510 research outputs found
Polymer rheology for melt processing : complex flows
No full textThe rheological behavior of polymer melts plays an important role during processing since it not only determines the extra stresses and pressure gradients and can reduce flow instabilities that yield process limits, but it also couples material, process, and production properties. Examples of (final) product properties determined by the process are frozen-in orientation, the structure of semicrystalline polymers, and long-term dimensional stability. For quantitative predictions of these qualities, the availability of reliable constitutive models is a prerequisite. Here, the performance of a new generation of constitutive differential equations (Verbeeten et al. 2000 W.M.H. Verbeeten, G.W.M. Peters and F.P.T. Baaijens, Differential constitutive equations for polymer melts Proc. XIII Int Congress on Rheology, British Society of Rheology, Glasgow, UK (2000).Verbeeten et al. 2000) based on the Pompom model (McLeish and Larson 1998) is presented
A stretch-based model for flow-enhanced nucleation of polymer melts
No full textA phenomenological model for flow-enhanced nucleation in crystallizing polymers is developed and validated by short-term shear experiments from Hristova, Peters et al. (2004) and Housmans, Steenbakkers et al. (2009). The model extends earlier work on flow-induced oriented crystallization [ Custodio, Steenbakkers et al. (2009); Peters (2003); Peters, Swartjes et al. (2002); Swartjes, Peters et al. (2003); Zuidema (2000); Zuidema, Peters et al. (2001)] to flow-enhanced pointlike nucleation, which can lead to number densities of spherulites multiplied by orders of magnitude. Excellent agreement between simulations and experimental data is obtained with only two free parameters: a prefactor to the creation rate of flow-induced nucleation precursors and a parameter that governs their influence on the relaxation dynamics of the high molecular weight (HMW) fraction of the melt. The two main conclusions of this paper are, first, that the creation of flow-induced precursors is driven by stretch, not by orientation, of the primitive path of chains in the HMW tail of the molecular weight distribution, and secondly, that nucleation of theseprecursors is impeded by flow
A model of human skin under large amplitude oscillatory shear
No full textSkin mechanics is of importance in various fields of research when accurate predictions of the mechanical response of skin is essential. This study aims to develop a new constitutive model for human skin that is capable of describing the heterogeneous, nonlinear viscoelastic mechanical response of human skin under shear deformation. This complex mechanical response was determined by performing large amplitude oscillatory shear (LAOS) experiments on ex vivo human skin samples. It was combined with digital image correlation (DIC) on the cross-sectional area to assess heterogeneity. The skin is modeled as a one-dimensional layered structure, with every sublayer behaving as a nonlinear viscoelastic material. Heterogeneity is implemented by varying the stiffness with skin depth. Using an iterative parameter estimation method all model parameters were optimized simultaneously. The model accurately captures strain stiffening, shear thinning, softening effect and nonlinear viscous dissipation, as experimentally observed in the mechanical response to LAOS. The heterogeneous properties described by the model were in good agreement with the experimental DIC results. The presented mathematical description forms the basis for a future constitutive model definition that, by implementation in a finite element method, has the capability of describing the full 3D mechanical behavior of human skin
Thermodynamic admissibility of the extended Pom-Pom model for branched polymers
No full textThe thermodynamic consistency of the eXtended Pom-Pom (XPP) model for branched polymers of Verbeeten et al. [W.M.H. Verbeeten, G.W.M. Peters, F.P.T. Baaijens, Differential constitutive equations for polymer melts: the extended pom-pom model, J. Rheol. 45 (4) (2001) 823–843; W.M.H. Verbeeten, G.W.M. Peters, F.P.T. Baaijens, Differential constitutive equations for polymer melts: the extended pom-pom model (vol 45, pg 823–843, 2001), J. Rheol. 45 (6) (2001) 1489] as well as its modified version [J. van Meerveld, Note on the thermodynamic consistency of the integral pom-pom model, J. Non-Newtonian Fluid Mech. 108 (1–3) (2002) 291–299] is investigated from the perspective of non-equilibrium thermodynamics, namely the General Equation for Non-Equilibrium Reversible–Irreversible Coupling (GENERIC) framework. The thermodynamic admissibility of the XPP model is shown for both its original and modified form. According to the GENERIC formalism, the parameter a introduced by Verbeeten et al. to predict non-zero second normal stress in shear flows must fulfill the condition 0 = a = 1
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