On the "viscosity maximum" during the uniaxial extension of a low density polyethylene

TL;DR

This study investigates the viscosity maximum during uniaxial extension of low density polyethylene, revealing that the observed viscosity overshoot is due to deformation inhomogeneity, not material rheology, across different strain rate regimes.

Contribution

It provides a detailed experimental analysis combining integral and local measurements, clarifying the origin of the viscosity maximum in polyethylene extension tests.

Findings

Viscosity maximum linked to deformation inhomogeneity and secondary necking.

No true viscosity overshoot occurs in homogeneous deformation regimes.

Different deformation regimes exhibit distinct stress and viscosity behaviors.

Abstract

An experimental investigation of the viscosity overshoot phenomenon observed during uniaxial extension of a low density polyethylene is pre- sented. For this purpose, traditional integral viscosity measurements on a Muenstedt type extensional rheometer are combined with local mea- surements based on the in-situ visualization of the sample under exten- sion. For elongational experiments at constant strain rates within a wide range of Weissenberg numbers (Wi), three distinct deformation regimes are identified. Corresponding to low values of Wi (regime I), the tensile stress displays a broad maximum. This maximum can be explained by simple mathematical arguments as a result of low deformation rates and it should not be confused with the viscosity overshoot phenomenon. Corre- sponding to intermediate values of Wi (regime II), a local maximum of the integral extensional viscosity is systematically observed. However, within this regime, the local viscosity measurements reveal no maximum, but a plateau. Careful inspection of the images of samples within this regime shows that, corresponding to the maximum of the integral viscosity, sec- ondary necks develop along the sample. The emergence of a maximum of the integral elongational viscosity is thus related to the distinct in- homogeneity of deformation states and is not related to the rheological properties of the material. In the fast stretching limit (high Wi, regime III), the overall geometric uniformity of the sample is well preserved, no secondary necks are observed and both the integral and the local transient elongational viscosity show no maximum. A detailed comparison of the experimental findings with results from literature is presented.