Primary normal stress coefficient measurement

We note that the primary normal stress coefficient P 1 is positive, whereas the secondary normal stress coefficient P2 is negative, but with a lot of scatter in the data. It is difficult to measure (r22 — T33) and its value is in doubt, but the ratio — (tn — X22)/ x22 — T33) appears to be about 0.1. 

The normal stresses c,/ cannot be specified on an absolute basis because of the arbitrary hydrostatic pressure in equation 59, but their differences are predicted by continuum mechanics and several molecular theories and can be measured. The primary and secondary normal stress differences are defined as (Th — an and 022 — an respectively. Data are often expressed in terms of the primary normal stress coefficient... 

The elasticity of polymer melts is manifested through two material functknis, namely, the primary normal stress coefficient ii and the secondary normal stress coefficient ijia. The secondary normal stress coefficient is not as well duuacter-ized as the primary normal stress coefficient due to its small magnitude. The primary normal stress measurements are themselves difficult and require highly... 

The cone-plate rheometer. The cone-plate rheometer is often used when measuring the viscosity and the primary and secondary normal stress coefficient functions as a function of shear rate and temperature. The geometry of a cone-plate rheometer is shown in Fig. 2.47. Since the angle Oo is very small, typically < 5°, the shear rate can be considered constant throughout the material confined within the cone and plate. Although it is also possible to determine the secondary stress coefficient function from the normal stress distribution across the plate, it is very difficult to get accurate data. 

The cone-plate rheometer is used when measuring the viscosity and the primary and secondary normal stress coefficient functions as a function of shear rate and... 

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