An investigation of the laws of plastic flow

Experimental results 312 III.Types of viscous and plastic flow 319 Case I. Viscous flow-Homogeneous fluids 319 Case 2 .Viscosities additive-Emulsions 320 Case 3. Fluid mixtures-Fluidities additive 321 Case 4. Suspensions-The simplest case 326 Case 5. Seepage 326 Case 6. Nonplastic suspensions 327 Case 7. Plastic suspensions 329 IV. Discussion of experimental data 331 1 .Law of pressures 331 2 .Friction constant 335 3 .Mobility 336 4. Slipping 340 V. Relation of results to earlier work on plasticity 341 VI.Models of plastic flow 351 VII.Summary 352 3IO BvJletin of the Bureau of Standards [Voi.13Since glass and other similar bodies are often regarded not as solids but as very viscous liquids, the demarcation of viscous flow from plastic flow has not been sharply made.In fact, attempts have been made to glv^numerical values to the viscosity of ice, menthol, glass, and pitch, and Tammann^defines plasticity in a perfectly definite manner as the reciprocal of viscosity-in other words, plasticity and fluidity are synonymous.Unfortunately, for the sake of simplicity, this definition is clearly untenable.If any finely divided solid, such as clay, be suspended in a liquid, the fluidity is lowered rapidly and in a perfectly linear manner, so that at a comparatively low concentration of clay the fluidity, as measured in the ordinary viscometer, approaches zero.Thus Durham and Bingham ^found that a certain clay suspended in water gave a zero fluidit}^when the volume percentage had reached 6.95 (14.6 per cent by weight), this being independent of AJJ Jl