THE PLASTICITY OF SOLIDS

223

lastic flow and it has already had the attention of Buckiiig-
(1921)- In plastic material confined between two parallel
es of indefinite extent which are being sheared over each
r? th_e shearing stress F will be identical at every point. But
low tfctrough. a capillary tube according to Buckingham
is n.ot the case; the shear increases continually from the
sr of tfa.e capillary outward and only at a certain distance r0
the shearing; force become sufficient to overcome the friction,
"efore the material at the center of the capillary moves as a
plug with the velocity v0, and beyond the radius TQ the mate-
m.o\res in telescoping layers. This results in the flow not
? a linear function of the pressure.

it tbiere are other possible causes of the peculiarity which
be mentioned here. The plastic material next to the
ma/y have a lower concentration of solid than elsewhere
ting in apparent slippage. Or the shearing stress may cause
Lquid to flow bet~ween the particles of solid, seepage.
Lcklngham suggests that the friction between the particles
ig flow may not be the same as the static friction. It
s further possible that the friction will need further definition
. the individual particles of the plastic material are of very
•ent sl^es. We shall at first assume that slippage and
<ge a,re both absent and that the particles of solid are uni-
.y small.

e total force producing the flow through a capillary tube
R2 a,nd since there is no acceleration, this is opposed by a
in the opposite direction ZirRlF. If p is the pressure
i is used up during the flow in overcoming the friction, the
m/ Is defined by the equation

/-|p (77)

o follows that

r°

F =

(78)
(79)

ce tlie speed decreases as the radius increases, Eq. (73)
les

dv =

(80)