Energy-based yield criterion for PMMA from large-scale molecular dynamics simulations

We use molecular dynamics (MD) with the DREIDING force field to characterize the ultimate mechanical response of amorphous poly(methyl methacrylate). We characterize how volumetric and deviatoric strains contribute to yield for a wide range of loading conditions from pure deviatoric, volume-conserving cases to isotropic volume expansion. We propose and apply an energy-based yield criterion to define yield consistently for all cases. Our results show that permanent deformation occurs when either the deviatoric or volumetric strains reach critical values, except in a narrow region around the transformation between deviatoric- and volumetric-dominated yield where the two strain invariants interact. In contrast, the pressure-modified von Mises criterion is only applicable to shear-dominated loading conditions. These results provide insight into the physics of yield in amorphous polymers and provide quantitative information and guidance for physics-based yield criteria for polymer-matrix composite materials