Shear stress growth

Keywords. Adherent cells. Chemical stress. Shear stress. Growth inhibition. Stress protection... 

Mason, P. L., Bistany, K. L., Puoti, M. G., and Kokini, J. L. 1982. A new empirical model to simulate transient shear stress growth in semi-solid foods. J. Food Process Eng. 6 219-233. 

Fig. 1. Shear stress growth on the start up of shear flow for 60 mole % PHB/PET at 240°C and 250°C.

The shear stress growth on the inception of shear flow may reflect the orientation of the liquid crystalline domains. Orientation seems to occur within less than 2 strain units in shear flow. This primary normal stress difference can exhibit different phenomena from the shear stress response. In particular for the 60 mole % PHB/PET system, values of N are positive and rise gradually to the equilibrium values whereas the 80 mole % PHB/PET system can exhibit negative values of N. Ericksen s transversely isotropic fluid theory can qualitatively handle some of the observed phenomena. Further studies which couple the transient flow behavior to the orientation and morphology need to be carried out. 

Fig. 7.19. Shear stress growth function at several shear rates for solution of polybutadiene M = 3.5 x 10 ). In this sample is about 1 sec. Reproduced from...

Out-of-phase component of complex viscosity Shear stress growth coefficient Shear stress decay coefficient Tensile stress growth coefficient Tensile stress decay coefficient... 

Figure 8.14 Shear stress growth function of a low density polyethylene filled with two different fillers. (Reprinted from Ref. 91 with kind permission from Gulf Publishing Co., Houston, Texas, USA.)...

FIG, 17-35. Shear stress growth and relaxation for successive impositions of a constant shear rate with intervening rest periods of different lengths, for a 3% polyisobutylene solution (Mw = 2.8 X 10 ) at 25°C. (Stratton and Butcher. ) Reproduced, by permission, from the Journal of Polymer Science. 

Next, let us consider the predictions for a shear stress growth experiment. In a stress growth experiment, a linearly increasing shear is imposed, i.e. 

In a third test, a step fimction shear rate is suddenly applied att = 0 (start-up flow). In this case, the shear stress a is measrmed as a fiinction of time and the shear stress growth coefficient (stressing viscosity) +( ) can be calculated ... 

Figure 9.9 Normalized shear stress growth coefficient (s, ) and normalized first normal stress growth coefficient (s, ) as a function of the normalized time s at a shear rate of 2s (T = 20°C, 15%-MDMA0-HN03, pH = 3.5). Predictions of the Giesekus model...

Figure 12.35 shows variations of shear stress growth a+(y, /)) as a function of shear strain (yt) upon startup of shear flow at 260°C and y = 1.0 s for 95.7/4.3 PC/MMT nanocomposite and 95.7/4.3 PC/Cloisite SOB nanocomposite, where three separate runs, each using a fresh specimen, were made. Thus, the three curves given in Figure 12.35 represent variability of a+(y,t) on three repeated measurements. The following observations are worth noting in Figure 12.35. Upon startup of shear flow, both 95.7/4.3 PC/MMT and 95.7/4.3 PC/Cloisite SOB nanocomposites exhibit an overshoot followed by monotonic decay reaching steady state, and the magnitude of overshoot peak 95.7/4.3 PC/Cloisite SOB nanocomposite is...

The lower limit on the integral is zero rather than minus infinity, since the sample is known to be in a stress-free state at t = 0. The ratio of the stress to the shear rate is called the shear stress growth coefficient and has units of viscosity ... 

In start-up of steady simple shear, the measured stresses are divided by the imposed shear rate or its square to obtain the shear stress growth coefficient and the first normal stress growth coefficient, which are defined as follows ... 

At short times, when the deformation is small, we expect the shear stress growth coefficient to follow the prediction of the Boltzmann superposition principle. And if the shear rate is sufficiently low, the entire transient should be governed by this principle, and rf (t, y) becomes the same as r) t), which was shown as Eq. 4.8 ... 

As in the equation for evolution of fiber orientation, a closure approximation is needed in Eq. 3.102 to express A4 in terms of A. Using Eq. 3.102 it is straightforward to show that the shear stress growth coefficient, +, and the first normal stress growth function, N, are... 

Whereas the standard measurements provided no clue as to the differences in the samples, the nonlinear measurements provided some insight into the differences in the samples. In Figure 3.31 is shown the stress growth behavior of the three polymers. The shear stress growth curves of the three samples at the same shear rate are essentially the same. However, Niiy, f) for sample A apparently rises to a higher value than it does for either sample B or C. This is the first material property which indicated there was a difference in the three samples. 

Viscosity T), first normal-stress coefficient /i, second normal-stress coefficient /2 Shear stress growth coefficient T1+, first normal-stress growth coefficient t t[, second normal-stress growth coefficient /J Shear stress decay coefficient Tj", first normal-stress decay coefficient /f, second normal-stress decay coefficient /j Shear creep compliance J... 

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