Study On Mechanical Properties Of Cnt Composites Based On Cohesive Zone Model

Carbon nanotube(CNT) has wide application because of its wonderful physicaland mechanical property. It has been thought as a promising candidate as the idealreinforcing fibers for advanced composite with high strength and toughness. However,the poor interfacial bonding between CNTs and polymer has been hindering thecomposites to fully achieve the desired mechanical properties. Research on theinterface of the CNT composites will necessarily promote the application of CNTcomposites.Due to the multi-scale features of CNT composites and the limitations ofexperimental technology, investigation on their interfaces in aspect of modeling andnumerical simulation is necessary. In this thesis, cohesive zone model (CZM) and thehomogenization method were employed to study cohesive force of the interface phase.Then, mechanical properties of the CNT composite as well as damage of theirinterfaces were predicted. In addition, the effective mechanical properties andstrengthening mechanism of CNT composites were obtained. The contents of thisthesis can be divided into three parts:(1) The transverse mechanical model of the CNT composites reinforced bycontinuous CNT was established, and the mechanical properties and damagemechanism of the interface was analyzed at the microscopic scale. The longitudinalmodels of the CNT composites reinforced by discontinuous CNT were establishedunder different arrangement. The results show that the damage initiation anddebonding process can be influenced by different arrangement of CNT in a greatextend.(2) The stress-strain curve was employed to analyze the role of interfacialmechanical properties in the composite performance. By changing the interfaceparameters such as cohesive strength and cohesive energy, it’s found that the cohesivestrength decides the strength of the composite, and the cohesive energy decides theductile-brittle performance of the composite material. Under the same volume fractionratio and aspect ratio, the performance of the composite can be impacted by thearrangement of discontinuous CNT significantly, composite with staggeredarrangement of CNT has the best enhancement.(3) the arbitrary crack initiation and propagation in matrix or the interface wassimulated along random path, and the zero thickness cohesive elements were insertedinto all boundaries of the entity element. the different composite failure modes withdifferent interfacial strength were analyzed. It is shown that when the interfacestrength is relatively high, the damage and destruction occurs at the matrix position close to CNT ends, and the crack direction is vertical to the stretching direction. onthe contrary, when the interface strength is relatively weak, the injury appears firstlyat the end of the interface, then the crack was expanded to the matrix body and thetangential interface, and finally the composites broke along the interfacecross-sectional. the mechanical properties of the composites containing the initialmicro crack were analyzed. The results demonstrated that the micro crack located atthe region without enhancement, and perpendicular to the stretching direction is mostdangerous.This work can help to understand the reinforcement and failure mechanism ofCNT composites, which is significant in the design and performance analysis of CNTcomposites.