Intrinsic brittleness of polymers

Before discussing the fracture resistances of a selection of polymers we note broadly that most, if not all, unoriented polymers are intrinsically brittle solids, in terms of the criterion of Kelly et al. (1967) whereby a defining condition is considered at an atomically sharp crack tip under mode I loading as in Fig. 12.2. As discussed in Section 12.2.2, all stresses are concentrated at the tip of such cracks as 

Consider the case of amorphous polypropylene as a generic polymer, the ideal plastic response and cavitation response of which have been simulated in considerable detail (Mott et al. 1993a, 1993b) under imposed pure shear deformation and under pure dilatation, respectively. As stated by those investigators, the otherwise ideal plastic shear resistance r and the ideal cavitation resistance a were determined to be 

The and values of the polymers are their back-extrapolated values at 0 K. and 

However, in the mode I field, around an atomically sharp crack the concentrations of the deviatoric shear stress and the mean normal stress a are given as special cases of eqs. (12.4) as 

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See also in sourсe #XX -- [ ]

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