Constant rate period flow stress

There are other more complicated experimental situations where viscoelastic behavior can also be predicted in terms of the relaxation and retardation spectra or other functions. These include deformations at constant rate of strain and constant rate of stress increase, stress relaxation after cessation of steady-state flow, and creep recovery or elastic recoil, all of which were mentioned in Chapter 1, as well as nonsinusoidal periodic deformations. In referring to stress a, strain y, and rate of strain 7, the subscript 21 will be omitted here although it is understood that the discussion applies to shear unless otherwise specified. 

The attained value of crinax at y = const (Fig. 3 presents the data for y = 11s- ) and the rise time of the stationary flow ts, are dependent upon the break time tbr between experiments points 1-4 in Fig. 3 correspond to the break times of 1200, 600, 60, and 24 s, respectively. Upon the break time increase, the values of crmax and tsl also increase however, at times periods of longer than 15 min (experiments were performed up to 1 h) they become independent of time. The behaviour is qualitatively the same at fixed break times and temperatures but for various y. Moreover, the ratio of deformation rate (see Fig. 4). For the same tbr and different T the functions cr(t) differ one from another by a constant number. Therefore, temperature (just as y) has no influence on the ratio a/crsl as a function of t, which implies that the influence of T on qualitatively studied in stationary flows. 

This section draws heavily from two good books Colloidal Dispersions by Russel, Seville, and Schowalter [31] and Colloidal Hydrodynamics by Van de Ven [32] and a review paper by Jeffiey and Acrivos [33]. Concentrated suspensions exhibit rheological behavior which are time dependent. Time dependent rheological behavior is called thixotropy. This is because a particular shear rate creates a dynamic structure that is different than the structure of a suspension at rest. If a particular shear rate is imposed for a long period of time, a steady state stress can be measured, as shown in Figure 12.10 [34]. The time constant for structure reorganization is several times the shear rate, y, in flow reversal experiments [34] and depends on the volume fraction of solids. The viscosities discussed in Sections 12.42.2 to 12.42.9 are always the steady shear viscosity and not the transient ones. 

To characterize Newtonian and non-Newtonian food properties, several approaches can be used, and the whole stress-strain curve can be obtained. One of the most important textural and rheological properties of foods is viscosity (or consistency). The evaluation of viscosity can be demonstrated by reference to the evaluation of creaminess, spreadability, and pourability characteristics. All of these depend largely on shear rate and are affected by viscosity and different flow conditions. If it is related to steady flow, then at any point the velocity of successive fluid particles is the same at successive periods of time for the whole food system. Thus, the velocity is constant with respect to time, but it may vary at different points with... 

The aim of a stress relaxation experiment is to observe how the stresses decay with time (i) after cessation of steady shear flow or (ii) after a sudden shearing displacement. In case (i) the fluid sample trapped in a small gap between two parallel plates is allowed to maintain constant shear rate that was started long before f = 0 so that all the transients during the stress growth period have evened out. Then at t = 0, the flow is stopped suddenly and the decay of the stress is monitored with respect to time till it becomes insignificant or dies out. The stress would relax monotonically to zero and more rapidly as the shear rate in the preceding steady shear flow is increased. 

Plastic bodies are those in which the apphed shear stress has exceeded the yield point. Under these conditions, the deformation of the plastic body is progressive with time until, at some point, it is able to sustain a constant applied shear stress. On the removal of the shearing stress, any elastic component of the total strain is recovered however, the plastic component of the strain is not recovered. When the body deforms over a measurable period of time, i.e., flows under the action of a constant stress, one speaks of creep occurring. An elastic-plastic body having a near-zero yield point can be called a viscous body. Such a body can not sustain shear stress over a finite period of time, since the stress relaxes with time in other words, the body flows to relieve shear stresses. At constant strain, the stress could initially build up instantaneously as an elastic response, however, such stress would ultimately decay through internal relaxation mechanisms. If the externally applied stress is maintained, the deformation will continue to occur with time, since the applied stress is never balanced. The slower the rate of stress relaxation, the more viscous the body is. 

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