92168.pdf

256 Rheology

Work-hardening can occur as a result of mechanical entanglement

or jamming of the structural elements on shearing, an example of this

being the 'necking' and corresponding toughening of metal rods when

subjected to a tensile stress. A technically important rheological

property, which is related to strain-hardening (and to flow elasticity),

is spinability - i.e. the facility with which a material can be drawn into

threads.

Viscoelasticity^12

When a typical elastic solid is stressed, it immediately deforms by an
amount proportional to the applied stress and maintains a constant
deformation as long as the stress remains constant - i.e. it obeys
Hooke's law. On removal of the stress, the elastic energy stored in
the solid is released and the solid immediately recovers its original
shape. Newtonian liquids, on the other hand, deform at a rate
proportional to the applied stress and show no recovery when the
stress is removed, the energy involved having been dissipated as heat
in overcoming the internal frictional resistance.
When viscoelastic materials are stressed, some of the energy
involved is stored elastically, various parts of the system being
deformed into new non-equilibrium positions relative to one another.
The remainder is dissipated as heat, various parts of the system
flowing into new equilibrium positions relative to one another. If the
relative motion of the segments into non-equilibrium positions is
hampered, the elastic deformation and recovery of the material is
time-dependent (retarded elasticity).

Experimental methods

Numerous instruments (plastometers, penetrometers, extensiometers,
etc.) and procedures have been devised for measuring the rheological
behaviour of various viscoelastic materials. However, the results
obtained from most of these instruments are of little fundamental
significance, because the applied stress is not uniformly distributed
throughout the sample, and the way in which the material behaves
towards a particular apparatus is measured rather than a fundamental
property of the material itself. Nevertheless, such empirical instru-
ments are indispensable for control testing purposes in industry,