MULTIWALLED CARBON NANOTUBE COMPOSITES AND THEIR MECHANICAL BEHAVIOR

Abstract

After the discovery of carbon nanotubes (CNTs) by Iijima (1991), they have attracted the attention of scientists for their application in various technical fields. CNTs are finding increasing applications in the field of nanocomposites because of their low density, high modulus, resilience and strength. Previous studies report that the addition of CNTs in matrix significantly increases the strength of composite. The reinforcement effect of CNT on matrix depends not only by their content within the hosting system but also by the level of dispersion, alignment of fibres and load transfer between fibre and matrix. CNT can be visualized as a graphene sheet that has been rolled into a cylindrical tube. CNTs exist as either single walled or multiwalled structures. Multiwalled carbon nanotubes (MWCNTs) are simply composed of two or more than two concentric single walled carbon nanotubes (SWCNTs). The mechanical behavior of CNT based composites is needed to be studied to explore their application in various fields. In this work, MWCNTs are used as fillers in nanocomposite to determine their mechanical behavior. From the available literature, it is expected that the presence of inner walls in MWCNTs enhance their bending, buckling and compression properties than SWCNTs. It is also expected that they can disperse easily than SWCNTs. Incorporating MWCNTs in matrix can enhance their mechanical properties for various applications. First, the extensive review of the literatures related to background of both SWCNT and MWCNT based composites and their modeling and simulation methodologies are performed. Finite element model of MWCNT composite is generated using continuum mechanics approach. A three dimensional representative volume element is constructed which can determine the load transfer mechanism, nanotube- matrix interaction and the effect of boundary conditions on mechanical behavior of nanocomposite. The mechanical properties of MWCNT based composites not only depend on the load transfer between matrix and MWCNT but also on the load transfer between the walls of MWCNTs. Therefore the inter-wall interaction between the walls of MWCNT and also between matrix and outer wall of MWCNT are investigated. The inter-wall interactions in MWCNT are described using van der Waal forces. The continuum mechanics based analytical formulations are used to validate the continuum approach based FEM simulation model. To study the properties of MWCNT composite, it is important to understand the structure and morphology of MWCNT. The structure and morphology of MWCNT include the diameter of MWCNT, number of walls in MWCNT, waviness of MWCNT, the interlayer van der waals forces between the walls of MWCNT, chirality of MWCNT and length of MWCNT. MWCNTs can appear in straight or wavy form, when incorporated in matrix affects the reinforcement of ii nanocomposite. Therefore the effect of degree of waviness of MWCNT on elastic response of composite is investigated. For understanding the effect of chirality, the atomistic structure of MWCNT embedded in matrix is studied. Multiscale modeling approach is used in which, MWCNT and interface are modeled at nanoscale, while the matrix material is treated at microscale. Overall load transfer in MWCNT composite also depends on the load transfer between the walls of MWCNT. Therefore the number of walls of CNT in MWCNT also determines the overall elastic response of nanocomposite. Besides structure of MWCNT, the reinforcement capabilities of MWCNT in composite also depend upon interfacial bonding between MWCNT and matrix, alignment and orientation of MWCNT in matrix. Therefore the effect of all these factors is also investigated. Through proper alignment of MWCNT layers in matrix the structural properties of composite can be optimized in different direction. Sandwich approach is used for the alignment of MWCNTs in form of layers in composite. Axial, lateral, vertical alignment of MWCNT layers is performed to enhance the strength of nanocomposite in various directions. Effect of inclination of MWCNT on transverse and longitudinal elastic property of composite is also investigated. Variation in inclination of MWCNT is considered from 0° to 90°. Vibration characteristics of MWCNT reinforced composites are important and needed to be studied for various structural applications. The application of MWCNT reinforced composites for adhesive bonding in pipes in the offshore oil and gas industry is investigated. The parameter used for examining the strength and reliability of the joints are Von mises stress and Shear stress distribution along adhesive thickness. To understand the factors affecting the adhesive joint, tensile and torsional loading conditions are considered. It is observed that the interphase serves as a buffer between MWCNT and composite. Its properties are critical to overall composite performance. MWCNT based composites provide better value of stiffness in compressive loading as compared to tensile loading. This can be explained as, during tensile loading, slippage occurs between shells of MWCNT due to which the load is not transferred to the inner layers. But in compressive loading, load is transferred to inner layers of the MWCNT through easy buckling and through the bent sections of the nanotubes. The proposed simulation approaches are found to be efficient in determining the mechanical behavior of MWCNT based composite and their application in epoxy adhesives