A COMPUTATIONAL STUDY OF MECHANICAL PROPERTIES OF CNT COMPOSITES FOR VARIOUS DEFECTS

Abstract

Carbon nanotubes (CNT) have unique mechanical properties that make them excellent reinforcing materials. Incorporating CNTs in matrices can potentially enhance the stiffness and strength of composites. Due to its extraordinary properties CNT has the potential to provide extremely strong and ultra light composite material. In this thesis work, the effective elastic material properties of carbon nanotube reinforced composites are evaluated with single CNT based model using hexagonal representative volume element under axial and lateral loading conditions. Numerical equations are used to evaluate the effective material properties. An extended rule of mixtures is applied for validation of the proposed model. It has been observed that the addition of the CNTs in a matrix at volume fractions of only about 3.6%, the stiffness of the composite is increased by 33% for long CNT at Et/E`° = 10. The results indicate that the short CNTs in a matrix may not be as effective as the long CNTs in reinforcing a composite. The better value of stiffness in axial direction is found as compared to stiffness in lateral direction for long CNTs. Also, the multiple long and short CNTs are used to evaluate the effective material properties of CNT composites. Aligned and random orientation of multiple short CNTs have also been considered for analysis. Models with multiple CNTs are found to be more efficient as compared to models with single CNTs. Young's modulus of composite increases with an increase in length and volume fraction of CNT. Aligned CNTs provide excellent Young's modulus as compared to randomly oriented CNTs. Finally, considering the defects in carbon nanotubes, analysis of crack propagation in CNT composites has been done using XFEM. The fracture parameters like stress intensity factor (SIF) and J-Integral are calculated using contour integral method. Results show that the highest propensity of crack propagation is found at the mid length of the composites. The SIF decreases with the increase in length of composite and increases with increase in diameter of CNT.