delay. To avoid this time-
dependent start-up effect which is also called transient effect, the user should wait sufficiently long at each measuring point (see also Chapter 3.3.1b and Figure 2.9: no. 5). For samples showing clearly time-dependent behavior, differing measuring times for an otherwise identical preset test profile may result in different yield point values. A yield value also depends on the sample preparation before the test (e. g. concerning shear load, time effects, temperature) [3.26].
Summary: A yield point is not a material constant. Since it is time-dependent, it depends on the conditions during the preparation of the sample as well as on the test conditions.
Note: Structural strength at rest and frequency sweeps
When determining structural strength or consistency-at-rest, frequency sweeps (oscillatory tests) are the better way of testing in principle since they take best into account the influence of time. Here, when presetting frequencies, time-dependent results are obtained since a frequency is an inversed time (see also Chapter 8.4.4a).
3.3.4.2.2b) Interaction forces and network of forces
Dispersions and gels are showing yield points due to intermolecular forces (van-der-Waals forces). This includes dipole-dipole interactions between particles, and between particles and the surrounding dispersion agent. There are different kinds of interaction forces (with specification of the typical bonding energy per mol): Electrostatic interactions between permanent dipoles (Keesom forces; < 30 kJ/mol), interactions due to induction between permanent and induced dipoles (Debye forces; < 2 kJ/mol), and dispersion forces between mutually induced dipoles (London forces; < 40 kJ/mol) [3.22] [3.27] [3.64] [3.82]. They are all based on physical-chemical bonds (secondary bonds) between the molecules and have a considerably lower bonding energy, usually below 20 kJ/mol, compared to the chemical primary valency bonds which are acting within the molecules. These primary bonds are covalent electron-pair bonds, ionic or metallic “electron gas” bonds which are usually showing energy values of 50 to 400 kJ/mol, and maximum values of 1000 kJ/mol [3.20] [3.28]. Bonds via intermolecular hydrogen bridges (< 50 kJ/mol) are an exceptional type of physical-chemical secondary bonds. In large numbers, however, they can have a great effect on rheological behavior.
Interactions may build up a three-dimensional network of forces. In the low-deformation range, this network occurs as a stable and solid-like structure resulting in elastic behavior (gel-like character, or gel-like state, see also Chapter 8.3.2a).
3.3.4.2.3c) Plastic behavior
DIN 1342-1 and -3 states the following: “For a plastic material, rheological behavior is characterized by a yield point.” And: “Plasticity is the ability of a material to show remaining deformation (and flow) only if the yield point is exceeded. Below the yield point occurs no or only elastic deformation.” Further: “A deformable material is called plastic if it behaves in the range of low shear stresses as a rigid, elastic or viscoelastic solid, in a higher shear stress range however, as a liquid. The shear stress value at which the transition takes place is called the yield point (or yield stress).” Sometimes further terms can be found in literature such as “plastic deformation”, “plastic creep”, or “plastic flow”.
In 1916, Eugen C. Bingham (1878 to 1945) described the behavior of dispersions showing a yield point [3.4]. He called this behavior to be “plastic”. Ideal-plastic behavior was illustrated using the Saint Venant model (A. J. B. de Saint Venant, 1797 to 1886 [3.29]); see also DIN 1342-1. Drawings of this model are shown in the meanwhile withdrawn DIN 13342 of 1976, and in [3.10] [3.30]. This model consists of a friction element (or “sliding shoe”), which does not begin to move until the shear force overcomes the resistance caused by static friction. Then the structure of the stressed sample yields, showing an increasing deformation (creep or flow), but the motion is still slowed down by the sliding friction of the friction element. In the past, various rheologists designed a lot of different concepts to explain the behavior in the transition range between rest and flow:
1 Some rheologists assumed that a material remained completely rigid and undeformed under increasing shear load until the yield point was exceeded. This behavior was called “plastic-rigid” or “inelastic” [3.9] [3.24]. Above the yield point the material showed “plastic flow” and finally, under higher shear load, “viscous flow”. This behavior was described in 1919 using the Bingham model [3.4] (see also Chapters 3.3.6.4a and 14.3: 1916). Both steps of this behavior were termed “viscoplastic ” (as in the redrawn DIN 13342) and [3.30], or as “plastic-viscous” [3.10]. After the load is removed, no reformation occurs at all.Other rheologists were convinced that the sample under increasing shear load first showed reversible elastic behavior in a very limited deformation range until the yield point was exceeded. Then an irreversible plastic deformation occurred. This behavior was described in 1924 by the Prandtl model [3.30], or in 1930 by the Prandtl/Reuss model (DIN 1342-3 and [3.10], see also Chapter 14.3: 1924). Both stages of that behavior were termed “elastoplastic” (withdrawn DIN 13342), as “elastic-plastic” [3.10], or as “plastoelastic” [3.31]. The extent of reformation after removing the load represents the elastic portion. Using modern terms, these kinds of materials should be called viscoelastic liquids.Another concept was that under a low shear load below the yield point the sample was deformed reversible-elastically (deformation behavior according to Hooke). After exceeding the yield point, the material showed plastic behavior (slow flow, slowed down by the friction element according to Saint Venant), and finally, under increased shear load it showed viscous flow (flow behavior according to Newton, without any effect of the friction element). This behavior was described using an extended Bingham model [3.30], and all three stages were named “elastico-plastico-viscous” or “elasto-visco-plastic” or similar terms were used [3.32] [3.33]. The extent of reformation after removing the load corresponds to the elastic portion. Using modern terms, these kinds of materials should be called either viscoelastic liquids if there is only partial reformation even after a sufficiently long period of time, or viscoelastic solids if they are recovering completely even if this may take a longer time.
For people working scientifically, the term “plastic behavior” used in rheology is a synonym for “inhomogeneous behavior”. For practical users performing rheological tests, the following can be stated: Plastic behavior is shown by materials which do not exhibit