Molecular dynamics study of brittle fracture in epoxy-based thermoset polymer

Abstract

A novel molecular dynamics (MD) simulation methodology to capture brittle fracture in epoxybased thermoset polymer under mechanical loading is presented. The ductile behavior of amorphous polymers has been captured through traditional MD simulation methods by estimating the stress-strain response beyond the yield point; however, brittle fracture in highly crosslinked polymer materials such as epoxy thermoset has not been addressed appropriately and is the primary objective of this work. In this study, a numerically cured epoxy system comprising molecules of epoxy resin and hardener is generated. During the virtual deformation test, it is observed that the inherent molecular vibration due to temperature re-equilibrates the elongated covalent bonds and this molecular vibration impedes further stretching the covalent bond leading to scission. In order to overcome the influence of thermal vibration, an approach that employs deformation tests at absolute zero temperature conditiona concept borrowed from the quasi-continuum method, is developed. Bond dissociation energy is measured to quantify the extent of failure in the system by calculating the bond potentials during the deformation tests. Applying zero temperature condition to the deformation test, however, requires a large amount of computational time due to intermediate energy minimization processes. To improve the computational efficiency, an ultrahigh strain rate (UHSR ≈ 10 13 s -1 ) approach is developed by which the thermal vibration is decoupled from the deformation test using a strain rate higher than molecular vibration frequency. Note that the deformation test performed by traditional MD simulation methods used strain rates ranging 10 8 -10 10 s -1 . Simulation results show that the UHSR approach successfully captures brittle fracture in epoxy polymer due to covalent bond dissociation with high computational efficiency.

Molecular dynamics-based multiscale damage initiation model for CNT/epoxy nanopolymers

“…The higher the bond vibration frequency, the higher the energy required for breaking the bond. When a simulation volume is deformed by an external force, the new position of the atoms and the extension/contraction of the bonds in the system are dictated by the local force experienced by the atoms; however, it has been demonstrated in our previous work that the bond length variation is re-equilibrated between timesteps due to thermal vibration [17]. This re-equilibration of bond lengths is attributed to the frequency of bond vibrations being much higher than the rate at which the strain is experienced by the simulation volume.…”

“…A recent study by the authors on the simulation of molecular chain sliding and bond breakage in neat epoxy polymer demonstrated the use of a novel, computationally feasible MD approach as an alternative to expensive zero-temperature simulations [17]. In this paper, we extend the use of this atomistic simulation approach to investigate CNT-enhanced epoxy systems and further integrate the nanoscale J Mater Sci (2018) 53:2604-2617 results to a comprehensive micromechanics-based matrix failure model.…”

“…However, zero-temperature simulations are computationally intensive and time consuming. An approach whereby a combination of high strain rate and low-temperature MD simulations is performed to extract bond dissociation energy is adopted in order to better understand damage initiation [17]. The dissipated energy due to bond breakage calculated at the atomistic level is used to develop a material parameter that defines damage at the continuum level.…”

Atomistically derived cohesive behavior of interphases in carbon fiber reinforced CNT nanocomposites

“…Hence, the bond order based potential parameter set from Singh et al [31] is introduced to capture bond elongation and subsequent dissociation of covalent bonds in the interphase models during the virtual pullout tests. The novel QC-equivalent MD simulation approach using which covalent bond dissociation is captured and quantified at the interphase, is presented in detail in Ref [19,20]…”

“…The mechanism of damage initiation is also significantly different in crystalline and amorphous materials. In a previous effort, the authors employed a hybrid force field MD simulation methodology to capture bond breakage in epoxy polymers [19]. This methodology was used to simulate damage caused by successive bond breakages using a combination of high strain rate and low temperature MD simulations and was compared to quasi-continuum (QC) simulations showing excellent correlation [20].…”