Comparison of the craze-growth model with experiments

9 Comparison of the craze-growth model with experiments 

A cursory examination of the experimental results for the craze-growth rate and their stress dependence presented in Fig. 11.15 compared with model predictions obtainable from eqs. (11.50), (11.51), and (11.52) using the specifie material parameters of Table 11.2 shows that the model significantly overpredicts the craze-growth rates and their stress dependences both for PS and for PMMA. This suggests that some basic parameters of the model are very different from those present in the experimental behavior. To understand the causes of this differenee the general form of the model expression for the craze-growth rate vc = dc/dt is stated simply as 

Tabie 11.2 Material parameters for commercial PS and PMMA, for evaluation of the craze-growth rate at 7 = 293 K 

The values of (To/T e and were adopted to parallel the experimental results in Fig. 11.15. The values of and were determined from material information in Tables 8.2, 8.3, 

The magnitudes of Av listed in Table 8.3 for PS and PMMA are those for plastic relaxations occurring in fully constrained bulk material. On the other hand, the bases of the fibrils where active drawing occurs in narrow flow zones are near free surfaces of the fibrils and are only weakly constrained. This suggests that the energy barriers to plastic relaxations at these sites can be considerably lower than 

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