Failure mechanisms, nanotube

Ren Y, Fu YQ, Liao K, Li F, Cheng HM (2004). Fatigue failure mechanisms of single-walled carbon nanotube ropes embedded in epoxy. Appl. Phys. Lett. 84 2811-2813. 

As-synthesized MWCNT and SWCNT exist as bundles or ropes and tend to agglomerate due to strong van der Waals forces (13) (Figure 7.1). Unless the CNTs are separated in to individual tubes and dispersed in the polymer matrix, the interactions of the nanotubes with the polymer will be weak. The mechanical failure of such composites will occur due to slippage of the tubes in the bundle that are not bonded to the matrix. In addition, the aggregates or bundles reduce the aspect ratio of the reinforcement which affects electrical properties as well (15). Because of these factors the first step will be to open up these bundles to separate individual tubes by using different techniques to increase the volume of interface between the CNT and the matrix (40). 

Nanocomposites with carbon nanotubes have been an area of considerable R D ever since the excellent electrical and mechanical properties of carbon nanotubes were demonstrated. However, attempts to prepare carbon nanotube RPs often result in phase separation of the CNT and polymer phases causing premature material failure. Researchers at Nomadic Inc. and Oklahoma State University developed a layer-by-layer (LBL) assembly process that permits preparing polyelectrolyte/CNT RP with a CNT loading greater than 50 wt%. The excellent mechanical properties of these materials can be improved further by additional chemical action crosslinking of the CNT and polymer phases and by parallel alignment of the CNTs. The LBL method has been used to prepare various types of RPs. 

SWNTs are an important kind of carbon nanotube due to most of their properties change considerably with the (n, m) values, and according to Kataura plot, this dependence is unsteady. Mechanical properties of single SWNTs were predicted remarkable by Quantum mechanics calculations as Young s modulus of 0.64-1 TPa, Tensile Strength of 150-180 GPa, strain to failure of 5-30% while having a relatively low density of 1.4-1.6 g/cm. ... 

Furthermore, it is expected that the dispersion of fillers with dimensions in the nanometer level having very large aspect ratio and stiffness in a polymer matrix could lead to even higher mechanical performances. These fillers include layer silicates and carbon nanotubes. Carbon nanotubes have a substantially higher aspect ratio (-1000) in comparison with layered silicates (-200) and they also have very high strain to failure. 

Because of the covalent sp bonds between individual carbon atoms, a nanotube can have aYoung s modulus between 1.2-5.5 TPa, atensile strength about a hundred times greater than that of steel, and can tolerate large strains before mechanical failure. A CNT can be a metal, semiconductor, or small-gap... 

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