UCLA

Currently, the advancement of nanotechnology provides new insights into the structures with the characteristic length in the nanometer scale. Due to their large specific area, the nano-structures may possess desirable features. Therefore, scientists attempt to employ the nano-structures as the reinforcements in the composite materials; i.e., nanocomposites, to achieve improved properties. It is well known that the local atomic environment at the matrix-reinforcement interface is different from its setting associated with the interior due to the accommodation of two different materials. As a consequence, the free energy associated with the interface is different from that associated with the interior. Since the nanocomposites have much larger interface area than the traditional composites, the interface energy effect becomes one of the main factors that determine the mechanical properties. The objective of the present study is to research on the effective (overall) elastic, elastoplastic and elastoplastic damage behavior of nanocomposites by considering the interface energy effect. Firstly, a nanomechanical framework is proposed in Chapter 3 to investigate the effective elastic behavior of nanocomposites containing randomly distributed spherical particles. The interface energy effect is simulated by the zero-thickness membrane interphase between the matrix and the reinforcement together with the interface stress. In addition, classical micromechanical homogenization procedures are incorporated to determine the volume averaged effective properties. Secondly, the elastic nanomechanical framework in Chapter 3 is extended to consider the more sophisticated spheroidal particle reinforced nanocomposites in Chapter 4. The spheroidal particles are assumed to be aligned and randomly distributed in the matrix. Thirdly, the effective elastoplastic behavior of the spherical particle reinforced

nanocomposite is studied in Chapter 5. The effective secant moduli are obtained for the nanocomposite with the elastoplastic matrix and the elastic reinforcements. In Chapter 6, the elastoplastic damage performance of the continuous fiber reinforced nanocomposites is

investigated. Interface debonding is considered as the damage type that occurs in the nanocomposites. The progressive debonding of the interface and the volume fraction evolution of debonded fibers are presented. The results show that the effective mechanical properties of nanocomposites are dependent upon the total interface area. The interface energy effect increases with the rising total interface area in the composite and becomes negligible when the dimensions of the reinforcements are in micrometer scale. Further, classical micromechanical solutions can be obtained when the interface energy effect is neglected.