A test sample to standardize measurements of normal stress
In the early 1940’s, Garner, Nissan and Wood (1)1) and Weissenberg (2) made the first attempts to measure normal stress2) in viscoelastic fluids undergoing shear. Fifteen years later, by the late 50’s, a considerable body of data on various fluids and techniques had been generated. Markovitz and his colleagues, recognizing the importance of collecting different measurements on one sample, completed an extensive program on a solution of polyisobutylene. In surveying this work in 1958, Markovitz (3) said ... “The data lead us to the conclusion that two contradictory types of theory are needed to correlate different pairs of experiments. Obviously, there is something wrong. We need more and better experiments to decide among the various possibilities.” The statement could well be made at this Congress, although the 14 years intervening have been filled with activity, devising new methods, collecting data and increasing our understanding.
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