str (109)

PLASTICITY

PLASTICITY

method.

at junctions in the apparatus. These can ł effectively investigated by the chronophMHHj

The real naturę of plasticity will next b I considered. There has been much confusion as t the exact meaning of this word, which, ariginatin! ' in the Greek and Latin, really means “mould? ability.” The Chinese potters of ancient times age<J \ their clay, so that it could be moulded into morę I elaborate shapes.¹ The complaint of the Israelites when they were forced to make bricks without straw, may possibly have been morę than a protest at being deprived of a mechanical reinforcement for the clay they used in brickmaking, for it is | possible that certain substances in the straw helped to render the clay morę plastic. Other examples of the use of straw in brickmaking are found in the ancient and the mediaeval world, as for instance in the bricks used to build Saladin’s (Sala-ud-din, 1138-1193) tomb. A process in which plant juices are used to plasticise clay is worked &ommercially now in America, under the name ||| the “ Egyptianisation process.”

Although ceramists have been interested in plasticity for so many centuries, it is still as tnie to-day as it was in 1844, when Brongniart madę the statement, that "on a souvent parlś dh dette proprićte, on semble la connaitre, mais on n’en a qu'une vague idee.” Perhaps the best modern definition is that of Wilson, who describes it as | that property which enables a materiał to be

1HB continuously and permanently without during the appHcation of a force that ^ceeds’ the yield-value of the materiał"; but •fshould be noted that this definition gives no idea how to measure plasticity, nor any idea of its dimensions. An attempt to define plasticity morę precisely might describe it in terms of a critical strain, lite ductility—" the.    to which a materiał

can be deformed continuously and permanentły without rupture, during the appłication of a force that exceeds the yield-value of the materiał.” This modification though attractive is dangerous, because, for metałs, the work of Taylor makes it elear that it is not a strain which is critical, but rather a series of stresses.¹ Scott Blair, however, uses an empirical test in which clay cylinders are rolled until they crumble, at which point their thirmp-ss is measured. This test is far from perfect, and depends partly on the ratę of evaporation of the water.

Although plasticity cannot yet be defined in absolute units, yet like honesty, it is "although undefinable, associated with certain ąualities.”

The word implies a Iow viscosity, at least at higher stresses (sińce the materiał is easy to mould), combined with a yield-value such that shapes can be retained under the stress of gravity. A plastic materiał must also have a high shear modulus, otherwise it would be "springy/' (These aspects will be discussed later.) Although this is true, much confusion has been caused by attempts to define plasticity in terms of mobility and yield-value. " Stiffness,” which is defined as the slope of the straight linę part of Bingham's curve and the

| g_ome workers believe that it is not stress, but some property having other dimensions, that is critical. See Bridgman, J.

Appl. Pftys., i93®» IX., 517.