Technikatörténeti szemle 10. (1978)

A MÉRÉS ÉS A MÉRTÉKEK AZ EMBER MŰVELŐDÉSÉBEN című konferencián Budapesten, 1976. április 27–30-án elhangzott előadások II. - Tálos Gy.: A reológiai mérési módszerek fejlődésének története Newtontól napjainkig

of the measurements. These are the special viscometric flows of the incompressible simple fluids, which represent the steady flow behaviours of these fluids. To begin with I should like to mention that the rheometry is such a field of sciences for which the physical and chemical properties of the systems to be measured, the mathematical flow model describing their deformations and the instruments constructed for the determination of theirs are in an inalienable relation with each other, they presume each other. So, they cann't be separated in this lecture either. In the viscometers, some force induces stress in the inner parts of the fluid and the proper stress-functions or their time-derivatives are brought into connection with the raising strain-functions or their time-derivatives. It is the relation of these consistency-variables, that is described with the rheological equations, which already contain the proper viscosity-coefficients or different rheological moduli. If this function is linear; the fluid is Newtonian, if it isn't linear; the fluid is non-Newtonian. But the term of the non-Newtonian flow behaviour isn't as simple as that. Apart from the non-linear coefficients (shear dependant viscosity), normal stress effects, the dependancy of the stress from the higher order time derivatives of the shear rate and the phenomenon of the stress relaxation (viscoelasticity) might taken place too. To describe them, not only the deformation in that moment, but the earlier histories of the deformations are important. All these phenomena are summarized in the theory of the simple memory fluids, for special flow situations we can give the exact mathematical terms of the theory of materials with fading memory. As for the instrument; the essence of design is that in the gap of the apparatus, the established flow field has to correspond to that supposed in the theory. If it doesn't, the readings of dials on the most precise and modern force- and time-measuring attachments are of little significance. In addition to the construction of the instruments such points of views have to be considered as the physical and chemical natures of the specimens the apparatus must tolerate, the range of stresses and shear rates to be covered, the level of accuracy to be sought and the available time and money. To harmonize all of them—that is to produce a simple steady flow of the material—is very difficult. (Which is the aim of the rheometry.) So, it can be seen that the researchers practically were making no headway until the beginning of this century, because—with the exception of a few of them—opposite to the above mentioned they were dealing with the complex deformation of the ideal material. (Which is the aim of the hydrodinamics.) After Newton, at the beginning of the 19. century the researches on fluids have only been commenced. Amongst the physicists of this time Navier, Stokes and Poiseuille overtook the others. The dynamic equations of the ideal fluid had been come into being on the basic experimental works of Navier and Stokes and therefore these equations were denominated after them. Stokes had set up the equation of the falling ball viscometers too. Poiseuille made blood and some different fluids flowed through narrow cylindrical capillary tubes and with Hagen set up the law named after them, which was the basic equation of the capillary viscometry. In the second half of the century, it was modified by Hagenbach and Couette and changed into its present form, which was used for works of the viscosity standards. At the same time Couette had constructed the first rotational viscometer in 1888. Amongst the experimenters of the second part of the century Maxwell's and