Investigating the Influence of Nanoparticle Size and Orientation on Failure in Notched Epoxy Nanocomposites Using the Atomistic J-Integral

Abstract

The goal of this paper is to (a) investigate the validity of using continuum-based linear elastic fracture mechanics (LEFM) methodology to model fracture processes at the "discrete" atomic scale and (b) investigate the impact of nanographene platelet size on the rupture strength of an edge-cracked polymer block. EPON 862 epoxy polymer with a cross-link density of 85 percent was selected for this study. Furthermore, an atomistic J-integral is used as a nanoscale fracture metric to explore flaw-tolerance at the nanoscale, which several other studies have reported, and to build a methodology to evaluate and predict the material's initiation fracture toughness. For this purpose, five different molecular dynamics (MD) models of edge notched specimens that are similar to Compact Tension (CT) specimens were developed, each with a different size of graphene nanoplatelet (GNP) and different orientations, and they were all embedded in EPON 862 epoxy polymer with 85 percent cross-link density. All simulations in this work are performed using the molecular dynamics software LAMMPS and the bond order based ReaxFF potential available in LAMMPS. Significant deviation of peak Jintegral values from LEFM is detected at crack lengths below a critical threshold crack length. GNP size and orientation significantly affect far-field stress vs. strain plots and near-tip stress-fields. The atomistic J integral plots also revealed that our computational methodology is robust enough to capture weak singularity at the subcritical crack-tip, and that J vs. K data approximately obeys the quadratic relationship predicated by LEFM even at the nanoscale.