Micromechanics Analysis Of Carbon Nanotube Reinforced Magnesium Matrix Composites

It is one of the future orientations of material science in the21st century for the design of overall properties of material according to the theory of micromechanics Building the connection between the efective properties and microstructure of composites is a key task to structural optimization and function design of composites. Methods for calculation of the effective properties of composites and their history, development and current application are reviewed.The stress field and the stress transfer efficiency of carbon nanotubes reinforced magnesium matrix (CNTs/Mg) composites were analyzed based on the shear-lag model by the Micromechanics. The interphase should be included in the three-phase concentric cylindrical shell model to analyze the stress of CNTs/Mg composites. Analysis showed that the effective load was transferred from the matrix to CNTs at the the embedded end of the CNTs by the interfacial shear stress, and the stress concentration of CNTs/Mg composites is Mainly distributed at the embedded end of CNTs. This result is consistent with the experimental results. An optimum the effective thickness of the interphase is observed to be approximately 9nm for the stress transfer efficiency of CNTs/Mg composites. The aspect ratio of CNTs influences significantly for the stress transfer efficiency of the CNTs/Mg composites, when the aspect ratio is small(βt<9).The effective Young's modulus of CNTs/Mg composites were analyzed based on the shear-lag model by the Micromechanics.The effects of the parameter on the effective Young's modulus of CNTs/Mg composite were investigated. The results show that the effect for the effective Young's modulus was larger when the interface Poisson's ratio and Young's modulu was smaller; the influence of the length and the volume fraction of CNTs is limited on the effective Young's modulus of CNTs/Mg composites; the effective Young's modulus of CNTs/Mg composites increases with increasing the interface thickness .A theoretical model on frictional pull-out properties of CNTs/Mg composite was presented to describe the interfacial elastic stress transfer, then the stress and strain of respective components of CNTs/Mg composite were derived. Including the Poisson's effect and friction stress at the debonded interface, and based on the energy equilibrium and the interphase strain criterion in the interfacial debonding process, an expression for the energy release rate and the crack opening displacement profile were derived.The effects of the parameter on fracture characteristics of CNTs/Mg composite were investigated by the relative energy release rate ratio and crack opening displacement profile. The results show that the effect for the interface debond characteristics was thickness was smaller; The more Young's modulus and the Poisson's ratio of interphase are, the larger the crack opening displacement profile becomes,which have the optimum effect for the relative energy release rate ratio.The stress of respective components in CNTs/Mg composites was examined according to the shear-lag model approach. Considering the variety strengthening mechanism of composite materials, The yield strength model was established to examine the influence of performance parameters of respective component on the yield strength of CNTs/Mg composites. The results show that the influence of the length of CNTs is limited on yield strength of CNTs/Mg composites; the more carbon nanotubes layers and the more sparsely distributed are, the less increase on yield strength of CNTs/Mg composites becomes; the yield strength of CNTs/Mg composites increases with increasing the temperature difference; There is the optimum of the volume fraction of CNTs on yield strength of CNTs/Mg composites exists. The values of yield strength of CNTs/Mg composites predicted by the model are well agreed with the experimental values.