On Viscoelastic Viscoelastic Viscoelastic Modelling of Poly Ethylene Terephthalate Behaviour during Multiaxial Elongations Slightly over the Glass Transition Temperature
The mechanical response of Polyethylene Terephthalate (PET) in elongation is strongly dependent on temperature, strain and strain rate. Near the glass transition temperature Tg, the stress-strain curve presents a strain softening effect vs strain rate but a strain hardening effect vs strain under conditions of large deformations. The main goal of this work is to propose a viscoelastic model to predict the PET behaviour when subjected to large deformations and to determine the material properties from the experimental data. To represent the non–linear effects, an elastic part depending on the elastic equivalent strain and a non-Newtonian viscous part depending on both viscous equivalent strain rate and cumulated viscous strain are tested. The model parameters can then be accurately obtained trough a comparison with the experimental uniaxial and biaxial tests.
On the rheological modeling of viscoelastic polymer liquids with stable constitutive equations
Various approaches to the theology of polymer solutions and melts have resulted in an overabundance of constitutive equations (CEs). None of these equations can describe the entire set of available rheological data in a consistent fashion, even for simple flows. Additionally, almost all the CEs suggested in the literature have recently been determined to be unstable in either the Hadamard or dissipative sense without any physical reason. In this paper, we attempt to describe simple flow data for LDPE Melt I/IUPAC A/IUPAC X, HDPE II. PS I, PIB P-20 and PIB L-80 with a certain class of differential CEs, while complying with the global isothermal stability constraints. For simplicity, only one or two nonlinear parameters additional to the discretized linear viscoelastic spectra are introduced for the description of data.
The risk of Listeria monocytogenes infection in beef cattle operations
Aim:  To determine the prevalence of Listeria monocytogenes and associated risk factors among beef operations (cow‐calf and feedlot) in central and southern California.