Shear rate experiment, build

A step shear rate experiment which studies the build-up of structure at low shear rate after the sample has been "sheared out" at a high shear rate. 

In the experiments, two main types of time-dependent flows have been studied start-up flows and stress relaxation. In the start-up flow experiments, shear flows with constant shear rates and elongational flows with constant elongational rates are started in the system in equilibrium under no external force, and the time-dependent stress build-up in the system is measured. In the stress relaxation experiments, constant deformations are applied to or removed from the system, and the time-dependent relaxation of the stress is measured. In this section, we study these two types within the framework of transient network theory. 

The above behaviour can be explained from consideration of the structure of the system. If, for example, the suspension is weakly flocculated, then on applying a shear force on the system this flocculated structure is broken down (and this is the cause of the shear thinning behaviour). On reducing the shear rate back to zero the structure builds up only in part within the duration of the experiment (30 s). 

Figure 9.18 refers to two other standard experiments. It depicts the results of stress growth experiments, conducted again on a polyethylene melt. The figure includes both measurements probing shear and tensile properties, thus facilitating a direct comparison. Curves show the building-up of shear stress upon inception of a steady state shear flow at zero time and the development of tensile stress upon inception of a steady state extensional flow. Measurements were carried out for various values of the shear rate 7 or the Hencky rate of extension ch ... 

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