Developing Strain Rate Dependent Mode-I Traction Law for the Glass Fiber-Epoxy Interphase using Molecular Dynamics Method

The properties of the glass fiber reinforced epoxy composites are greatly influenced by the interaction of the fiber with the epoxy matrix. This interaction is quantified using the interphase traction-separation response, called traction law. Using molecular dynamics (MD) simulations, we develop strain-rate dependent Mode-I traction laws for the glass fiber-epoxy interphase. 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). Quasi-static traction-separation response is predicted from stress-relaxation simulations 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, however interphase with 2 nm-2 silane number density gives the highest energy absorption capability having mixed-mode failure. Strain rate dependent peak traction and energy data are fitted with mathematical functions to use them in the finite element-based micro-mechanical modeling of composite.