Model material parameters used in viscoelastic constitutive equations

1 Model (material) parameters used in viscoelastic constitutive equations 

Stress relaxation time, obtained from rheograms based on viscometric flows, is used to define a dimensionless parameter called the Deborah number , which quantifies the elastic character of a fluid 

The rate of deformation tensor in a pure elongatioiial flow has the following form 

The elongation viscosity defined by Equation (1.19) represents a uni-axial extension. Elongational flows based on biaxial extensions can also be considered. In an equi-biaxial extension the rate of deformation tensor is defined as 

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