Extensional viscosity
With extensional viscosity is the viscosity in elongation, usually in the uniaxial stretching, meant. Tensile stress occurs not only in the solid state at (tensile test ), but also in industrially relevant flow patterns.
The extensional viscosity is defined as
In which
- The true stress
- The true strain rate is.
In many cases, the extensional viscosity is in a constant ratio to the shear viscosity. In the case of uniaxial strain, the rheologically main form, it is three times as high - the Trouton ratio:
This can be derived from the relationship between shear and tensile modulus for a Poisson's ratio of 0.5, ie an incompressible fluid.
For Hencky strains of typically greater than 0.7 occur in many non-Newtonian materials deviations from Trouton ratio on:
- For materials with fillers a lower extensional viscosity is commonly found than expected from the Trouton ratio, what is known as Dehnentfestigung ( in English strain softening referred to ):
- Materials with specific molecular properties esp. with long chain branches, however, have a higher extensional viscosity than would be expected from the Trouton ratio. This is called strain hardening (English strain hardening ) may refer to:
Extensional flows are dominant in the following manufacturing processes of plastics:
- Fiber spinning
- Film blowing
- Deep drawing
- Blow molding
In addition, extensional flows in the following processes essential:
- Reduction of cross-sections ( corresponding to the venturi effect ), also called Kapillareinlaufströmungen.
- Magnification of cross-sections, also called Kapillarauslaufströmungen.