J. deC. Christiansen

Influence of push–pull injection moulding on fibres and matrix of fibre reinforced polypropylene

Abstract Push–pull-processing (PPP) is a particular method of injection moulding with enlarged capabilities to control the melt flow during solidification. While the melt is solidifying from the mould wall to the mould centre an oscillating motion through the cavity maintains the melt flow. Aims were to investigate the capabilities of the process to control orientation of fibres and matrix and its effect on fibre length since this determines significantly the mechanical properties of plastic parts. Processing parameter variations were carried out on a Ferromatik K110/S60-2K injection-moulding machine. Sample plates were produced of short and long fibre reinforced polypropylene. The process and the control of it are described in the paper. Fibre orientation and fibre length were measured. Inter-laminar shear strength (ILSS) tests were performed to investigate the strength between shear layers. WAXD was applied to investigate orientation in the matrix. The results were correlated to process settings.

Modelling the viscoplastic response of polyethylene in uniaxial loading¿unloading tests

Abstract Two series of uniaxial cyclic tests are performed on low-density polyethylene at room temperature. In the first series of experiments, injection-molded specimens are stretched to several maximal strains ϵmax in the region of sub-yield deformations with a constant cross-head speed, ϵ =10 mm/min, and retracted down to the zero stress with the same strain rate. In the other series, loading–unloading tests are carried out with the maximal strain ϵmax=0.10 and cross-head speeds ranging from 5 to 200 mm/min. A constitutive model is derived for the viscoplastic behavior of a semicrystalline polymer at small strains. A polymer is modelled as an equivalent network of chains bridged by permanent junctions (entanglements, physical cross-links on the surfaces of crystallites and lamellar blocks). The network is treated as an ensemble of meso-regions connected by links (crystalline lamellae). Deformation of a specimen induces sliding of junctions with respect to their reference positions both at active loading and unloading (this process reflects sliding of junctions in amorphous regions and fine slip of crystalline lamellae). At retraction, sliding of junctions is accompanied by mutual displacements of meso-domains (that reflects coarse slip and fragmentation of lamellar blocks). The constitutive equations are determined by 5 adjustable parameters that are found by matching the experimental stress–strain curves.

The effect of annealing on the viscoplastic response of semicrystalline polymers at finite strains

Abstract A constitutive model is developed for the viscoplastic behavior of a semicrystalline polymer at finite strains. A solid polymer is treated as an equivalent heterogeneous network of chains bridged by permanent junctions (physical cross-links, entanglements and lamellar blocks). The network is thought of as an ensemble of meso-regions linked with each other. In the sub-yield region of deformations, junctions between chains in meso-domains slide with respect to their reference positions (which reflects sliding of nodes in the amorphous phase and fine slip of lamellar blocks). Above the yield point, this sliding process is accompanied by displacements of meso-domains in the ensemble with respect to each other (which reflects coarse slip and disintegration of lamellar blocks). To account for the orientation of lamellar blocks in the direction of maximal stresses and formation of micro-fibrils in the post-yield region of deformations (which is observed as strain-hardening of specimens) elastic moduli are assumed to depend on the principal invariants of the right Cauchy–Green tensor for the viscoplastic flow. Stress–strain relations for a semicrystalline polymer are derived by using the laws of thermodynamics. The constitutive equations are determined by six adjustable parameters that are found by matching observations in uniaxial tensile tests on injection-molded isotactic polypropylene at elongations up to 80%. Prior to testing, the specimens were annealed at various temperatures ranging from 110 to 163 °C. Fair agreement is demonstrated between the experimental data and the results of numerical simulation. The effect of annealing temperature on the material parameters is studied in detail.

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 Zie...

CYCLIC VISCOPLASTICITY OF SEMICRYSTALLINE POLYMERS WITH FINITE STRAINS: OBSERVATIONS AND CONSTITUTIVE MODELING

Observations are reported on low density polyethylene and isotactic polypropylene in uniaxial tensile cyclic tests (oscillations between maximum elongation ratios kmax and the zero minimum stress σmin with kmax increasing monotonically with number of cycles). Constitutive equations are developed in cyclic viscoplasticity of semicrystalline polymers with finite strains. Adjustable parameters in the stress–strain relations are found by fitting the experimental data. Ability of the model to describe the characteristic features of the mechanical behavior and to predict the viscoplastic response in cyclic tests is confirmed by comparison of the results of numerical simulation with observations in additional tests.