Creep behavior asymmetric

FIGURE 20,5-1 Demonstration of typical permeability creep behavior observed for asymmetric membranes. Both cellulose acetate and polysulfone display this behavior, which is believed to be related to densification of the dense separating layer or compaction of the support foam structure shown in Fig. 20.1-6. ... 

Experimental creep data for ceramics have been obtained using mainly flexural or uniaxial compression loading modes. Both approaches can present some important difficulties in the interpretation of the data. For example, in uniaxial compression it is very difficult to perform a test without the presence of friction between the sample and the loading rams. This effect causes specimens to barrel and leads to the presence of a non-uniform stress field. As mentioned in Section 4.3, the bend test is statically indeterminate. Thus, the actual stress distribution depends on the (unknown) deformation behavior of the material. Some experimental approaches have been suggested for dealing with this problem. Unfortunately, the situation can become even more intractable if asymmetric creep occurs. This effect will lead to a shift in the neutral axis during deformation. It is now recommended that creep data be obtained in uniaxial tension and more workers are taking this approach. 

Plots of the product permeability versus time on a log-log coordinate system are often linear over relatively long time periods, as shown in Fig. 20.5-2. Similar behavior is observed in asymmetric reverse osmosis membranes. The log-log plotting approach provides a simple and reasonably satisfactory means of predicting the performance change of fibers under long-term operation by exuapolation of short-term data. Mechanical creep and volume recovery in glassy polymers aftw an initi perturbation also ate known to be reasonably represented on such log-log plots. ... 

FIG. 18-4. Asymmetric, nonlinear volume creep and creep recovery, calculated from equation 5, with parameters given in text, plotted as (o — o,)/(d2 — /) (o. ) AP = 5 X 10 dynes/cm (6. b ) AP = 1.2 X 10 (c. c ) A/ sufTiciently small so that equation 5 reduces to equation 3 (linear behavior with single retardation time). 

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