Multiscale mechanisms of the nonlinear Poisson's ratio in viscoelastic particle-filled composites

The Poisson's ratio of polymer-based particulate-filled composites exhibits nonlinearity and viscoelasticity. In this study, experimental investigations were conducted to characterize the nonlinear viscoelastic Poisson's ratio. The Poisson's ratio slightly increases during the relaxation tests but linearly decreases (from 0.5 at the initial state to 0.35 at 60% strain) during the tension tests, remaining insensitive to the temperature and strain rate. The Poisson's ratio exhibit ratcheting behavior but no Mullins effect. Quasi in situ micro-experiments and representative volume element simulations reveal that Poisson's ratio decreases with increasing micro-interfacial extended displacement (Stage II of the cohesive zone model). A multiscale constitutive model integrates the normalized interfacial displacement, crack length, and volume ratio as damage internal variables for the Poisson's ratio, shear and bulk moduli. The accuracy and applicability of the constitutive model were validated via center-holed specimens. Ignoring Poisson's ratio nonlinearity leads to an overestimation of the shear modulus damage and an underestimation of the postdeformation volume. The methodology and findings provide insights for multiscale analysis of particle-filled composites.