Stress growth behavior

Another similarity in the rheological response between CPNCs and LCPs is the stress growth behavior at startup [Metzner and Prilutski, 1986 Utracki, 2004]. For a constant applied shear rate, the shear stress, crn, goes through a maximum. Its magnitude depends on the shear history the longer the specimen is undisturbed, irest. the larger is the stress overshoot ... 

The interrupted shear test described in Figure 3 may be useful in identifying the time for the orientation to relax. In this test a LCP is sheared until steady state stresses are obtained. The flow is then stopped and the fluid allowed to rest for various lengths of time before flow is started up again. The stress growth behavior after various periods of relaxation is then compared with the initial response. Some representative data for the 60 mole % PHB/PET system is presented in Figure 17. After three minutes of rest time in the melt we see that the first peak is not recovered. However, the second peak is nearly fully recovered. In fact we have observed that the second peak recovers in about 6 seconds. 

As a test of this model, calculated results are compared with experimental data reported in the literature (2,6,7). Figure 1 gives the stress growth behavior of a monodisperse polystyrene solution (M = 1.8 x 10, 4% by weight concentration, solvent Aroclor, 25 C), while Figure 2 shows the corresponding stress relaxation behavior. 

In this chapter, we describe the steady-state and transient viscometric behavior of the blend solutions of HPC and EC in m-cresol, mainly, as a function of blend composition. As the transient behavior, the stress growth behavior was focused. Furthermore, in order to estimate the phase of the blend solutions, we observed the textures of blend solutions at rest and undergoing shear with a polarized microscope and determined the refractive indices with an Abbe refractometer. We also noted the miscibility of HPC and EC. 

Rheological behavior is divided into two sections one is steady-state behavior and the other is transient (stress growth) behavior. 

We have proposed the following equation for characterizing the stress growth behavior of the HPC liquid-crystalline solution [45] ... 

Fig. 9 Schematic diagram of stress growth behavior for our blend solution (a) monotonous increase, (b) stress overshoot, and (c) shouldering.

Fig. 10 Typical stress growth behavior of reduced stress versus time for the 20 wt% solution 50/50.

Fig. 11 Schematic representation of stress growth behavior for the liquid-crystalline solutions which exhibit the stress overshoot.

The dependence of steady-state viscometric behavior on blend composition greatly depended on the solution phase For the isotropic solution, the trends of behavior changed at the composition of 50/50 for the biphasic solution, the viscosity exhibited a maximum around 15/85 for the single-phase anisotropic solution, the behavior of the H PC-rich compositions was different from that of the EC-rich compositions. With respect to the stress growth behavior, there were two retardation times for the isotropic solution, and three or four retardation times for the anisotropic solutions. Those data showed that HPC and EC are immiscible. 

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. 

FIGURE 331 Stress growth behavior of three LDPE samples at 150 °C (—) A (—) B (—-) C. (Reprinted by permission of the publisher from Meissner, 1975.)... 

The utility of K or any elastic plastic fracture mechanics (EPFM) parameter to describe the mechanical driving force for crack growth is based on the ability of that parameter to characterize the stress-strain conditions at the crack tip in a maimer which accounts for a variety of crack lengths, component geometries and loading conditions. Equal values of K should correspond to equal crack tip stress-strain conditions and, consequently, to equivalent crack growth behavior. In such a case we have mechanical similitude. Mechanical similitude implies equivalent crack tip inelastic zones and equivalent elastic stress fields. Fracture mechanics is... 

An example of the growth behavior of crazes in a liquid environment is shown in Fig. 20 which is taken from the results of Williams and Marshall They measured the craze length versus loading time at different Kj-values in PMMA specimens immersed in methanol. The time-depettdent craze-behavior was interpreted in terms of a plasticisation mechanism incorporating the effect of the fluid Due to its porous nature the craze has a very high area to volume ratio so that penetration of the fluid by only a small distance leads to a complete plasticisation of the fibrils and a subsequent drop in the load carrying capacity (t of the fibrils the material effectively behaves as one with a lower craze stress oojc (a < 1). 

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