Mechanics of Nanoropes

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Nanorope Mechanics, Rod Ruoff and Wing Kam Liu, Northwestern UniversityCarbon nanotubes, CNTs, are a multifunctional material that may find applications as a reinforcing component in a variety of composites, where the matrix could be polymer, ceramic, or a metal, including ductile metals such as aluminum. Additionally, CNT's may be applied as a new type of cable material that would exploit the high stiffness and potentially the high strength. There are two types of carbon nanotubes, the "single walled carbon nanotubes," SWCNTs, and the "multiwalled carbon nanotubes," MWCNTs. This grant from the National Science Foundation addresses the mechanics of SWCNT bundles with a combined experimental and modeling effort. In particular, the goal of this project is to develop a detailed understanding of the mechanics of both parallel and twisted SWCNT bundles. Inspiration for this effort comes from the rather well established fields of twisted wire, and textile, mechanics. These disciplines have treated the mechanics of twisted wire structures, or textile fabrics, typically by continuum mechanics. One may expect that twisting a bundle, or achieving a "woven" bundle, of SWCNTs will enhance the load bearing capacity of the SWCNT "rope." The extent of load transfer between individual tubes in the bundle is a crucial aspect of their potential application in structural applications in either composites, as cabling, and even for example, as windings in electromagnets. Our experimental effort involves the use of a nanomanipulator/testing stage in which we will pick up SWCNT bundles, mount them for tensile loading, and apply twists with a component of this testing stage, which can undergo 1800 individual steps per 360 degree revolution, and can continue to "wind up" a SWCNT bundle through n turns. The stiffness as a function of applied twist, and also the bundle strength as a function of applied twist, will be studied with this tool, which has been previously used to study the tensile loading of individual MWCNT's and of untwisted SWCNT bundles. Our modeling effort involves using a variety of approaches, including molecular dynamics (MD), molecular mechanics (MM), and continuum mechanics, to study such issues as load transfer as a function of both twist and contact length, for both idealized bundles (for example, where every tube in the bundle is identical, such as all tubes being (10,10) tubes with perfect closest-packing) and bundles that might more closely mimic those actually tested in experiment, such as having different diameter tubes in the bundle, without perfect closest packing. There is a close collaboration between the groups doing theory and experiment, and each effort is meant in part to guide the other, and to provide deeper overall understanding.