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(curve d in Fig. 155), the yield point is not explicitly defined; it is
determined by extending the linear part of the curve to the stress
coordinate; the lower (rj = cc) and upper yield points may possibly also
With an anomalous liquid of the pseudoplastic type, i; decreases with
increasing shear stress and deformation rate, while i/ increases with a
dilatant liquid. There is no dependence oftj on the stress and
deformation rate with a Newtonian liquid.
In the case of substances exhibiting dilatancy (e.g. aqueous
suspension of ground quartz or feldspar), low-strength water films form
between the particles and flow brings about friction between the solid
surfaces. Substances of this type are unsuitable for ceramic forming, as
the resistance to forming deformation increases with increasing stress.
On the other hand, with a plastic material, such as clay with water,
deformation occurs readily when the yield point is exceeded. However, one
should avoid exceeding the critical deformation rate, above which real
plastic materials become brittle and tend to crack. A suitable yield
value ensures stability of formed ceramic ware in the sense that they do
not deform under gravitational forces. This rheological behaviour of clay
bodies is related to the nature of water films formed on the hydrophilic
surface of clay mineral particles.
In the case of thixotropic materials, the yield point depends on the
conditions of measurement and on the previous history. At standstill, it
acquires high values, while on stirring it may be close to zero. In
practice, it means that vibration during handling of green ware may cause
a decrease in yield point and subsequent deformation. Thixotropy is due
to the formation of gel-type structures which are destroyed by dynamic
When attempting to describe more accurately the rheological behaviour
of ceramic plastic mixes, one should also take into account the elastic
behaviour above the yield point. If a plastic body is abruptly stressed
by a constant load, there first occurs rapid clastic deformation followed
by delayed elastic deformation and irreversible flow. Similarly, instant
as well as delayed relaxation take place after stress relief. If a formed
product has only a limited possibility to relax, it retains some internal
stress which may be the cause of drying defects.
Despite the various complexities involved in the behaviour of real
plastic bodies, approximation by means of the Bingham model is useful.
Apart from the parameters
included in the model, there is another significant factor of the
maximum deformation the material will withstand without cracking. Along
with the yield point and the plastic viscosity, it is one of the basic
parameters characterizing the behaviour of plastic bodies during forming.
0.1 0.2 03 OA 0.5 06 relative deformation
FIG. 156. Stress-deformation curves of a clay plastic mass (from Norton,
water in % of dry clay
FIG. 157. Workability of monodi'sperse kaolin fractions (from Norton,
1970). The numbers at the curves indicate mean spherical particle size in
In technological practice, use is made of a number of tests and
methods for checking or establishing Theological parameters determining
the forming properties. Mention can, for instance, be made of the method
for determining the relationship betw'een deformation and continuously
increasing stress. For this purpose, a device can be used in which a
cylinder or tube of plastic body is stressed in torsion. The deformation
is recorded continuously with increasing stress (cf. Satava, 1954). In
this way. typical curves similar to those shown in Fig. 156 for clay
mixtures with various water contents can be obtained.
Elastic reversible deformation is involved roughly up to point a. If
the stress acts just for a short period of time, the deformation is
relieved on unloading. At point a, the yield point is attained and rapid
irreversible deformation occurs. For this reason the curve includes an
approximately horizontal section the length of which corresponds to the
maximum deformation the material is capable of withstanding without
failure. Cracks begin to form at point b.
* From Fine Ceramics by F. H. No ton. Copyright ?) 1970 by McGraw-
Hill, Inc. Figures 10.10 and 10.20 used with permission.
From these curves, it is possible to obtain two parameters that have
the greatest effect on workability: yield point and maximum deformation.
Good workability requires an adequately high yield value which will
eliminate spontaneous deformation due to gravity forces. A further
requirement is the highest possible value of maximum deformation, which
guarantees that no cracks will form on shaping. The two characteristics
are mainly affected by water content, although this influences the two
values in the opposite directions. Workability can then be expressed