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Center for Nonlinear Studies |
I will present two studies of non-equilibrium behavior in complex materials. First, I will discuss our application of the recent mesoscale peridynamic theory of continuum mechanics to the dynamic propagation of a martensitic phase boundary. Peridynamics is a continuum formulation that works directly with the displacement field, providing some freedom from tracking discontinuities, as well as incorporating long-range effects that can be important at small scales. Our study of martensitic interfaces in the peridynamic framework suggests that the theory is closed with respect to kinetics and nucleation and does not require additional information as in classical continuum mechanics. In particular, kinetics can be understood in the framework of traveling waves, and nucleation as a dynamic linear instability. Further, the computational ease of the theory allows us to examine the interaction between a phase boundary and an inclusion. I will then discuss our use of the method of Objective Structures to the non-equilibrium deformation and failure of carbon nanotubes at imposed strain rates of 10^4/s - 10^8/s. Objective Structures generalizes the notion of crystallinity to important non-crystalline nanostructures such as carbon nanotubes. This allows the calculation of properties of these nanostructures through efficient techniques developed for crystals. Our simulations with different strain rates, temperatures and unit cells show that Stone-Wales defects do not play a significant role in failure (though partials are sometimes seen just prior to failure), a variety of failure mechanisms are observed, and most simulations give a strain at failure of 15--20%, except those done with initial temperature above about 1200 K and at the lower strain rates. The latter have a strain at failure of 1--2% We also observe unexpected effects of pre-tensioning the nanotubes.
Host: Turab Lookman, txl@lanl.gov