Strain hardening, from cyclic loading

As could be expected, the mechanical properties of a crazed polymer differ from those of the bulk polymer. A craze containing even 50% microcavities can still withstand loads because fibrils, which are oriented in the direction of the load, can bear stress. Some experiments with crazed polymers such as polycarbonate were carried out to get the stress-strain curves of the craze matter. To achieve this aim, the polymer samples were previously exposed to ethanol. The results are shown in Figure 14.24 where the cyclic stress-strain behavior of bulk polycarbonate is also illustrated (32). It can be seen that the modulus of the crazed polymer is similar to that of the bulk polymer, but yielding of the craze occurs at a relatively low stress and is followed by strain hardening. From the loading and unloading curves, larger hysteresis loops are obtained for the crazed polymer than for the bulk polymer. 

Figure 3 shows the stress-strain curves of the first specimen, all taken at 4.2 K. The first load cycle of the as-received specimen was stopped at 0.2% strain and about 13 MN/m. After warming to 77 K and holding for one day, the second load cycle showed an increased yield stress from the as-received value of 6 MN/m to about 13 MN/m. Due to work hardening during the following 1000 strain cyclics... 

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