Abstract
We show the importance of incorporating material nonlinearity for accurate determination of spatial buckling of nanorods and nanotubes. Both the nanorods and nanotubes are modeled as a special Cosserat rod whose nonlinear material laws are obtained using the recently proposed helical Cauchy-Born rule. We first present Euler buckling of solid diamond nanorods whose normalized buckling load, obtained from fully atomistic calculations, exhibits an interesting trend. The buckling load starts from unity at large aspect ratio of the nanorod, then as the aspect ratio is decreased, the buckling load increases slowly and finally decreases rapidly. We attribute this trend to material nonlinearity of the nanorod’s core at large compressive strain. We also discuss how surface stress affects buckling in nanorods. We then present the effect of compression and twist on buckling of single-walled carbon nanotubes. Interestingly, for highly twisted nanotubes, fully atomistic calculations show the first buckled mode to be different from a typical Euler buckling mode. Both the observations about nanorods and nanotubes are accurately replicated in the finite element special Cosserat rod simulation when the material nonlinearity is also incorporated. However, the simulation results exhibit completely different trend when only linear material laws are incorporated.
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Notes
Throughout the paper, the differentiation of a field variable with respect to arc-length is denoted by a superscripted prime, i.e., \(\frac{d}{ds}\equiv(\cdot)^{ \prime}\).
The Einstein’s summation convention is implied unless stated. Repeated Latin indices always sum from 1 to 3 whereas repeated Greek indices sum from 1 to 2.
We refrain from calling the smallest repeating cell as “unit cell” since the nanorod does not retain translational periodicity when deformed. The generalization to the concept of fundamental domain was first proposed by James [18].
For detailed discussion on constraints, see the paragraph following Eq. (19) in Kumar et al. [22] and also Sect. 5.1 there which illustrates the role of constraints in imposing uniform bending-stretching deformation in a nanorod.
The stiffnesses have been normalized with their corresponding values in the stress-free state (at relaxation strain).
The “Young’s modulus” is defined as the second derivative of the diamond’s unit cell energy with respect to compression divided by the undeformed area of the unit cell’s face whose normal is along \([1\, 0\, 0]\) direction.
The stiffnesses in Fig. 9(a) are normalized by their continuum formulae for which we use bulk diamond’s Young’s and shear modulus in its stress-free state. Similarly the bulk elastic moduli are normalized by their respective values in the stress-free state.
Liang et al. [23] also reported the variation in the extensional stiffness for a copper nanowire to be largely due to nonlinear elasticity of the nanowire’s core.
The buckling load diagram similar to Fig. 11 were also presented in Fig. 9 in Kumar et al. [22]. The key differences here are: (i) the inclusion of buckling data corresponding to shell buckling mode to highlight the failure of rod model for hollow tubes at low aspect ratio and (ii) the aspect ratio being used here on the horizontal axis in order to facilitate its comparison with the solid diamond rod data from Fig. 9(b).
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Acknowledgements
P. Gupta acknowledges the financial support received from DST-INSPIRE fellowship and A. Kumar acknowledges the support from SERB, India through the grant YSS/2014/000023.
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Gupta, P., Kumar, A. Effect of Material Nonlinearity on Spatial Buckling of Nanorods and Nanotubes. J Elast 126, 155–171 (2017). https://doi.org/10.1007/s10659-016-9586-1
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DOI: https://doi.org/10.1007/s10659-016-9586-1