Note: Method for overcoming ductile failure in Munstedt-type extensional rheometers
McGrady, C. D.
Baird, Donald G.
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The purpose of this note is to introduce an experimental technique to overcome ductile failure in uniaxial extensional flow and show a simple illustration of its effectiveness. Ductile failure prevents the rheological measurement of transient stress growth at higher strains for certain strain-hardening materials. This reduces the accuracy of nonlinear parameters for constitutive equations fit from transient stress growth data, as well as their effectiveness in modeling extensionally driven processes such as film casting. We propose an experimental technique to overcome ductile failure called encapsulation in which the material that undergoes ductile failure is surrounded by a resin that readily deforms homogeneously at higher strains. The materials are arranged in concentric cylinders and are deformed in parallel. A simple parallel model is shown to calculate the viscosity of the core material and is tested for its robustness in a linear low-density polyethylene/low-density polyethylene system in which the viscosities of each resin are known at all strains, strain rates, and temperatures investigated. Agreement is found at each strain rate investigated across all strains studied. The method is then extended to measure the viscosity of a sparsely branched high-density polyethylene (HDPE) system that undergoes ductile failure at higher strain rates. The technique shows good agreement with experimental results up to the onset of ductile failure in the pure resin and predicts viscosity growth of the HDPE resin well beyond the critical strain for the onset of failure seen in experimental measurements of the pure HDPE. Repeatability of the technique is also illustrated. (C) 2009 The Society of Rheology. [DOI: 10.1122/1.3109574]