Carbon Nanotubes (CNTs) are nanomaterials that have spectacular machanical, physical and thermal properties. From machanical point of view, they are the stiffest known materials. Nanotubes stiffness is several times of steel stiffness, while their weight is only one sixth of the steel weight. In addition, nanotube electrical conductivity is several times greater than the best conductors. Their magnetic sucebtibility is greater than many other materials. With increasing or decreasing temperature, they have a similar behavior to Silisum. They are stable up to 3000 o C in vacuum. To use them in a wide range, it is necessary to study and predict their behavior under different situation and loadings. According to theoretical studies, using numerical simulations, nanotube properties can be estimated. One of the methods that is largely used in modeling nanomaterials, is Molecular Dynamic. Altough molecular dynamic is a powerful toll for modeling nanomaterials behavior, due to high computatiomal operations, it has some limitations in modeling materials.Many researchers are investigating about new methods with high precision and low computational operations. In this research, using a model based on structural mechanics and finite lement methods, the behavior of CNTs is proposed. This method is based on equivalent molecular potential and strain energies to compute constants of elements. Then the behavior of CNTs under tensional and torsional loads at different temperatures for undefected and defected structures. In addition, vibration of clamped CNTs at different temperatures is modeled. It is illustrated that elastic and torsional modules of CNTs are in TPa and GPa range, respectively. Defects in CNTs structure decrease these values. In this research the effect of vacancy defect and 5-7-7-5 defect are studied. Results indicate that vacancy defect has more effect on CNTs properties. By increasing temperature, due to weakening bonds between carbon atoms, the elastic constants of CNTs are decreased. Simulation on buckling of CNTs, under compressive load, shows that by increasing aspect ratio, the critical load is decreased. Results show that y increasing aspect ratio of CNTs, the fundamental frequency of clamped CNTs decreases. In addition, it is illustrated that the chirality of CNT has no effect on their fundamental frequency. Defected carbon nanotubes have a lower frequency than undefected carbon nanotubes.
