Strain-rate dependent mode I cohesive traction laws for glass fiber-epoxy interphase using molecular dynamics simulations

Abstract

Using molecular dynamics (MD) simulations, we develop strain-rate dependent Mode-I cohesive traction laws for the glass fiber-epoxy interphase as a function of interphase structure. Interphase models with mono-layer glycidoxypropyltrimethoxy silane are prepared varying silane number density from 0.0 nm−2 to 3.9 nm−2. Interphase is formed through epoxy-amine diffusion in the silane layer followed by curing reaction. Traction laws are developed over a full range of strain rates from quasi-static to super-high strain rate (∼1e16/s) where a theoretical plateau strength limit is predicted. A stress-relaxation methodology is introduced to construct quasi-static traction-separation responses from high strain rate loading. Simulation results reveal that the interphase traction-separation responses are strain rate and silane number density dependent. Variations of peak traction and energy with strain rates show a characteristic S-shape pattern in a semi-log plot with a gradual increase in properties up to 1e12/s where a steep transition occurs between 1e13/s to 1e14/s followed by a strain rate independent plateau. Interphase properties (especially peak traction) increase almost linearly with silane number density for all strain rates. Mathematical correlations for the MD predicted traction versus strain rate and energy versus strain rate data are established to bridge length scales providing input micromechanics models for the prediction of fiber-matrix debonding in composites.