On the Normal Stress Measurement in Couette Flow

Abstract

We examine the principle of the normal stress measurement with a sliding plate rheometer proposed by Oakley and Giacomin (Polym. Eng. Sci., 34, 580 (1994)). In this rheometer, a sheet of material is held between a pair of plates and a simple shear is applied by the parallel motion of the plates. They state that the pressure of the material on the plate is equal to the first normal stress difference, N1, and the negative second normal stress difference, -N2. This statement is evidently wrong. It is shown here that the pressure at the rim of the sheet can be related to the normal stress differences. The pressure at any point is equal to N1 if the sheet is infinitely long in the neutral direction and it is equal to -N2 if the sheet is long in the shear direction. These conditions can be fulfilled with a coaxial cylinder system if the radius is large and the gap is small. The first normal stress difference in oscillatory shear may be evaluated from the pressure in translational (piston-like) motion of a coaxial cylinder system.