ABSTRACT
In this chapter, we will analyze and summarize the results of the preceding investigations of statistical features of fracture of polymers.
8.1. Lifetime and Fracture Mechanism of Bulk (Low-Strength) and Thin (High-Strength) Polymer Films and Fibers
The physical nature of the scale factor of strength was analyzed in the preceding chapters of this monograph and in [45, 48, 402, 407 - 411]. As was found, fracture of bulk samples is different from that of thin samples (see Chapters 4 - 7), and this difference is related to the fact that, due to the technological conditions of their processing and scale factor, small-sized samples contain no rough defects. Furthermore, as was shown, the samples based on thin polymer films and fibers are grouped near certain values of strength or lifetime which are referred to as discrete strength or lifetime levels and are graphically presented as maxima in the distribution curves. Strength levels are formed under certain conditions. The first strength level corresponding to the lowest strength is associated with edge defects whereas all higher strength levels are related to surface and volume defects. They are unequivocally related to the defects of certain potency such as macrocracks, microcracks, and submicrocracks. Stability of strength levels is the most striking feature. With varying the testing temperature, scale of the sample or breaking stress, the position of strength levels remain almost unchanged; in this case, only re-distribution of the samples between strength levels is observed, and the value of mean strength is changed. In bulk samples (with thickness dQ > 50 ^m), the distribution curve shows one maximum corresponding to rough edge defects of the samples as strips. In this case, the position of the maximum in the distribution curve depends on temperature and stress and on the action of various external factors. Hence, the kinetics of fracture of thin films and fibers is different from the fracture kinetics of bulk samples (see Chapters 4-7).
