MECHANICAL PROPERTIES ANALYSIS OF CARBON NANOTUBE REINFORCED POLYMER COMPOSITES

Abstract

Since the discovery, carbon nanotubes (CNT) have generated tremendous interest in the field of Science and technology due to their exceptional physical, chemical and electronic characteristics [90]. CNTs are single or nested structure having tube diameter in nanometer scale and important material responses are observed at the discrete level. The tensile strength of the CNTs has been found close to close to a few 100 GPa and their Young’s modulus are in the range of terapascal, which is far better than conventional carbon fibres [225]. The exceptional properties of carbon nanotube have attracted composite fraternity to use them as potential reinforcement material primarily in polymer based composites. The present work deals with mechanical properties analysis of CNT reinforced polymer composites. In the present study, both computational and experimental work has been carried out and effects of geometrical properties, CNT defects and interfacial crack on the mechanical properties of nanocomposites are studied. Continuum mechanics based approach has been adopted to analyze the effect of volume fraction with varying thickness/diameter of CNTs on the elastic properties of three phase polymer nanocomposites. The effects of pinhole defects present in single wall carbon nanotubes (SWNT) and double wall carbon nanotubes (DWNT) are studied using multi scale modeling approach. The effect of interfacial crack between CNT and polymer matrix is studied using analytical and extended finite element method (XFEM) approach. Experiments are conducted to study the effects on mechanical properties of epoxy carbon fibre composites due to reinforcement of multi wall carbon nanotubes (MWNT). The experimental result shows that tensile strength and strain at failure have been substantially enhanced due to addition of MWNT. The potential applications of polymer nanocomposites in composite structure, bio-medical, sensors/actuators are also studied. The current research in composites has indicated that CNT reinforcement in polymer based composites can substantially enhance the various mechanical properties, as it plays the role of matrix additives and provides multifunctional properties to the composites. The CNT reinforced polymer composites requires macro level prediction. Experimental characterizations of nanocomposites are expensive and time consuming task. Computational methods have been increasingly used in the recent years and the difference in length scale is compensated using computational techniques. Modeling and simulations of nanocomposites are being achieved on desktop computer with relatively less cost. The accuracy of mathematical models will determine v the results obtained through computer simulations. The important approaches for the modeling of nanocomposites are molecular mechanics, continuum modeling and multiscale modeling. Continuum mechanics approach can model materials at the nanoscale as the theories of continuum mechanics are robust enough to treat even infinitesimally discrete objects. However, continuum mechanics approach is not able to cater for nanotube chirality and atom to atom interactions are also neglected. Based on continuum mechanics, a nanoscale representative volume element (RVE) has been proposed for evaluation of effective mechanical properties of CNT reinforced composites [125]. Multiscale modeling approach is a new research paradigm emerged in recent time. It utilizes certain features of both atomistic and continuum mechanics approaches and can model systems that vary on time or length scales with large orders of magnitude [119, 147, 227]. The effect CNT reinforcement on elastic modulus of two phase polymer composite is studied using both finite element analysis and rule of mixture theory. The carbon nanotubes are constructed as hollow cylindrical shape tube structure and mechanical property of CNT reinforced nanocomposites have been investigated using 3D represented volume Element (RVE). The matrix materials have been taken epoxy and polyether ether ketone (PEEK), whereas carbon fibres and glass fibres are being used as primary reinforcements. The mechanical properties have been evaluated for different volume fractions of CNTs and for different configuration of matrix and fibres. The representative volume elements based synthesis has been adopted for study of the CNT based composites at the nanoscale [125]. Effective elasticity property has been evaluated with hexagonal and Square RVE under an axial stretch [33, 101]. The analysis results of both long and short CNT clearly indicates that the nanotube reinforcement substantially improves the composite modulus of a two phase composites. It is also found that short CNT inside RVE provides better reinforcement to composite as compared with long CNT. The Young’s modulus of nanocomposites has been evaluated for varying wall thickness and varying internal diameter of CNTs for different combination of matrix and fibre. It has been observed that with 2% reinforcement of CNT results in 40% increase in elastic modulus. It has been found that with increase in thickness from 0.4 nm to 1.6 nm, the composite stiffness is increased by 20.2%. Also, an increase in internal diameter from 2 nm to 4.4 nm results in 24.6% increase in composite stiffness. Multiscale FE modeling approach has been implemented for analyzing the vacancy defects and its effect on the mechanical properties of the polymer nanocomposites are studied. The composite RVE consists of CNT, matrix and matrix-CNT interphase region. Two configurations viz. zigzag vi nanotubes and armchair nanotubes are used for analysis. For study of atomic vacancy in SWNT, two distinct CNT structures are investigated viz. zigzag (9, 0) and armchair (5, 5) nanotubes, each having an approximate diameter of 0.7 nm. For the DWNT, armchair (5,5), (10,10) and zigzag (9,0), (16,0) is selected for defect analysis. The C-C bonds at atomic scale are modeled as Euler beam. The presence of chemical covalent bonding between functionalized nanotube and matrix are modeled as elastic cross links. The multiple cylindrical layers of an MWNT are held together with the help of non-bonded Van der Waals forces. The constitutive interactions between two deforming surfaces are modeled as cohesive interaction using special interaction element in FEM software by defining the CNT waals as master and salve surfaces. The composite RVEs are studied under an axial load condition and influence of the pinhole defects on the elastic modulus of nanocomposite are investigated as a function of stiffness ratio and nanotubes chirality. It has been found from simulation results that armchair DWNT provides better reinforcement to nanocomposites compare to zigzag DWNT in presence of defects both at inner and outer wall. The simulation result shows that elastic properties are significantly affected by the defect at inner wall than at outer wall for both armchair and zigzag DWNT. The effect of crack on the polymer/CNT interface is analyzed using fracture mechanics approach. Two approaches have been adopted to analyze the delamination problem. In the first approach, an analytical method has been adopted applied to find the stress intensity factor (SIF) and energy release rate [86]. Analytical approach given by Hutchinson and Suo is considered for the study of interfacial crack of the nanocomposites [87]. The matrix and CNTs are modeled as 2D isotropic solids and the crack is considered on the interface of polymer matrix and CNT. In the second approach, XFEM technique is used to evaluate the stress intensity factor and energy release rate [192]. A square RVE has been considered with suitable boundary condition to study the effect of interface crack between matrix and CNT. Both long CNT and short CNT based nanocomposites are studied. The SIF and energy release rate is evaluated using AbacusTM software. The XFEM solver has computed energy release rate in terms of contour J integral and the average value of energy over the enriched nodes are computed. Energy release rate is evaluated for various stiffness ratio and crack length. The results obtained from both analytical and XFEM are compared. . It has been found that varying stiffness significantly affect the energy release rate. It has been observed that for the change in stiffness ratio from 0.01 to 0.06, the energy release rate is reduced to approximately 16% of the original value. It can be seen that for the same crack length ratio, stress intensity factor and energy release rate for short CNT is substantially less compared to long CNT. vii The effects of multi wall nanotubes (MWNT) reinforcement on epoxy-carbon polymer composites are investigated using experiments. MWNTs with high purity are synthesized by chemical vapor deposition (CVD) technique and amino functionalizations are achieved through acid - thionyl chloride route [27]. Diglycidyl ether of bisphenol - A (DGEBA) epoxy resin with diethyl toluene diamine (DETDA) hardener has been used as a matrix. T-300 carbon fabric is used as the primary reinforcement. The TEM studies show that MWNTs have typically 20-30 walls and their diameters in the range of 20-30 nm. Three types of test specimen of epoxy carbon composite are prepared with MWNT reinforcement as 0%, 1% and 2% MWNT (by wt). The analytical results obtained from ROM shows good agreement with the experimental results. It has been found that both tensile strength and strain at failure have been substantially enhanced due to addition of MWNT. The experimental result shows that 1% MWNT reinforcement results in 23.6% and 10.8% increase in tensile strength and failure strain, respectively. It has been found that 2% MWNT reinforcement results in 40.2 % and 16.2% increase in tensile strength and failure strain, respectively.