Numerical and experimental investigation of temperature- and strain-rate-dependent double yielding in mLLDPE dumbbell specimens based on a three-network model

Double yielding is a typical tensile response of semi-crystalline polymers (SCPs), reflecting the coupled evolution of amorphous and crystalline phases. In this work, the double yielding behavior of metallocene-catalyzed linear low-density polyethylene (mLLDPE) is examined through polarized optical microscopy (POM), Fourier-transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), X-ray diffraction (XRD), and tensile tests. Owing to its multiple short-chain branches, mLLDPE exhibits a broad lamellar thickness distribution that governs the emergence of double yielding. Combined DSC–XRD analysis further confirms lamellar fragmentation, melting, and recrystallization during deformation. DSC peak deconvolution quantitatively distinguishes high- and low-stability crystalline domains, clarifying their sequential transformation during the two yielding events. A three-network model (TNM) is adopted to describe amorphous yielding, lamellar failure, and rubber-like chain deformation. Finite-element simulations reproduce the double yielding behavior of mLLDPE specimens (NRMSE < 3%). Both yield stresses decrease linearly with temperature (R ² > 0.99) and follow the Ree–Eyring relation with strain rate (R ² > 0.94). These results deepen the understanding of structure–deformation relationships in SCPs and offer a quantitative foundation for tailoring polymer microstructures to achieve targeted mechanical performance in engineering applications.