  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.
