Mechanisms of nanoindentation on single-walled carbon nanotubes: The effect of nanotube length
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The mechanisms of nanoindentation on single-walled carbon nanotubes (SWCNTs) have been studied by using molecular dynamics simulation and continuum analysis during which a flat layer of diamond atoms is pressed down incrementally on a vertically aligned SWCNT. SWCNTs are divided into three distinct categories based on their aspect ratios, such that the nanotube behavior transits from a shell (short tube) to a beam (long tube). Molecular dynamics simulations are used to explore the diverse indentation characteristics in each domain, where the relationships between the strain energy and indentation depth during loading, unloading, and reloading are continuously recorded. The nanoindentation mechanisms are characterized by the critical indentation depth, maximum strain energy and force associated with buckling, as well as with the evolution of carbon bond length and morphology of the SWCNTs. Bifurcation behaviors are explored by investigating the loading-unloading-reloading behaviors of the nanotubes. Parallel finite element simulations are also used to study the pre- and post-buckling behaviors of SWCNT by incorporating the van der Waals interaction into the continuum code. It is found that, for the most part, continuum analysis can effectively capture the overall indentation characteristics, yet some details related to the atomic characteristics of nanoindentation may only be revealed by molecular dynamics simulation. Finally, an indentation mechanism map is derived by comparing behaviors of SWCNTs with different aspect and section ratios. Focusing on the effects of nanotube length, this paper is the first of a series of numerical studies on the indentation mechanisms of carbon nanotubes, which may be used to determine the intrinsic mechanical properties of SWCNTs by means of nanoindentation.
I. INTRODUCTION A. Motivation
Since their discovery in 1991,1 carbon nanotubes (CNTs) have been the subject of intensive research. Single-walled carbon nanotubes (SWCNTs) have attracted particular attention because of a wide range of their potential applications. Among these applications are their uses: (i) as structural materials with extraordinary mechanical properties2; (ii) in nanoelectronic components3; (iii) as probes in scanning-probe microscopy4 with the added advantage of a chemically functionalized tip; (iv) as high-sensitivity microbalances5; (v) as gas detectors6; (vi) in hydrogen storage devices7; (vii) as field-emission type displays8; (viii) as electrodes in organic light-emitting diodes9 and (ix) as tiny tweezers for nanoscale manipulation.10 a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0128 1048 J. Mater. Res., Vol. 21, No. 4, Apr 2006 http://journals.cambridge.org Downloaded: 15 Mar 2015
The mechanical properties of the SWCNT must be fully understood to fulfill its promises. Perhaps the most fundamental phenomenological mechanical property of CNTs are its Young’s modulus when the CNT is modeled as a continuum hollow cylinder. A variety of experimenta
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