Yield point flow stress

In the case of gel-like samples (G > G" in the viscoelastic linear region), this test is frequently used to determine the yield point (yield stress) and flow point (flow stress). The yield point corresponds to the limiting value of the linear viscoelastic region. The flow point corresponds to the stress where G = G". 

Under compression or shear most polymers show qualitatively similar behaviour. However, under the application of tensile stress, two different defonnation processes after the yield point are known. Ductile polymers elongate in an irreversible process similar to flow, while brittle systems whiten due the fonnation of microvoids. These voids rapidly grow and lead to sample failure [50, 51]- The reason for these conspicuously different defonnation mechanisms are thought to be related to the local dynamics of the polymer chains and to the entanglement network density. 

Powder Mechanics Measurements As opposed to fluids, powders may withstand applied shear stress similar to a bulk solid due to interparticle friction. As the applied shear stress is increased, the powder will reach a maximum sustainable shear stress T, at which point it yields or flows. This limit of shear stress T increases with increasing applied normal load O, with the functional relationship being referred to as a yield locus. A well-known example is the Mohr-Coulomb yield locus, or... 

Figure 3 gives an example of a typical force profile. The force is increased continuously and reaches the point - at the end of the first part of the force profile - where the pectin preparations start to flow. The so-called yield point is reached. The further increase leads to the continuous destruction of the internal structure and the proceeding shear thinning. The applied stress in part 3 of the stress profile destroys the structure of the fruit preparations completely. Now the stress is reduced linearly, see part 4 and 5, down to zero stress. The resulting flow curves 2, 3 and 4 and the enclosed calculated area from the hysteresis loop give important evidence about the time-dependent decrease of viscosity and a relative measure of its thixotropy. 

The rheological characteristics of AB cements are complex. Mostly, the unset cement paste behaves as a plastic or plastoelastic body, rather than as a Newtonian or viscoelastic substance. In other words, it does not flow unless the applied stress exceeds a certain value known as the yield point. Below the yield point a plastoelastic body behaves as an elastic solid and above the yield point it behaves as a viscoelastic one (Andrade, 1947). This makes a mathematical treatment complicated, and although the theories of viscoelasticity are well developed, as are those of an ideal plastic (Bingham body), plastoelasticity has received much less attention. In many AB cements, yield stress appears to be more important than viscosity in determining the stiffness of a paste. 

The second effect of ram movement usually works in the opposite direction (slower ram speeds plus a dwell at the peak press may cause, an increase in density). This effect is due to the fact that, at loading pressures usually used, expls are stressed beyond their yield points and creep or flow plastically. This effect, of course, becomes more important at vary high pressures, such as those used for delays. In addition to increasing the density, slower speeds plus dwell of the ram result in a more uniform density 4) Pelletizing... 

Concentrated particle suspensions may also show a yield point which must be exceeded before flow will occur. This may result from interaction between irregularly shaped particles, or the presence of water bridges at the interface between particles which effectively bind them together. Physical and chemical attractive forces between suspended particles can also promote flocculation and development of particle network structures, which can be broken down by an applied shear stress [2]. 

It was found that increasing Ca caused the yield stress and yield strain to increase, along with cell deformation at the yield point. At sufficiently high values of Ca, cell distortion is so severe that film thinning and rupture can occur, resulting in mechanical failure of the foam (Fig. 6). This implies the presence of a shear strength for foams and HIPEs. The initial orientation of the cells was also found to affect the stress/strain behaviour of the system in the presence of viscous forces [63]. For some particular orientations, periodic flow was not observed for any value of Ca. 

The results of the latest research into helical flow of viscoplastic fluids (media characterized by ultimate stress or yield point ) have been systematized and reported most comprehensively in a recent preprint by Z. P. Schulman, V. N. Zad-vornyh, A. I. Litvinov 15). The authors have obtained a closed system of equations independent of a specific type of rheological model of the viscoplastic medium. The equations are represented in a criterion form and permit the calculation of the required characteristics of the helical flow of a specific fluid. For example, calculations have been performed with respect to generalized Schulman s model16) which represents adequately the behavior of various paint compoditions, drilling fluids, pulps, food masses, cement and clay suspensions and a number of other non-Newtonian media characterized by both pseudoplastic and dilatant properties. 

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 interna stress w hich may be the cause of drying defects. 

Stmcture formation during flow is an additional complication. Stmcture breakdown occurs also and is evident particularly in clay suspensions, which are generally flocculated at rest. Under flow there is a loss of the stmcture and the suspension exhibits thixotropy and a yield point. The viscosity decreases with increasing shear stress (Fig. 7.33). 

Fig. 14 Typical flow curves for a concentrated emulsion sheared between two rough surfaces closed circles the line is a fit to the Herschel-Bulkley equation), and between rough and smooth surfaces open circles hydrophilic glass surface open squares hydrophobic polymer surface). 7a denotes the apparent shear rate, and is the value of the apparent shear rate at the yield point (cr = CJy). CJs is the sticking yield stress below which the emulsion adheres to the surface...

The arrival at the yield point on the stress-strain curve is revealed at the most gross structural level by a permanent change in the body s shape. However, there are more subtle structural changes as well. One of the intriguing features that arise with the onset of plastic flow and a hint in favor of the claim that dislocations are the carriers of plastic change is the occurrence of micron-sized steps on the crystal faces. In particular, if an optical microscope is used to examine the crystal surface... 

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