Constitutive equations in finite viscoplasticity of semicrystalline polymers A.D. Drozdov * , R.K. Gupta Department of Chemical Engineering, West Virginia University, P.O. Box 6102, Morgantown, WV 26506, USA Received 25 June 2003; received in revised form 25 June 2003 Abstract Three series of uniaxial tensile tests with constant strain rates are performed at room temperature on isotactic polypropylene and two commercial grades of low-density polyethylene with different molecular weights. Constitutive equations are derived for the viscoplastic behavior of semicrystalline polymers at finite strains. A polymer is treated as an equivalent network of strands bridged by permanent junctions. Two types of junctions are introduced: affine whose micro-deformation coincides with the macro-deformation of a polymer, and non-affine that slide with respect to their reference positions. The elastic response of the network is attributed to elongation of strands, whereas its viscoplastic behavior is associated with sliding of junctions. The rate of sliding is proportional to the average stress in strands linked to non-affine junctions. Stress–strain relations in finite viscoplasticity of semicrystalline polymers are developed by using the laws of thermodynamics. The constitutive equations are applied to the analysis of uniaxial tension, uniaxial compression and simple shear of an incompressible medium. These relations involve three adjustable parameters that are found by fitting the experimental data. Fair agreement is demonstrated between the observations and the results of numerical simulation. It is revealed that the viscoplastic response of low-density polyethylene in simple shear is strongly affected by its molecular weight. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: Semicrystalline polymers; Low-density polyethylene; Isotactic polypropylene; Viscoplasticity; Finite strains 1.Introduction This paper is concerned with the viscoplastic response of semicrystalline polymers at isothermal de- formations with finite strains. The experimental part of the study focuses on the rate-dependent behavior of isotactic polypropylene (iPP) and two grades of low-density polyethylene (LDPE) with different molecular weights. The choice of iPP and LDPE for the investigation is explained by (i) their numerous industrial International Journal of Solids and Structures 40 (2003) 6217–6243 www.elsevier.com/locate/ijsolstr * Corresponding author. Tel.: +1-304-293-2111; fax: +1-304-293-4139. E-mail address: aleksey.drozdov@mail.wvu.edu (A.D. Drozdov). 0020-7683/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0020-7683(03)00414-1