The subjective judgement of the elastic and plastic properties of soft bodies; the "differential thresholds" for viscosities and compression moduli

Abstract

In measuring the softness of materials which show both elastic and plastic properties, it is customary either to assess the softness and elasticity by means of purely empirical tests, or, more frequently in industrial processes, to rely on the subjective judgment of some experienced person. Little is known of the capacity of individuals to measure such properties by tactual, kinaesthetic and visual observations, save that Katz (1937) has made a preliminary study of the capacity of bakers to judge the properties of flour dough, and concludes that their special skill is not in the main due to any abnormal capacity to judge flow and deformation (“rheological”) properties. Problems of this kind are to be found in many industries, notably bread and biscuit making, cheese and butter manufacture and ceramics. On the theoretical side, the data from such complex materials have been fitted into a double framework originally devised for true fluids and elastic solids respectively. Equations were used containing two terms, one an essentially Newtonian term, but involving a variable viscosity, and the other an essentially Hookian term suitably modified to allow for elastic hysteresis and after-effect. There is much to be said for this treatment. The properties so evaluated are obtained in c. g. s. Units, and are reproducible so long as fixed arbitrary conditions of stress and strain are maintained. Moreover, physicists are accustomed to thinking in terms of viscosities and elastic moduli, and more readily assimilate information presented in such familiar terms. On the other hand, it must be remembered that to divide the phenomena of deformation into viscous and elastic parts involves a division of the strains produced into recoverable and non-recoverable components; and, since some materials show exceedingly slow elastic after-effects, such a distinction is often quite arbitrary. Moreover, for very complex systems such as are met with in nature and dealt with in industry, the Newtonian-Hookian framework is decidedly artificial.