Local and non-local behavior and coordinated buckling of CNT turfs

The mechanical properties of arrays of curved, intertwined, but nominally vertical carbon nanotubes (CNTs), referred to as turfs, have been measured using nanoindentation. The elastic properties appear to be non-linear; as noted in prior studies the observed tangent modulus decreases with increasing strain. Decreasing adhesion between the turf and probe lowers the perceived stiffness of the material. The elastic properties do not vary significantly between the top and the bottom of a particular carbon nanotube turf; both ends of the turf exhibit an effective modulus on the order of 50 MPa when several cubic microns of material are tested. Within a single turf the spatial variation in elastic properties is less than 10%, turfs from different growth runs can vary by up to 50%. These observations, in conjunction with in situ compression tests of turfs that buckle near the base rather than in the center of the structure (as would be found during Euler buckling), justifies the use of average mechanical properties for a given vertically aligned turf for design purposes without the need to account for spatial variation in structure. A turf's mechanical properties may depend on imposed strain, and may exhibit local buckling without a gradient in structure. (C) 2010 Elsevier Ltd. All rights reserved.