| It is well known that non-Newtonian behavior may manifest when the liquid under study is a high molecular weight polymer, a mixture of polymer and low viscosity liquid, or a suspension. However, it will be shown that at sufficiently high shear stress even traditionally "Newtonian" liquids become highly non-linear and that they display a yield stress analogous to that of solids and of roughly the same magnitude. This phenomena has implications for any engineering application for which a very large shear stress must be sustained by a liquid such as metalworking, bearing lubrication, liquid jet machining and ballistic impacts.;Included in this work are measurements of the variation of viscosity with pressure and temperature and the determination of the glass transition temperature and dielectric relaxation time at elevated pressure. These studies motivated the search for "solid-like" response of liquid lubricants at high pressure and subsequently, measurement of elastic modulus and yield strength. The observed variation of yield strength of these liquids with shear rate led to the conclusion that a rate independent limiting shear stress model is appropriate. Measurements on liquids above the glass transition temperature showed that the limiting stress concept governs the high shear stress rheology in the liquid state as well as for amorphous solids. A constitutive law is presented which includes viscous, elastic and plastic behavior and appears to be consistant with all observed behavior in uniaxial shear.;In the progress of this work, many unique rheometers were developed. Shear stresses were generated in liquids essentially free of thermal effects to levels about one hundred times greater than had been achieved before. Viscosities as high as 10... |
