purely elastic material. The material is instantaneously deformed upon
applying the stress and it instantaneously returns to its original shape.
Note Actually, instantaneous deformation cannot occur; in
practice the deformation rate corresponds to the sound velocity in
the material, often of the order of a km?s^1.The material thus has a perfect ‘‘memory’’ for its original shape. The energy
applied to achieve deformation is not dissipated but stored: upon removal of
the stress, the stored energy is recovered. The ratio of stress over strain is
calledmodulus.
In Figure 5.8c we see what happens with apurely viscousmaterial, i.e.,
a Newtonian liquid. As soon as a stress is applied, it starts to flow, and after
removal of the stress, flow stops, and the deformation attained remains. The
liquid has no memory for its original shape, and the energy applied to cause
flow is dissipated into heat.
In Figure 5.8d an intermediate behavior, called viscoelastic, is
depicted; such a relation is often called a creep curve, and the time-
dependent value of the strain over the stress applied is called creep
compliance. On application of the stress, the material at first deforms
elastically, i.e., ‘‘instantaneously,’’ but then it starts to deform with time.
After some time the material thus exhibits flow; for some materials, the
strain can even linearly increase with time (as depicted). When the stress is
released, the material instantaneously loses some of it deformation (which is
called elastic recovery), and then the deformation decreases ever slower
(delayed elasticity), until a constant value is obtained. Part of the
deformation is thus permanent and viscous. The material has some memory
of its original shape but tends to ‘‘forget’’ more of it as time passes.
It should be noted that viscoelastic behavior varies widely among
materials. The magnitudes of the instantaneous elastic modulus, the
apparent viscosity and the elastic recovery, and especially the time scales
involved, vary widely. Some viscoelastic substances, like cheese, seem on the
face of it to be solids, but they are observed to flow over longer time scales.
Others, like egg white, appear to be liquids but show elasticity on closer
inspection. Closer inspection may involve sudden acceleration of the liquid,
for example by rapidly giving a turn to the beaker containing it. If the liquid
then shows oscillatory behavior, best seen in the motion of a few enclosed
air bubbles or small particles, it also has some elastic property.
Such aviscoelasticormemory liquidis another example of a non-
Newtonian liquid. Nearly all viscoelastic liquids are also strain rate
thinning, but not all strain rate thinning liquids show significant elasticity.
Deformation can, of course, be in shear or elongation, etc. However, for
viscoelastic liquids, the Trouton ratios [see Eq. (5.2)] are higher, often much