The Classification of the Rheological Properties of Industrial Materials in the light of Power-Law Relations between Stress, Strain and Time

An earlier empirical classification of rheological properties is discussed in the light of a power-law relation between stress, strain and time. Partial differentiations of stress, strain and time products with respect to stress and strain (but not to time) give expressions which tally comparatively well with the earlier empirical classification. Reasons are given for preferring the power-law treatment to the more usual process of dividing strains arbitrarily into recoverable and non-recoverable parts, both where purely physical concepts are in question and also in studying such conceptions as `firmness' when judged subjectively by handling materials. A tentative explanation is given for the fact that while the elasticity of rubber can be judged by handling about three times as accurately as can the viscosity of bitumen of the same order of firmness, the subjective and objective time units involved in a direct comparison between these materials are also related in the ratio of three to one.