PLASTICITT* BY E C G E S E C . BINGHAJI
The term p l m t i c is much inore familiar in our language than inany scientific terms. such for example as the term L'Z'SCOILS. It seems to have long been recqnizetl that plasticity is exclusively a property of solid5 anti that it is complex in cha.racter; i. e. a substance is plastic which nJt only can be readily molded or deformed under pressure or inore exactly under shearing stress, but, also n-ill hold its shape when the shearing stress is removed. Clay has been conceived to be the plastic substance pnr excelleiice hecause it is easily shaped in tjhe hands of the pott,er and once given an exact shape retains it. Ruttcr noultl seen1 to offer ail equally good exaniple. This liehavior is in sharp contrast to that of pit,ch which flies to pieces when one tries to deform it quickly and when riven an exact shape will not hold it. pernimentmlyunless const rained. Our discussion of plasticity therefore concerns itself with the flow of solids, which is certainly broad enough to suit the most, catholic taste. for the Greek philosopher IIera.klitus was literally correct when he said t'hat "E~'erjgt h i n g ~flows". I t is t,herefore necessary t o limit our discussion by excludiiig the flow of those things which we are accustomed to refer t o as fluids, the pure liquids and gases. Rut the circle of our lives is not concerned priiicipall?- wit,h the fluids even air and water but with plastic materials. Our very bodies. the foods we eat, and the materials which we fashion i n our industries are largely p1ast)ic solids. It, may not lie obvious at, once that) all of these are plastic or that a knowledge of the laws of plastic flow will enaldrx man tc) fashisn an[! use matt,er more intelligently. Nost products are soft, at some stage of manufacture or use or have a definite tensile strength and if this strength is exceeded the materials will berid or break. Why say that a knowledge of tjhe laws of plast'ic flow will enable us to nianuf'acture hetter art'icles ant1 more economically? The st,atenient may be true but it is unscientific. Let, us put it t,his way. Investigation leads us to the belief that plastichy is made up of t)wo funtlainental properties which have been samecl yield rnltte and mobility. the former being dependent. upon the shearing stress required to start tlhe deformation and the mobility being proportional to t8he rat'e of deformation after the yield value has been exceeded. Although the conception of plasticity is as old as language itself, it must be kept in mind that plasticity has never been measured until recently, due to the property being complex. If time proves the above reasoning to be correct', we have now- available two ~'zindainentnlproperties of matter which can be measured. Given t'wo new fundamental propert'ies of matter, what need is there t,o ask whether a consideration of these properties may be of importance t'o science or industry'?
* Introduction t o Plasticity Symposium, Lnfayrtte College. Oct.
17
(19243.
One might amuse one’s self hy inaking a list of those fields of industry or science in vhich plastic flow pla a part and it niight provc suggestivr. It ~vouldinclude a coiisitleratioii of suspensions and cinulsions with smokes, filnis and foains, etc.. constituting practically all that we clciioiiii~iat~c under the head of Colloid (,*lieinistry. Under what we n1ay call ~ o ~ - p o l aco1loid.s. r it would include paint, h i e and plaster. foritlant, cia!.. e~ianiels.dyes. carbon black, asphalt ancl other bitmninous products, iiietals and dloys, slags, cements. and rocks deep in the earth’s crust. I7ntlerpolrrr col/oidS it might incluclc, st8arch,cellulose, nitrocellulose, cellulose esters, viscose. plastics ant1 condensation products, soaps, fats and preases. r u l h c ~ gelntiw , a i d glue, leather, gluten and flour. chocolate. chcning puin, hlootl and albunien. For pootl nieasure we may add that lubricating oils, glass, varnish, dental iinpression compounds are not yet excluded, since they are not aln-ays viscous liquids. From one point of view this classification by industries is unsatisfactorJsince it might lead to elliphasis h i n g placed on dissimilarities. On the cont’rary it i s hoped that this Syniposium will bring about exactly the oppositc result and prove that the problems of widely diverse intlustries and workers are essentially the same. By establishing a clearing house of t*heinformation which n e already have, it may be fourid possible to devise hetter plans for further attack and t o conserve effort. 3Iore useful than the clussification 11)- industries is the one according to properties. There are many properties of inaterids which depend upon plasticity aiid they have never 1)eeIi sharply clefined and ineasurerl due t o the lack of any met8hodfor measuring plasticity i-tself. It inay I)? stated with some eriiphasis that, this state of things is not due to the properties in question being unimportant. The terms hnrdrier,s, terisilt streizgtii, d i ~ c t i l i t ~toughness, /, malleabilitg ancl elasticity are of the utmost iriiportnnce in the w h ~ l efield of tic flon plays a r61e in connection with each of these procg is a tlerm used in connection with clay, lime, greases, cement. etc. To l i e sure ~ v espeak of n suhstmce having the consistency of butter as if consistency was a well-defined quantity. Iiut the consistency of butt8er varies enormously Ti-ith the temperature. II-heii a vegetable oil is hydrogenated to resemble lard is it the yield value that is important or the mobility or must, both properties accord with those as found in lard:’ K h a t is meant, by painting consistency and trowel consistency’.’ Length and short,ness are anot’her pair of important properties in plastic flow which we recognize in connection with glue, rubber, paint, steel, as well as t>hedough of the baker. It seems probable -thsit substances wish a high yield value and high mobility arc always short, while substances with a very low yield yalue and Tery low inohility are long. It is possiblc therefore tha.t t,he measurement of plasticity of a substance will enable one to determine the length or shortness of a material. The flow of plast,ic substances has proved iiiost tantalizing for whereas the flow seemed to be very characteridic. the viscosity as measured by the st,andard methods was found to vary n-itlely xiih the shearing stress. Such
PLASlICITY
I203
inexplicable behavior inade the viscosity valueless; but by using the plasticity, the anomaly is esplained and progress seems possible. lIist,ures do not, generally have a definite melting point but a melting renge through which the material is plastic. There may tie a eutectic temperature and a temperature of complete fusion. 11.hen the melt is very visCOUP 2s is the case in many industrially iniportant substances such as asphalt. pi;ch. parafin. fats, the standr:rcl methods for obtaining melting points are highly inaccurate due to overheating or undercooling, even when the methods give reproducible results. Experiments indicate that this difficulty can be obviated by keeping the nieterial at R cnnstant temperature for a long time and determining its plasticity. Thc temperature at which tmheyield value just tiecanie zero would tie tlie teniperature of coiiiple.te fusion. At present, the method is a tetlioiis onc h i t it can tloul>tlcss be iniproved.
Solubility in colloids is not true solubility and cannot be measured by the standard methods. It is connected with the plasticity but it is perhaps .too soon t o say just how. 11-hen a given concentration of tn-o suhstances -% and R are peptized in t h p sani? solvent, it is probable that the less soluble one is the one n.hich displays the grea-tcr yield value. If this proves to he t,he case, it will become possitilc to nieasiire solubility in colloids n-ith precision just as weztlo in classical chemistry. Finally as a basis for our discussion of plasticity, we may consider methods of measurement with the mathematical theory of plastic flow in different kinds of material. 11-e have no reason to assume that the flow in a polar colloid follows the same laws as in colloids of the non-polar type. Similarly flow along the slip planes of a pure crystal offers new and interesting conditions. 11-e must consider also the effects of ternperature, concentration, H-ion concentration, deflocculating agents etc. 11-e are quite well agreed on tlie fundamental Law of P3iseuille for viscous fluids, derived froni the simple espressjon v
=
4iFr
where 1' is the velocity given to a surface by a shearing stress F, the siirface kieing a t a distance i' froiii another siirface considered at^ rest, the space be-
tween being filled nith the viscous flmd. The conqtant @ represents the fluidity. This formula gives a family of linear curves which all start from the origin as shown in Fig. I . For plastic substances the formula
v
= p
(F+) r
has been proposed where p i h the niohility and I' tlic >ield value. It gives t8hecurves shown in Fig. 2 and accordi with our kno\T-letlge that soft solids hold their shape unchanged under small shearing stresses Just as correction terms for kinetic energy etc., were required to hc norked out for thP Law of Poiseuille so we may assume that correction terms may have t o be developed in connection with the ahove formula for plastic flow. It hac been suggested that corrections for seepage and slippage mal- be required. In polar colloids there is a breaking clown of the filaments or structure during the process of flow nhich does riot seem to resemble what takes place in the flon of a suspension. Thc curve obtained is also very different, as shonn in Fig. 3 The yield value seems to depend upon the niagnitude of the shearing stress. the substance apprariiig to be w r y viscous at very low shearing stresws but quite fluid at high shearing stresses. l\lorc1 over the yielci value appears to tlepenc~upon tile radius of the capillary used in measuring the flov . I I(. 3 This seems t o make necessar?- the addition of some new variable u-liich 19 dependent upon the strength of the filament*. Finally it is noted that elastic deformation follons a lan analogous to the above, s = e F r
1
I
where s is the distance sheared by the shearing streas F. It is particularl: emphasized that elasticity is not dependent on the tiiiie factor as is true of viscous and plastic flow since 1- = s t. Elasticity 12 a property which is entirely independent of plasticity and it may therefore be separately determined. This conception was not gained by the early elasticians and resultt .! 1 in much confusion which still persists, It is for example quite poqsible to conceive of a fluid which possesses high elasticity of shape. Glass is perhaps the best example. I n the consideration of these matters here outlined the time at our disposal and much more may be profitably employed.
