Sample Length Variable During the Test, Constant Stress

Sample Length Variable During the Test, Constant Stress 

Vinogradov et used a more sophisticated setup compared to that of Cogswell, and showed the equivalence of extensional viscosity data obtained using a constant stretch rate instrument and a constant stress instrument for molten polystyrene. An unexpected feature of the constant stress results was that the strain rate was found to decrease initially, as expected, but it then exhibited a minimum before becoming constant. This implies a maximum in the tensile stress growth coefficient and, in that respect, this behavior was similar to that of linear low density polyethylene as reported by Schlund and Utracki ) among others. 

The major advantage of a constant-stress rheometer over a constant-strain-rate rheometer is that, for a given polymer, a steady state is frequently achieved in the former mode but not in the latter one.(28) Even when a steady state is obtained with the use of both instruments, the total strain needed to achieve a steady state is lower for the constant stress viscometer. This extends the range of strain rates at which the extensional viscosity can be determined for an apparatus of a given size. Finally, it has been observed(4 20) that the stress tends to decrease slightly in a constant stretch rate experiment even after a plateau appears to have been reached. The physical significance of this last observation is not entirely clear. (29) 

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