The Mechanical Properties Of Carbon Nanotubes And Their Composites

Owing to their unique and superior physical and mechanical properties, Carbon nanotubes (CNTs) hold a great promise of extensive applications in various fields. Among others, CNTs have been thought as a very promising candidate as the ideal reinforcing fibers for advanced composites with high strength and low density. In the present thesis, the fracture and buckling behaviors of CNTs as well as the effective elastic property of CNT-reinforced composites were investigated theoretically by using the methods of micromechanics and multiscale mechanics.First, some main factors that influence the stiffening effect of carbon nanotubes were analyzed quantitatively, including the effects of waviness, agglomeration and interphase. Correspondingly, three micromechanics models were established, that is, the waviness model of spiral shaped CNTs, the agglomeration model of effective inclusions, and the effective interphase model. It was found that the waviness and agglomeration of CNTs have significant influences on the properties of CNT-reinforced composites, while the influence of the interphase properties is relatively weak.We developed a hybrid quasi-continuum/atomistic model to study the Stone-Wales defect nucleation and fracture problems of CNTs. In this method, the atomistic method was used to consider the local region around the defect nucleation and fracture position, while the atoms far away from the defect were characterized by a quasi-continuum theory based on the modified Cauchy-Born rule. The critical strains of defect formation and fracture occurrence of CNTs with different diameters and chiral angles were obtained.Based on the above model, a multiscale mechanics method was developed to simulate the deformation and fracture behaviors of CNTs in composites. The unit cell containing a CNT embedded in a matrix was divided in three regions, which were simulated by the atomic-potential method, the quasi-continuum method based on the modified Cauchy-Born rule, and the classical continuum mechanics, respectively.The effect of CNT interaction was taken into account via the Mori-Tanaka effective field method of micromechanics. The critical strains of Stone-Wales transformation and fracture of CNTs embedded in a composite were determined, and the effect of residual strains was also examined.The axial buckling problem of CNTs of different diameters and chiral angles were considered finally. Using the method of structural mechanics, a CNT was modeled as a three-dimensional frame. The buckling forces were given analytically for armchair and zigzag CNTs. Furthermore, the influence of internal pressure on the critical load of buckling was analyzed using the finite element method, and the obtained results were compared with those of molecular dynamics simulations.