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

In this paper, we establish a methodology to predict strain rate-dependent mixed-mode traction-separation responses (i.e., traction laws) for glass-epoxy composite interphase using molecular dynamics (MD) simulations. Glass-epoxy interphases with monolayer glycidoxypropyltrimethoxy silane are prepared by varying silane number density from 0.0 nm(-2) to 3.9 nm(-2) following the epoxy-amine diffusion and curing reactions. To established the effects of strain rate and mode-mixity on the interphase traction laws, the nano-meter size interphase domain is loaded in various mode-mixity (theta = 0 degrees (Mode - II), 15 degrees, 30 degrees, 45 degrees, 60 degrees, and 90 degrees (Mode - I)) with full range of strain rates from quasit-static to high strain rate (similar to 1e16/s) where a theoretical plateau strength limit is predicted. Following our previous work on Mode-I [Chowdhury et al., Composites Part B 237 (2022) 109877], mathematical model is developed for Mode-II as function of strain rate for different interphase structures (i.e., silane number density). The continuum equivalent bi-linear cohesive traction law is developed using the MD results to determine the mode-mixity quadratic functions and associated exponents for peak tractions, energy absorption, crack initiation and crack opening displacement from the mixed-mode simulations data. The MD predicted traction laws can be used to model interphase in micromechanics finite element analysis to bridge the length scale for the prediction of fiber-matrix debonding in composites.