On rheological relations
Those who are serious about their desire to formulate constitutive equations which actually describe the behavior of visco-elastic media naturally wish to be able to have their equations subjected to experimental test. To this end, it is fruitful to seek to derive from a given theory relations among observable quantities. By an observable quantity in the context of our subject we mean a quantity which may be defined operationally in terms of a given flow or deformation history which may be carried out experimentally to within acceptable accuracy, i.e. a quantity which may be determined by rheological measurements. Steady state shearing stress, normal stress and complex dynamic shear modulus are examples of such quantities as are also elongational viscosity and the infinitesimal shear-relaxation modulus. We do not believe that there would be any objection if we referred to such quantities as “rheological quantities” and to relations among them as “rheological relations”.
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