Normal Stresses at High Shear Rates in a Polymer Solution

Abstract

Based on an examination of the characteristics of shear viscosity versus shear rate, it was postulated that high tensile and compressive stresses might exist in certain liquids at very high shear rates. If obtainable, these stresses could be important as load-bearing mechanisms in high-speed machine elements, and as a sealing mechanism in radial face seals. Such stresses should be evident in a polymer fortified oil, or in a liquid comprised of molecules possessing an appreciable length to width ratio. Therefore, a jet reaction viscometer reaching 107 sec−1 shear rate was developed to explore this possibility. Tests with polyisobutylene dissolved in a kerosene showed that elastic stresses were dominant with respect to viscous stresses at high shear rates. Tensile stresses up to more thn 1000 psi were obtained. However, the life of the polyisobutylene molecule was short. Hence it is concluded that normal stresses of appreciable magnitude can exist in high-speed machine elements under favorable conditions to affect their operation.