Mechanical behavior of a carbon nanotube turf

by 1981- Radhakrishnan, Harish

by Harish Radhakrishnan, M.S. Washington State University December 2006 Chair: Sinisa Dj. Mesarovic Carbon nanotubes grown on a substrate form a turf – a complex structure of intertwined, mostly nominally vertical tubes, cross-linked by adhesive contact and a few bracing tubes. The mechanical behavior of such a turf is analyzed and the physical mechanism of deformation deduced on the basis of (a) experimental results – standard and continuous stiffness nanoindentation tests and (b) micromechanical scaling analysis. The objective here is to develop a phenomenological model to simulate the behavior of the turf in nanoindentation tests, taking into account the various features observed during the experiments. Under moderate strains and deformations, the turf is fully reversible (non-linear elastic response) with a small Kelvin-Voigt type relaxation. Also observed in the experiments is the strong adhesion between the turf and indenter tip – due to the van der Waals forces of attraction. A finite element model has been developed and is able to reproduce the results from the indentation tests with reasonable accuracy. iv While the adhesive contact in the indentation of linear elastic solids is thought to be well understood through analytical models, a finite element model for the problem is yet to be developed. To benchmark the developed contact algorithm, runs were done to compare the results with existing theories of adhesion. The comparison revealed an unanticipated deviation from the existing models and is thought to be due to the finite deformation in the elastic material, not considered by the existing theories. v