The relationship between viscosity, elasticity and plastic strength of soft materials as illustrated by some mechanical properties of flour doughs, I

Flour dough belongs to a group of materials in which a high degree of plasticity is combined with considerable elasticity. Owing to their great industrial importance, a number of technological investigations have been carried out on these materials (of which unvulcanised rubber is another important example); but the problem of bringing the description of their behaviour within the scope of a general theory of viscosity and elasticity has hardly been tackled. The time during which a stress is applied is as important as the magnitude of the stress in determining how much of the deformation is elastic (recoverable) and how much plastic (non-recoverable). This fact suggests that a formulation based on Maxwell’s “time of relaxation” should be of value in this connection. The formulation as Maxwell gave it applies to a true fluid, for which case the relaxation is exponential and for which the rate of dissipation of internal stress is proportional to the stress, the constant of proportionality being the reciprocal of the relaxation time. Thus ─ds /dt = 1/t r S, or 1/t r = ─1/Sd S/dt =d (loge S)/dt , where S is the shearing stress, andt r the relaxation time.