216

FLUIDITY AND PLASTICITY

exceeding a certain value, the substance is called a solid, however soft
it may be. When the very smallest, stress, if continued long enough,
will cause a constantly increasing change of forrn, the body must be
regarded as a viscous fluid, however hard it may be.

"Thus a tallow candle is much softer than a stick of sealing wax;
but if the candle and the stick of sealing wax are laid horizontally
between two supports, the sealing wax will in a few weeks in summer
bend under its own weight, while the candle remains straight. The
candle is therefore a soft (or plastic) solid, and the sealing wax is a very
viscous liquid.

"What is required to alter the form of a soft solid is sufficient force,
and this, when applied, produces its effect at once. (This is, of course,
only relatively true, because plastic deformation is a function of the
time, as will appear later.) In the case of a viscous fluid, it is time which
is required, and if enough time is given the very smallest force will
produce a sensible effect, such as would be produced by a very large
force if suddenly applied.

"Thus a block of pitch may be so hard that you can not make a dent
in it with your knuckles; and yet it will, in the course of time, flatten
itself out by its own weight and glide down hill like a stream of water."

The italics and parentheses are ours. Butcher (1876) has
expressed views quite similar to those of Maxwell.

We may now define plasticity as a property of solids in virtue

of which they hold their shape
permanently under the action of
small shearing stresses but they
are readily deformed, worked or
molded, under somewhat larger
stresses. Plasticity is thus a com-
plex property, made up of two inde-
pendent factors, which we must
evaluate separately.

Reverting to our fundamental
conception of flow between two
parallel planes separated by a

Shearing Stress

FIG. 76.—Typical flow-shear
diagram for a series of viscous
liquids.

distance dr and subjected to a shearing force F, we have found
that in a viscous fluid

dv = <?Fdr

so that if we were to plot the volume of flow through a tube or
the rate of shear against the shearing stress, we would obtain

in.
;plo

IRe

vs