Functionality-dependent yield surface asymmetry in crosslinked epoxy polymers

Abstract

In this study, we investigate how epoxy functionality, a practical molecular design variable, governs the morphology and asymmetry of yield surfaces in thermosetting epoxy polymers. Molecular dynamics (MD) models of amorphous epoxy networks with systematically varied functionalities were constructed and subjected to multiaxial deformation to extract equivalent stress–strain responses and yield surfaces across multiple loading paths. The results show that increasing functionality elevates the equivalent stress under all loading modes and drives an asymmetric expansion of both the initial and post-yield surfaces, with the most pronounced growth occurring under biaxial compression. Mechanistically, higher functionality strengthens network confinement, suppresses chain mobility, and amplifies non-bonded stress contributions, thereby enhancing the capacity to sustain compressive-dominated stress states and intensifying tension–compression asymmetry during post-yield evolution. These functionality-dependent trends are corroborated experimentally through specimen synthesis and quasi-static uniaxial tension and plane strain compression (PSC) tests, which exhibit consistent stress–strain behavior and yield-surface evolution. Finally, to facilitate engineering use, we integrate the MD and experimental datasets to derive pressure-modified yield functions suitable for continuum-scale implementation, providing a practical pathway from molecular-level structure–property relationships to constitutive modeling of epoxy systems.