Robertson model, yield stresses

Comparison with Eq. (6.24) shows that the Herschel-Bulkley model is the power-law model with the addition of a yield stress. Another such derivative model is the Robertson-Stiff [379] model ... 

C0 yield stress constant, Robertson-Stiff model (s 1)... 

The model can be used to predict the shear rate as a function of shear stress for a range of temperatures. In the original paper a lack of shear stress data meant that the predictions were compared to tensile stress-strain data hy decomposing the tensile data into shear and biaxial components. The point on the tensile stress-strain data that Robertson took to be the most appropriate stress to compare with the computed strain rate was the yield point. The model gives values for the yield stress and the temperature dependence of the yield stress that agree well with experiment for a number of polsrmers. 

The Robertson model was extended by Duckett and co-workers in 1970 (29) to account for the pressure dependence of the yield stress. By their argument, if the two states are trans and cis and the effect of stress is to increase the number of cis conformations, then this implies a lower resulting density since the packing... 

Molecular Theories of Yielding. Several models of yielding of amorphous polymers are based on continuum mechanics invoking the concepts of free volume, springs and dashpots, and viscosity. Some of the theories take molecular point of view. Robertson (26,27) assumed chain bending as the fundamental mechanism for yielding. He postulated that the 3rield point corresponds to the state where the amount of bends produced by the stress were equivalent to the number of bends that occur at Tg. 

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