Constant Strain Rate

Constant rate extensional flow corresponds to a deformation in which the Hencky strain increases linearly with time according to ∈ = ∈ t where ∈ is the Hencky strain rate. Constant rate extensional flow will result in very large deformations of the material and will typically bring forth material features that are not revealed in small amplitude oscillation. Constant rate extensional flow is frequently employed to test the predictions of continuum or molecular models for the nonlinear behavior of polymeric systems.

In extensional rheology polymer melts and solutions typically undergo strain hardening, which is denoted by a upwards deviation of stress as a function of time from the linear prediction. This is clearly seen in Figure 1 below.

Typical data collection for this type of experiment is steady state extensional viscosity as function of strain rate.