Several soft solids showyieldingat low and fracture at high strain
rates, although the critical strain rate for fracture may vary widely. Most
plastic fats, including butter and margarine, show yielding at high stress
under most conditions, but at quite high deformation rates, sayC>10 s^1 ,
fracture can occur. Another example is given in Figure 17.8 for two types of
cheese. The ‘‘short’’ cheese (1) shows virtually no dependence ofefron the
strain rate, whereas the much ‘‘longer’’ cheese (2) exhibits flow at strain rates
of about 10^4 s^1 or smaller. This flow induces irreversible structural
changes, which means that yielding occurs. On the other hand, some
materials can show very slow fracture when a relatively small constant stress
is applied, as is illustrated in Figure 17.6b; this may represent the behavior
of some starch gels. Even the lowest curve, which suggests that an
equilibrium deformation has been reached, may start to increase in height
and finally become vertical when fracture occurs, which may be after quite a
long time (say, one hour).
This brings us to variation in fracture mode. In the case just
mentioned, small cracks presumably form in many places in the test piece
and slowly grow in size until they become connected over larger distances,
leading to visible fracture. (Differentiation between yielding and fracture
then is almost arbitrary.) In most cases, test pieces fracture either in shear or
FIGURE17.8 Fracture behavior of two cylindrical cheese samples subjected to
uniaxial compression. Sample (1) concerns a hard and fairly brittle cheese, (2) a
semihard green cheese that is quite extensible. (a) Stresssversus Hencky straineH,
resulting upon deformation at a strain rateCof 10^2 s^1 ; the dotted lines indicate
the strain at fractureefr. (b) Values ofefrobtained at variousC. (c) Fracture mode as
seen in cross sections through the test pieces at the moment of maximum stress; for
(1) it is a vertical cross section, for (2) a horizontal one.