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.
Hot-drawing of poly(ethylene terephthalate) under biaxial stress: application of a three-dimensional glass—rubber constitutive model
Abstract A study was made of the hot-drawing of amorphous poly(ethylene terephthalate) (PET) under biaxial stress in the temperature/strain-rate regime relevant to industrial film drawing. Film specimens were drawn at constant width at temperatures 75–120°C and strain-rates 1–16 s −1 , using a specially developed testing machine—the Flexible Biaxial Film Tester. The stress-strain data obtained, together with similar results from uniaxial tests, allowed a recently-proposed three-dimensional constitutive model for amorphous polymers to be tested and applied to PET. It was found that the yield, flow and strain-stiffening observed in drawing could be described well by the model at the lower temperatures. A procedure was devised for systematically fitting data to the model, and produced values of the model parameters which were interpreted in terms of the physical mechanisms of flow and rubber-like strain stiffening. The major inadequacy of the model was that it did not include flow resulting from entanglement slippage, modified in PET by stress-induced crystallization, that intervenes at the upper end of the temperature range.
The distribution of 54-Mn in the enamel and dentine of the rat incisor.
Abstract The distribution of 54 Mn was studied in the lower incisors of rats. Autoradiograms show 54 Mn to be present in the enamel and dentine 5, 10 and 15 days after its administration. In the dentine, the radioisotope is first localized in a zone near to and encircling the pulp and later becomes more diffuse. Pre-existing as well as the newly formed enamel incorporates the isotope. In both enamel and dentine 54 Mn appears to be principally associated with the inorganic portion.