Page 48 - Fiber Fracture

MODELS OF FIBRE FRACTURE 33

Fig. 3. Schematic model for a single-wall carbon nanotube. Nanotube structures are obtained by rolling a
graphene sheet into a cylinder, so that the lattice points 0 and 0’ fold onto each other.

will provide an asymptotic value. This theoretical maximum strength will be achieved
only for chain molecules of infinite length and fully aligned. Models intended for
explaining experimental results should consider the presence of defects (the chain ends
act as defects), the disorientation of the chains in relation to the fibre axis, the role of
secondary bonds between chains, the non-uniformity of the structure in the cross-section
and along the fibre, and the presence of voids and impurities.

Strong Bonds in Two Dimensions: Carbon Nanotubes

Strong bonding (with covalent bonds) in two dimensions can be achieved in nanotube
fibres. An ideal carbon nanotube can be considered as a hexagonal network of carbon
atoms rolled up to make a seamless hollow cylinder (Fig. 3). These fibres can be tens of
microns long but with diameters of only about 1 nm and with each end capped with half
a fullerene molecule. Carbon nanotubes, formed of seamless cylindrical graphene layers,
represent the ideal carbon fibre and should exhibit outstanding mechanical properties.
Single-wall nanotubes, having a shell of only one atom in thickness, are considered