deformation is permanent, and a second deformation (broken line) leads to
significantly smaller stresses. Frame (d) is a rather common type for several
soft solids; its shape implies that the elastic deformability is limited. The
hatched area between the compression and the decompression curve is a
measure of the energy that is dissipated into heat, largely due to structure
breakdown. The area under the decompression curve (bounded by the
vertical dotted line) represents recoverable energy.
Frame (e) illustrates a system that showsyielding. This means that
above a certain stress, called theyield stresssy, flow occurs; see also Figure
5.11. In such cases the structure is strongly altered by the deformation. Some
materials exhibit partial or full recovery of the original structure after some
time, a phenomenon calledthixotropy; see also Section 5.1.3. This occurs
typically in moderately concentrated dispersions in which the particles
attract each other, though with forces that are not overly strong (can you
explain this?). Systems for which the yield stress is rather high, say between 1
and 10^3 kPa, are often calledplastic solids. The yield stress then appears to
be a good measure for what is generally called ‘‘firmness’’.
Time Effects. When one deforms a viscoelastic material, the stress
depends not only on the strain but also on thestrain rate. This is illustrated
in Figure 17.6a; it is seen that at a given strain,sincreases with increasingC
value. This is because bonds that have come under stress owing to the
deformation can break after some time; generally, new bonds are also
formed, though not as many as had been broken. These phenomena cause
stress relaxation. A material has a characteristic stressrelaxation timewhich
becomes manifest when it is kept at a given strain: see Figure 5.10 and Eq.
(5.13). IfCis small, deformation time is relatively long, and much of the
FIGURE17.5 Examples of stresssversus strainefor various materials that are
first compressed, then decompressed (both at constant strain rate). Examples are
meant to illustrate various kinds of behavior. The hatched areas indicate the
deformation energy that is dissipated (not recoverable). The scales are generally
different for the various frames.