Cessation of steady shear flow

After this rather extensive discussion of the experimental verification of eqs. (2.11), (2.20) and (2.22), where the interrelations of steady flow properties with dynamic properties and shear recovery are scrutinized, a few words should be added with respect to stress-relaxation after cessation of steady shear flow. As pointed out above, Lodge predicted a decrease of the extinction angle during stress relaxation (4). That this... 

Fig. 2.7. Drop off of doubled extinction angle 2"/ during stress-relaxation after cessation of steady shear flow according to Wales (59). Measurements on the melt of a high-density polyethylene (Marlex 6002) at a measurement temperature of 147° C. Shear rate of the steady shear flow q = 0.06 sec-1...

Stress Relaxation after Cessation of Steady Shearing Flow... 

Constrained Recoil after Cessation of Steady Shearing Flow [Bird, Evans, and Warner (3)]... 

A relationship quite similar to Eq. (19.15) is given in Eq. (12.18) in the section on constrained recoil after the cessation of steady shearing flow. 

In stress relaxation after cessation of steady shear flow, the elastic dumbbells give no dependence of the relaxation process on the steady-state shear rate, but the rigid dumbbells do. In addition the elastic dumbbells show the shear and normal stresses relaxing with exactly the same... 

Stress relaxation after cessation of steady shearing flow... 

Figure 31.4 Stress relaxation curves after cessation of steady shear flow for the PET/PEN-clay systems (shear rate is 10 s ). Source Reproduced with permission from Calderas F, Sanchez-Solis A, Maciel A, Manero O. Macromol Symp 2009 283 354 [83], Copyright 2009 Wiley-VCH Verlag GmbH Co. KGaA.

Shear stress relaxation after cessation of steady shear flow (which includes Mooney stress relaxation [130] now described in ASTM D1646)... 

Figure 6.5 Maxwell viscoelastic model and its prediction of stress relaxation in a melt after the cessation of steady shear flow. t<, is the relaxation time.

After the cessation of steady shear flow, the optical quantities and Ig always changed with time, while the shear... 

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. 

The aim of a stress relaxation experiment is to observe how the stresses decay with time (1) after cessation of steady shear flow or (2) after a sudden shearing displacement. In case 1 the fluid sample trapped in a small gap between two... 

Besides the sinusoidal oscillations, transient tests such as stress relaxation, start-up of steady shear flow, and cessation of steady shear flow are also important in the rheological characterization of polymeric liquids. The instrument limitations and features, and their effect on the data obtained in some types of transient tests, are examined in this section. Also, we shall illustrate the use of linear viscoelastic transformation to obtain, for example, stress relaxation data from tests such as start-up of steady shear flow and sinusoidal oscillations. 

Figure 11.17 presents the relaxation of normalized (a) shear stress and (b) the first normal stress difference upon cessation of steady shear flow at y = 0.5 s for PSHQ9 at 130 °C. Here, experimental data are shown by dots and fitting curve by dashed line. The normalizing values Oq and Ni are the steady values of shear stress... 

Figure 11.20 Relaxation of normalized (a) shear stress, a y,t)/a, and (b) first normal stress difference, Ny y,t)/Nijn, upon cessation of steady shear flow aty = 1 s for PI-14-5CN at 70°C notations are the same as in Figure 11.17.

In this experiment, on cessation of steady shear flow the stresses are monitored with time. The material functions are ... 

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