ABSTRACT
For a fluid at rest. it is easy to show Ill that all the surface stresses that are present within the fluid are normal stresses and that at any location. these are not only equal in magnitude but also independent of the orientation of the surface. This single normal stress is compressive in character, and it is commonly called the pressure and denoted by the symbol p. From the definition of a fluid. the absence of motion of course implies the absence of shear stresses. Conversely, for a fluid in motion, it must necessarily be true that either shear stresses are
present or the normal stresses are unequal. For the situation depicted in Figure I .I. we have seen that flow arises due to the application of a shear stress. If the liquid being sheared is polymeric. one finds that the normal stresses along the three coordinate directions (see Fig. 1.3) become unequal; for Newtonian fluids. however. normal stresses remain equal to each other. Due to the fact that most liquids can be considered incompressible for all practical purposes. the application of equal normal stresses (or pressure) along the three coordinate directions docs not lead to deformation or a change in volume: it is only unequal normal stresses that cause motion and deformation. As a consequence. one cannot determine the absolute value of the pressure from a measurement of 11uid deformation, which is a measure of the change in distances between material points or material planes: one can only deduce normal stress differences.
