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24 Y1ELD-V ALUES

he calls the " upper or maximum yield-value ” (y). This nomenclature has not been very generally followed, and is not very satisfactory.

It is elear that two materials having different shearing strengths and mobilities may reverse the order of their apparent viscosities at different stresses.

Thus in Fig. 5, at stress 1, A is morę viscous than B, while at stress 2, B is morę viscous than A.

The mixed effect of both these factors, which produces a generał impression of resistance to flow, is called “ consistency,” which is described as " that property of a materiał by which it resists pennanent change of shape, and is defined by the compłete force-fłow rełation.” This definition was proposed by the Society of Rheology; but the

PRACTICAL APPLICATIONS 25

term is very often used to include elastic (nom permanent) deformations as well.

Applying Bingham’s theory to practical problems, Wilson and Hall obtain flow curves for clay slips. These are fairly linear, and the mobilities and shearing strengths (using the modem terminology) are found to characterise the clays. These authors madę the mistake of supposing that if clays are compared at a constant mobility, by suitable adjustment of the moisture contents, the shearing strengths would be a measure of the plasticities; moreover, too smali a number of clays was tested. Shearer obtains similar curves, and studies the effect of electrolytes on them. Bleininger develops the whole subject and claims that in very dilute suspensions, where Poiseuille’s law is approximately obeyed, “ equal concentrations (of suspension) will show fluidities inversely proportional to the plastic naturę of the materiał.” Scott Blair and Crowther, and later Richardson, show that the Bingham treatment applies in generał to soil pastes, but that various modifications must be introduced. These will be discussed in the next chapter. Talwalker and Parmelee find that clay rods under torsion obey the Bingham principle, the yield-value being a good indication of the “ fatness ” of the clay. Moness and Giesy apply the same principle to tooth-paste, and obtain results which show that the Bingham treatment is generally applicable to this materiał: they use a Bingham-Murray plastometer, which will be discussed in Chapter V.

The Bingham principle has been applied to paints with yarying success. The flow properties of paint are naturally very important:    the brushing

properties must be satisfactory: the levelling property is important, sińce brushmarks must