The carbon fiber/polymer matrix interphase region plays an important role in failure initiation and accurate modeling techniques are integral to study the effects of this complex region on the composite response. In composites infused with nanoparticles such as carbon nanotubes (CNT), the interphase region is more complex due to the presence of multiple constituents and their interactions with each other. An atomistic methodology to simulate the constituent interphases in carbon fiber reinforced CNT/epoxy nanocomposites is presented in this paper. The interphase model consisting of voids in multiple graphene layers enable the simulation of physical entanglement between the polymer matrix and the irregular carbon fiber surface. The voids in graphene layers are generated by removing carbon atoms and hydrogenating the end carbon atoms, which better represent the roughness of the carbon fiber surface. The epoxy curing studies and the response of fiber/matrix interphase under mechanical loading are investigated through molecular dynamic (MD) simulations with appropriate classical/harmonic and bond order-based force fields. Furthermore, the atomistic force-displacement behavior is also extracted to formulate a traction-separation law for interface cohesive zone models. The cohesive behavior determined from molecular models is parameterized in equations that can be integrated with an atomistically informed multiscale modeling framework.

A comprehensive, point-information-to-continuum-level analysis framework is presented in this paper to accurately characterize the behavior of carbon nanotube (CNT)-enhanced composite materials. Molecular dynamics (MD) simulations are performed to study sub-nanoscale interactions of the CNT with the polymeric phase of the nanocomposite. The effect of cross-linking between the epoxy resin and the hardener on the mechanical properties of the polymer is investigated; furthermore, the effect of CNT weight fraction on the probability distribution of polymer cross-linking degree is also studied through stochastic models. The stochastic distributions obtained from MD simulations provide a basis to simulate local variations in the matrix properties in the continuum model at the microscale. The inclusion of an atomistically informed elastic-plastic model at the microscale reveals a significant deviation of the mechanical properties from those obtained based on classical homogenization approaches. Microstructural variability arising from heterogeneous cross-linking degree in the polymer phase and variations in fiber geometry and spacing is also found to cause deviations in the mechanical response when compared to the assumption of a perfectly ordered fiber-matrix microstructure.

This paper examines microscale and sub-microscale damage mechanisms in carbon nanotube (CNT) reinforced nanocomposites. A multiscale modeling framework with a damage model developed from molecular dynamics simulation, is employed to study the physical mechanisms of damage initiation and propagation in CNT nanocomposites at the sub-microscale. Two CNT arrangements, randomly dispersed and entangled agglomerates, are examined. This investigation offers insights into damage properties of particular configurations of CNTs in a polymer matrix, in addition to specific understanding related to damage concentration effects around the filler material at the sub-microscale. High spatial CNT concentration differential is observed to affect damage initiation and rate of damage. It is further shown to result in sub-microscale crack initiation at low global strains, a phenomenon that is also observed at agglomeration boundaries, which results in CNT agglomerations to behave as crack initiation sites.

A methodology that accurately simulates the brittle behavior of epoxy polymers initiating at the molecular level due to bond elongation and subsequent bond dissociation is presented in this paper. The system investigated in this study comprises a combination of crystalline carbon nanotubes (CNTs) dispersed in epoxy polymer molecules. Molecular dynamics (MD) simulations are performed with an appropriate bond order-based force field to capture deformation-induced bond dissociation between atoms within the simulation volume. During deformation, the thermal vibration of molecules causes the elongated bonds to re-equilibrate; thus, the effect of mechanical deformation on bond elongation and scission cannot be captured effectively. This issue is overcome by deforming the simulation volume at zero temperature—a technique adopted from the concept of quasi-continuum and demonstrated successfully in the authors’ previous work. Results showed that a combination of MD deformation tests with ultra-high strain rates at near-zero temperatures provides a computationally efficient alternative for the study of bond dissociation phenomenon in amorphous epoxy polymer. In this paper, the ultra-high strain rate deformation approach is extended to the CNT-epoxy system at various CNT weight fractions and the corresponding bond disassociation energy extracted from the simulation volume is used as input to a low-fidelity continuum damage mechanics (CDM) model to demonstrate the bridging of length scales and to study matrix failure at the microscale. The material parameters for the classical CDM model are directly obtained from physics-based atomistic simulations, thus improving the accuracy of the multiscale approach.

A multiscale-modeling framework is presented to understand damage and failure response in carbon nanotube reinforced nanocomposites. A damage model is developed using the framework of continuum damage mechanics with a physical damage evolution equation inspired by molecular dynamics simulations. This damage formulation is applied to randomly dispersed carbon nanotube reinforced nanocomposite unit cells with periodic boundary conditions to investigate preferred sites and the tendency towards damage. The continuum model is seen as successfully capturing much of the unique nonlinear trends observed in the molecular dynamics simulations in a volume 1000 times greater than the molecular dynamics unit cell. Additionally, application of the damage model to the continuum unit cell revealed insights into the failure of carbon nanotube reinforced nanocomposites at the sub-microscale.