(Rheology-Letrure 3 (Viscoplastic fluid behaviour

1.3.2Viscoplastic fluid behaviour

This type of fluid behaviour is characterised by the existence of a yield stress
(0) which must be exceeded before the fluid will deform or flow. Conversely,
nsuch a material will deform elastically (or flow en masse like a rigid body) whe
deuthe externally applied stress is smaller than the yield stress. Once the magnit
eof the external stress has exceeded the value of the yield stress, the flow curv
,emay be linear or non-linear but will not pass through origin (Figure 1.4). Henc
dernin the absence of surface tension effects, such a material will not level out u
gravity to form an absolutely flat free surface. One can, however, explain this
eerkind of fluid behaviour by postulating that the substance at rest consists of th
than dimensional structures of sufficient rigidity to resist any external stress less
eh0. For stress levels greater than 0, however, the structure breaks down and t
. substance behaves like a viscous material
A fluid with a linear flow curve for jyxj > j0j is called a Bingham plastic
shear fluid and is characterised by a constant plastic viscosity (the slope of the
stancebstress versus shear rate curve) and a yield stress. On the other hand, a su
natesipossessing a yield stress as well as a non-linear flow curve on linear coord
llustratesi(for jyxj > j0j), is called a ‘yield-pseudoplastic’ material. Figure 1.8
.viscoplastic behaviour as observed in a meat extract and in a polymer solution