Stress Analysis near a Slit Exit by Flow Birefringence Method

Abstract

Measurements of birefringence and streamlines near the exit of a slit were carried out for the steady laminar flow of high density polyethylene Sholex 6008 melt. Distributions of deviatoric stress intensity and its direction were obtained by graphical method from the isochromatic and isoclinic trajectories, respectively. The results provided basic informations on the mechanical feature of the Barus effect, i. e., the enlargement phenomenon of the cross-sectional area of the extrudate than that of the nozzle. Observed directional change of the streamline suggested that large swelling was developable around the center plane in spite of small first normal stress difference there. On the other hand, near the slit wall, particularly near the slit edge, the polymer melt was highly stretched to the direction of flow and the positive normal stress was generated to the same direction. These observations were compared with the computer results obtained by Tanner et al. under Stokes' approximation for a particularly slow Newtonian liguid flow about the exit of a circular tube. Normal stresses normal to the duct wall surface was clearly generated in a short distance just before and after the exit plane. This was in contrast to the present observation where the normal stress varied over a wide range of distance from the exit plane. Thus the distribution of deviatoric normal stresses near the slit exit, suggests that the Barus effect is a kind of solid-like retarded elastic recovery phenomenon in response to the rapid release from the restriction of the sustaining duct wall.