Finite viscoplasticity of semicrystalline polymers

Abstract

A series of uniaxial tensile tests is performed on isotactic polypropylene at room temperature with cross-head speeds ranging from 5 to 100 mm/min. Prior to mechanical testing, injection-molded samples are annealed for 24 h at the temperature 160 C that ensures that the specimens can be drawn up to the Hencky strain of 0.6 without necking. A constitutive model is derived for the viscoplastic behavior of a semicrystalline polymer at finite strains. The stress-strain relations are determined by five adjustable parameters that are found by matching the observations. Fair agreement is demonstrated between the experimental data and the results of numerical simulation. It is shown that the material parameters in the constitutive equations are strongly affected by the strain rate. IntroductionThis paper is concerned with the effect of strain rate on the viscoplastic behavior of the injection-molded isotactic polypropylene (iPP) in isothermal uniaxial tensile tests at finite strains. The interest to the mechanical response of iPP is caused by its many industrial applications, ranged from oriented films for packaging to non-woven fabrics and reinforcing fibres. The influence of strain rate on the yield stress, defined as the point of maximum on the engineering stress-engineering strain curve, has been the focus of attention of some papers on polypropylene in the past decade, [1][2][3][4].The iPP is a semicrystalline polymer containing three basic crystallographic forms, [5]: monoclinic a-crystallites, (pseudo) hexagonal b-structures, orthorhombic c-polymorphs, and smectic mesophase, which consists of arrays of chains with a better order in the longitudinal than in transverse chain direction. At rapid cooling of the melt, which is typical of the injection-molding process, a-crystallites and smectic mesophase are mainly developed, whereas b-and c-polymorphs are observed as minor components, [6].The characteristic size of a-spherulites in an injection-molded specimen is estimated at 100-200 lm, [6, 7]. These structures are formed by lamellae stacks with lamellar thicknesses ranging from 10 to 20 nm, [7,8]. A unique feature of the crystalline morphology of iPP is the lamellar cross-hatching: development of transverse lamellae oriented in the direction perpendicular to the direction of radial lamellae in spherulites, [5,8].Transmission electron microscopy (TEM), [8,9], and polarized optical microscopy, [9], show that annealing of iPP at elevated temperatures results in

Deformation response and constitutive modeling of PC, ABS and PC/ABS alloys under impact tensile loading

“…The well-known theoretical constitutive models to describe the strain rate-dependent deformation of polymers were proposed by Haward and Thackary [14], Edwards and Vilgis [15], Boyce et al [16], Wu and van der Giessen [17], Drozdov and Christiansen [18], etc. On the other hand, experiment-based phenomenological constitutive model is convenient for engineering applications.…”

Viscoplastic Behaviour of Polyamides

“…Their constitutive models for various thermoplastics and elastomers consider the total deformation being composed of a sliding part and an elastic part and the stress-strain relation being governed by a strain-energy density function. The resulting constants and parameters are eventually fitted with the experimental data [14][15][16].…”