Multiscale modeling of damage progression in nylon 6/clay nanocomposites

A hierarchical multiscale model for nylon 6/clay nanocomposites is adopted to study its damage and post-damage behavior. The 3D representative volume element (RVE) of the nanocomposites at the macroscale comprises a nylon 6 matrix with embedded silicate layers. Taking into consideration the interactions between nylon 6 and the clay particulates, a gallery inter-layer was inserted between the intercalated silicate layers and an interphase layer was added around the silicate. Both gallery and interphase layers were treated as interfaces in this study. Molecular dynamics (MD) simulations were performed to obtain the material behavior of the interfaces. Results from the MD simulations were used to parameterize a traction-separation law which was incorporated into the RVE to model interfacial degradation. The damage of bulk nylon 6, on the other hand, was governed by the Gurson-Tvergaard-Needleman (GTN) model. The finite element method (FEM) was used to simulate the RVE model under quasi-static uniaxial stress loading. The constitutive relationship and fracture patterns of the nylon 6/clay model with 2.5% weight fraction of clay were studied. The apparent yield stress is found to increase while the ductility decreases when the clay particle size increases or the number of silicate layers decreases. The effect of interfacial strength on the properties of the nanocomposites is also presented. When the interfacial strength was relatively low, debonding of the interphase layer is the cause for damage initiation; a higher cohesive strength of the interfaces will lead to damage initiation from the polymer matrix around the interphase layer. This is due to inherent weak adhesion strength of the polymer chains, which was also observed through experimental results.