The relationship between viscosity, elasticity and plastic strength of a soft material as illustrated by some mechanical properties of flour dough.—II

In an earlier communication Maxwell’s “time of relaxation” was given an extended and more general definition so that the conception might be used in describing the behaviour of plastic substances. From a study of such materials in steady flow it is well known that the viscosity defined as the ratio of the shearing stress, S, to the velocity gradient or rate of shear, G, is not a constant but usually decreases as S increases. The time of relaxation, tr, is related to the viscosity, n, thus tr = n/n, n being the rigidity modulus. Since n is normally independent of the stress, tr and n show parallel variations. For ordinary fluids tr is very small and no way has yet been devised for measuring it. In flour dough, however, we have a material in which high viscosities are combined with a low rigidity modulus, and consequently the relaxation of internal stress is slow enough to be easily followed experimentally. RESP-865,885,910