92168.pdf

Rheology 257
where an arbitrary number which bears some relation to the
mechanical property under consideration is usually quite sufficient^118.
To measure elastic and viscous properties which are characteristic
of the material under consideration and independent of the nature of
the apparatus employed, the applied stress and the resulting
deformation must be uniform throughout the sample. Concentric
cylinder and cone and plate methods approximate these requirements.
For materials which are self-supporting, measurements on, for
example, the shearing of rectangular samples are ideal,
Creep measurements involve the application of a constant stress
(usually a shearing stress) to the sample and the measurement of the
resulting sample deformation as a function of time. Figure 9.6 shows
a typical creep and recovery curve. In stress-relaxation measurements,
the sample is subjected to an instantaneous predetermined deforma-
tion and the decay of the stress within the sample as the structural
segments flow into more relaxed positions is measured as a function
of time.

Creep under

constant stress

Recovery after

removal of stress

Time

Figure 9.6 Creep and recovery curve for a typical viscoelastic material

The response of a material to an applied stress after very short
times can be measured dynamically by applying a sinusoidally varying
stress to the sample. A phase difference, which depends on the
viscoelastic nature of the material, is set up between stress and strain.