无结构细长时六方层状晶体材料的反常弹性屈曲
Anomalous elastic buckling of hexagonal layered crystalline materials in the absence of structure slenderness
摘要：
异质层状晶体材料，如石墨烯、氮化硼、硫酸钨等，由于其独特的原子结构和优异的热、机械和物理性能的组合，引起了人们的极大关注。利用机械屈曲是一种很有前途的途径来控制其结构形态，从而调整其物理性能，从而产生许多新的应用。在本文中，我们采用有限元分析（FEA），分子动力学（MD）模拟和连续介质建模研究的机械屈曲的层状晶体材料与晶体层平行的纵向轴线制成的列。结果表明，随着长细比的减小，材料的力学失稳模式由弯曲失稳模式逐渐过渡到剪切失稳模式。当长细比趋于零时，临界屈曲应变εcr收敛到一个远小于材料机械强度的有限值，表明在适当的实验条件下是可以实现的。这种机械屈曲模式是反常的和反直觉的。对于一组典型的六方层状晶体材料，连续介质力学模型预测的临界屈曲应变εcr与有限元和分子动力学模拟的结果吻合得很好。对石墨的MD模拟表明，连续介质力学模型适用于20 nm的尺度。该理论模型还揭示了高度的弹性各向异性是在没有结构细长的情况下由层状晶体材料制成的柱的异常机械屈曲的起源。这项研究提供了工程层状晶体材料在各种纳米材料和纳米器件通过机械屈曲的途径。
英文摘要：
Hexagonal layered crystalline materials, such as graphene, boron nitride, tungsten sulfate, and so on, have attracted enormous attentions, due to their unique combination of atomistic structures and superior thermal, mechanical, and physical properties. Making use of mechanical buckling is a promising route to control their structural morphology and thus tune their physical properties, giving rise to many novel applications. In this paper, we employ finite element analysis (FEA), molecular dynamic (MD) simulations and continuum modeling to study the mechanical buckling of a column made of layered crystalline materials with the crystal layers parallel to the longitudinal axis. It is found that the mechanical buckling exhibits a gradual transition from a bending mode to a shear mode of instability with the reduction of slenderness ratio. As the slenderness ratio approaches to zero, the critical buckling strain εcr converges to a finite value that is much smaller than the material's mechanical strength, indicating that it is realizable under appropriate experimental conditions. Such a mechanical buckling mode is anomalous and counter-intuitive. The critical buckling strain εcr predicted by our continuum mechanics model agrees very well with the results from the FEA and MD simulations for a group of typical hexagonal layered crystalline materials. MD simulations on graphite indicate the continuum mechanics model is applicable down to a scale of 20 nm. This theoretical model also reveals that a high degree of elastic anisotropy is the origin for the anomalous mechanical buckling of a column made of layered crystalline materials in the absence of structural slenderness. This study provides avenues for engineering layered crystalline materials in various nano-materials and nano-devices via mechanical buckling.
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