Lawrence Berkeley National Laboratory

A novel molecular dynamics (MD) simulation methodology to capture brittle fracture in epoxy-based 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 condition - a 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 ultra-high strain rate (UHSR ≈ 1013 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 108-1010 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.