Predicting the double-yield phenomenon in low-density polyethylene film using three-network viscoplastic model

Abstract

Several Semi-Crystalline Polymers (SCPs) exhibit a Double-Yield (DY) phenomenon in the stress vs. strain response when subjected to uniaxial tensile load. There are only few constitutive models that can predict the DY phenomena at different temperatures and strain rates. These models were developed considering no deformation of crystalline phase, when the amorphous phase gets deformed, whereas in reality all the phases deform concurrently. In this investigation, a Three-Network (TN) viscoplastic model was employed to capture the DY phenomenon in the Low-Density Polyethylene (LDPE) films by considering simultaneous deformations in the amorphous and crystalline (lamellar) phases. The TN model comprised of three parallel networks, A, B, and C, with A and B consisting of non-linear springs and dashpots in series with each other and C containing a non-linear spring solely. While network A of the TN model influences the inter-lamellar shear resistance and the first yield point in the engineering stress vs. strain curve, networks B and C operate together to predict the second yield point. Furthermore, network B regulates the resistance to intra-lamellar shear fragmentation and orientation of lamellar phases. Network C accounts for both the rubbery effect of the amorphous phase and the resistance to fibril deformation along the loading direction at higher strains. To retrieve the TN model parameters, uniaxial tensile tests were conducted on the LDPE films at two different strain rates and temperatures ranging from ambient temperature (25 °C) to 110 °C. The engineering stress vs. strain graphs exhibited the DY phenomenon for all temperatures considered. Stress vs. strain curves were calibrated using the TN model in MCalibration software at certain temperatures (25 °C, 50 °C, 70 °C, and 90 °C) and loading rates (∼10−3/s and 10−2/s). The calibration results corroborated the experimental findings, particularly in terms of capturing the DY phenomenon. Also, simulations using calibrated TN model parameters satisfactorily predicted experimental stress-strain plots at remaining temperatures (40 °C, 60 °C, 80 °C, and 110 °C), demonstrating its versatility in predicting stress and strain characteristics. For the wide range of temperature (25 °C–110 °C) and strain rates considered, an overall difference of ∼5% is observed between the experimental and predicted stress vs. strain data.