort to understand the microscale processes that govern the macroscale response. Here we provide a contribution to this effort through the development of a number of multiscale mechanism-based models that explore the fundamental nature of various microscale processes governing the macroscale thermomechanical response of materials in extreme environments. The extreme environments of interest here may include pressures on the order of the bulk modulus, shear stresses near the ideal strength, temperatures approaching melting, and timescales ranging from nanoseconds to the age of our Solar System (~5 billion years). We focus on materials in two particular extreme environments in this thesis. First and foremost, we explore the behavior of metals subject to very high rate deformation. Second, we study the behavior of planetary materials subject to the extreme thermomechanical environments in our Solar System. One of the main themes presented in the thesis is that the time-dependent failure of materials is governed, in part, by the kinetics of a hierarchy of microscopic material defects. Furthermore, the kinetics of one particular defect are often governed by lower length-scale defects. Examples of this are provided for twin boundary propagation at high loading rates, dynamic void growth in ductile materials, and fatigue crack growth in quasi-brittle asteroidal materials. A second theme is that simple mechanism-based models are powerful and instructive, particularly when it comes to building an intuition for dynamic failure processes. We make use of such simple scaling laws to help establish a deeper understanding of dynamic ductile failure of metals. We particularly focus on understanding how the rate-sensitivity of spall strength depends on a competition between the pre-existing material microstructure (e.g. second-phase particles and grain boundaries) and the shock-induced microstructure.
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Keywords
Multiscale, micromechanics, molecular dynamics, shock, impact, extreme, spall, dynamic, ductile, failure, void, vacancy, dislocation, emission, nucleation, drag, kinetics, clustering, microstructure, twin, grain boundary, Hall-Petch, strain-rate, thermal, fatigue, regolith, chondrule, asteroid
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