CHAPTER VIII
THE PLASTICITY OF SOLIDS
Only by the behavior of materials under shearing stresses are
we enabled to distinguish between a fluid and a solid. If a
body is continuously deformed by a very small shearing stress,
it is a liquid; whereas if the deformation stops increasing after
a time, the substance is a solid. This distinction is theoretically
sharp like the distinction between a liquid and a gas at the critical
temperature, but just as a liquid may be made to pass into a gas
insensibly, so a solid may grade insensibly into a liquid. Glass
and pitch are familiar examples of very viscous liquids. Paint,
clay slip, and thin mud in a similar manner must be classed as
soft solids. According to the experiments of Bingham and
Durham (1911) the concentration in which the fluidity becomes
zero under a very small shearing force serves to demarcate the
two states of matter.
This simple distinction is not always sharply drawn nor is its
significance thoroughly appreciated; and for this reason much
labor has been ill-spent in the attempt to measure the viscosity
of solids, on the assumption that solids are only very viscous
liquids and therefore that plasticity and the fluidity of solids
are synonymous terms. The results are unintelligible because
the viscosity as so determined in various instruments is widely
different.
The views of Clerk Maxwell expressed in his " Theory of Heat"
are especially noteworthy and are quoted at length:
"If the form of the body is found to be permanently altered when the
stress exceeds a certain value, the body is said to be soft or plastic and
the state of the body when the alteration is just going to take place is
called the limit of perfect elasticity. If the stress, when it is maintained
constant, causes a strain or displacement in the body which increases
continually with the time, the substance is said to be viscous.
"When this continuous alteration of form is only produced by stresses