Simulation Of Intralaminar Reinforcement And Interlaminar Toughness Enhancement Of CNT/CF Multiscale Composites

Carbon nanotubes (CNTs) are considered as an ideal reinforcement owing to their very high elastic modulus and strength. The CNTs were introduced into the polymer matrix to obtain the nanocomposite for structural applications in early researches. However, experimental results have not achieved desired effects because of the poor dispersion, irregular alignment and low interfacial strength of CNTs in the nanocomposite.In recent years, some researchers have used chemical vapor deposition to grow well-distributed CNTs directly on the surfaces of carbon fibers (CFs) to gain CNT/CF fine coupling composite for intralaminar reinforcement and interlaminar toughness enhancement. This new multiscale composite system is formed by nano-diameter CNTs, micro-diameter carbon fibers, and epoxy resin. Here, uniformly-aligned CNTs oriented along the radial directions of fibers can extend into both the inter-tow and interlaminar regions, providing three-dimensional reinforcement. Besides, the CNTs could help to resist the nano-scale or micro-scale crack extension by bridging between the both sides of crack surface. Significant toughness enhancement is attributed to CNTs pull-out mechanism which absorbs a lot of energy. The modeling of the mechanical property and damage response for this fine coupling composite is a significant research. It has important theoretical significance and engineering practical value to build the relationship between structure and properties of the multiscale composite through theoretical analysis and numerical simulation. Now, the research in this respect is very few still.On the base of micromechanics of composites and finite element methods, a multiscale modeling approach is used to build the mathematical models and calculate the mechanical properties of the fine coupling composite system. The key work is to solve the multi-level coupling problem of carbon nanotubes, carbon fibers, plies and laminate, and to study intralaminar reinforcement and interlaminar toughness enhancement effect of the multiscale composite.Firstly, in order to discuss intralaminar reinforcement effect, the first step requires to establish representative volume element (RVE) of the equivalent matrix formed by a single CNT surrounded by the epoxy resin, and then the second step requires to establish periodic RVE of the overall composite formed by the carbon fibers surrounded by the equivalent matrix. The effective elastic coefficients are calculated by the homogenizing method. In this simulation, CNTs are considered as the isotropic material, and the behaviors of two types of interfaces (CNT/epoxy and CF/equivalent matrix) are described using an exponential cohesive zone model. In this study, an important task was to study the influences of the CNTs volume fraction and the CNT/epoxy interfacial strength on intralaminar reinforcement effect. The main conclusions are as follows:a) Due to the existence of the CNTs, the matrix-dominated properties of the multiscale composite perpendicular to the fiber axis are greatly enhanced, whereas the fiber-dominated properties of the multiscale composite in the fiber direction have little change.b) The multiscale composite transverse elastic modulus gets an approximate linear improvement with the increase of the CNTs volume fraction. Whereas the CNTs volume fraction has little influence on the multiscale composite longitudinal elastic modulus.c) When the CNT/epoxy interfacial strength is relatively low, the multiscale composite transverse elastic modulus gets a distinct improvement with the increase of the CNT/epoxy interfacial strength. However, when the CNT/epoxy interfacial strength reaches a certain value, the variation of the multiscale composite transverse elastic modulus along with the CNT/epoxy interfacial strength is not distinct. Whereas the CNT/epoxy interfacial strength has little influence on the multiscale composite longitudinal elastic modulus.Second, in order to discuss interlaminar toughness enhancement effect, the first step requires to establish the CNTs pull-out mechanism model formed by a single CNT surrounded by the epoxy resin, and then the second step requires to establish the double cantilever beam model of the multiscale composite laminates. The mode I interlaminar fracture toughness is calculated by the beam theory. In the interlaminar region, the combination of pure epoxy resin is described using the cohesive zone model, and then CNTs bridging between the both sides of crack surface is represented by equally distributed nonlinear spring elements. In this study, an important task was to study the influences of the CNTs volume fraction and the CNT/epoxy interfacial strength on interlaminar toughness enhancement effect. The main conclusions are as follows:a) The matrix elastic deformation energy in the single CNT pull-out mechanism model increases in a certain range with reducing the CNTs volume fraction.b) The CNTs maximal pull-out force and the final pull-out displacement increase signally with the increase of the CNT/epoxy interfacial strength.c) Compare to the traditional composite laminate, the maximal load value and the mode I interlaminar fracture toughness of the double cantilever beam structure with the CNTs reinforcement increases remarkably through the process of mode I crack propagation.d) The CNTs volume fraction is bigger, and the energy absorbed by interface debonding in the same volume of interlaminar region is higher, so as to the CNTs bridging effect is more obvious.e) The CNT/epoxy interfacial strength is higher, and the energy absorbed by the debonding of the same interface area is higher, so as to toughness enhancement of the multiscale composite is more remarkable.