Modelling large strain deformation of semi-crystalline polymers with variable crystal content under cyclic loading and microstructure and texture evolution under monotonic loading -

Abstract

Semi-crystalline polymers are used in a large number of applications (Automotive, biomedical, electronics etc.) due to their varying semi-crystalline morphology that leads to a wide range of mechanical properties. Using semi-crystalline polymers in applications depends on understanding the microstructure and texture evolution under different deformation mechanisms and proposing phenomenological, physical-based or partly physical-based models able to capture accurately the mechanical response. The first part of this work is focused on the Mullins effect in polyethylene. An ultra-low-density polyethylene (ULDPE) with 0.15 crystal content, a low-density polyethylene (LDPE) with 0.3 crystal content and a high-density polyethylene (HDPE) with 0.72 crystal content are subjected to multi-step cyclic stretching over a large strain range. Experimental observations are first reported to examine how the crystal content influences the Mullins effect in polyethylene. It is found that the cyclic stretching is characterized by a stress-softening, a hysteresis and a residual strain, whose amounts depends on the crystal content and the applied strain. A unified viscohyperelastic-viscoelastic-viscoplastic constitutive model is proposed to capture the polyethylene response over a large strain range and its crystal-dependency. Experimentally-based material kinetics are designed by considering the Mullins effect crystal-dependency and are introduced into the constitutive equations to capture the experimental observations. The model was able to capture the olygo-cyclic mechanical response for HDPE and ULDPE using the same parameters. The only variable between in the model was the crystal content. A damage model was introduced for LDPE in order to capture the damage induced in the material affecting the Young’s modulus with strain and improve the model’s capability. The full model was able to capture the maximum stress and the unloading stress-strain curves at each cycle, but was not able to represent the full LDPE mechan