Constitutive modeling of the viscoelastic and viscoplastic responses of metallocene catalyzed polypropylene

Purpose - The purpose of this paper is to perform experimental investigation and constitutive modeling of the viscoelastic and viscoplastic behavior of metallocene catalyzed polypropylene (mPP) with application to lifetime assessment under conditions of creep rupture. Design/methodology/ approach - Three series of experiments are conducted where the mechanical response of mPP is analyzed in tensile tests with various strain rates, relaxation tests with various strains, and creep tests with various stresses at room temperature. A constitutive model is derived for semicrystalline polymers under an arbitrary three-dimensional deformation with small strains, and its parameters are found fitting the observations. Findings - Crystalline structure and molecular architecture of polypropylene strongly affect its time- and rate-dependent behavior. In particular, time-to-failure of metallocene catalyzed polypropylene under tensile creep noticeably exceeds that of isotactic polypropylene produced by the conventional Ziegler-Natta catalysis. Originality/value - Novel stress-strain relations are developed in viscoelastoplasticity of semi-crystalline polymers and applied to predict their mechanical behavior in long-term creep tests.