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Xiaodong Li's picture

Experimental Nanomechanics

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Colleagues and friends,

Professor

Department of Engineering Mechanics

Tsinghua University

Beijing 100084, China

Ting Zhu's picture

Linking Interfacial Plasticity to Ductility: A Modeling Framework for Nanostructured Metals

Ting Zhu, Ju Li, Amit Samanta, Hyoung Gyu Kim and Subra Suresh

Nano-twinned copper exhibits an unusual combination of ultrahigh strength and high ductility, along with increased strain-rate sensitivity. We develop a mechanistic framework for predicting the rate sensitivity and elucidating the origin of ductility in terms of the interactions of dislocations with interfaces. Using atomistic reaction pathway calculations, we show that twin boundary (TB) mediated slip transfer reactions are the rate-controlling mechanisms of plastic flow. We attribute the relatively high ductility of nano-twinned copper to the hardenability of TBs as they gradually lose coherency during deformation. These results offer new avenues for tailoring material interfaces for optimized properties.

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Xi Chen's picture

Why is molecular mechanics simulation at 0K useful?

Although it is more realistic to study the mechanical properties of nanostructures such as the carbon nanotubes (CNTs) at room temperature, atomistic simulations at finite temperature (such as molecular dynamics, MD) may cause the following problems: (1) Due to the limitation of the time scale achievable in MD (typically at the nanosecond scale), the loading rate in MD simulation at any finite temperature is not realistic. Very often, the loading rate used in MD simulations may well exceed 10m/s at 300K and thus many orders of magnitude higher than the real loading rate used in experiments. (2) A great advantage of simulation is to be able to turn on and turn off certain features and explore their effects, which is otherwise impossible in experiments. For example, the buckling behavior of CNTs is very sensitive to geometrical perturbations, which is prominent at room temperature and such perturbations causes severe uncertainties and makes it difficult to explore the intrinsic buckling behaviors. Therefore, by removing the temperature effect, we could better evaluate other key factors affecting the intrinsic buckling behavior, such as tube chirality, radius, and length, which could be otherwise covered by the thermal fluctuation effect. (3) Due to both time and length scale limitations, the MD simulations of large system are not yet possible, and thus the effective continuum models must be developed which need to be calibrated by atomistic simulations. At present, the temperature factor is still absent in most continuum models. Therefore, atomistic simulations at 0K or near 0K may provide a useful benchmark for the development of parallel continuum models, focusing on the most intrinsic and basic mechanical properties of nanostructures. Based on the above analysis, atomistic simulations at 0K by using the molecular mechanics (MM) method are still very useful, especially to us as mechanicians.

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