Yield behavior crazing

Double-grooved specimens were used to study the failure of PC, PC/PE, PET, ABS, and HIPS during transitions from plane stress to plane strain. The yield behavior of PC is consistent with a von Mises-type yield criterion plane strain reduces its elongation. The yield behavior of PC/PE is consistent with a Tresca-type yield criterion plane strain appears to be relieved by voiding around the PE particles. PET undergoes a ductile-to-brittle transition its behavior is consistent with a von Mises-type yield locus intersected by a craze locus. The yield behavior of ABS and HIPS is not significantly affected by the plane-stress-to-plane-strain transition. Plane strain alone does not necessarily cause brittleness. 

In the discussion of the shear yielding and crazing behavior of PC (in Sect. 3.2 and 4.1), the existence of characteristic extension ratios has become apparent (1) The extension ratio, 7. , after shear yielding referred to as natural draw ratio, (2) the extension ratio, V, of craze I fibrils and (3) the extension ratio, at craze II initiation. 

Molecular Criterion for Craze/Yield Behavior from Chain Structure Parameters... 

According to the Eq. 11.7 both and Coo provide a consistent prediction of the deformation behavior. It seems, however, that the entanglement density Vg can be considered as the primary parameter which controls the crazing behavior, whereas the chain stiffness parameter C o is predominant in controlling the shear yielding behavior. 

On the other hand, PP shows shear yielding without craze formation and yielding behavior (i.e. necking) with a plateau in a plane stress state. The stress for yielding is also affected by the density of tie molecules because the tie molecules are pulled out from the lamellar fragment during the yielding process [4]. 

Figure 12.21 The strain rate dependence of the octahedral shear stress r at atmospheric pressure using data from torsion (o), tension (A), and compression. (Reproduced with permission from Duckett, R.A., Coswami, B.C., Smith, LS.A. et al. (1978) Yielding and crazing behavior of polycarbonate In torsion under superposed hydrostatic-pressure. Brit. Polym. J., 10, 11. Copyright (1978) Society of Chemical Industry.)...

Duckett, R.A., Goswami, B.C., Smith, L.S.A. et al. (1978) Yielding and crazing behavior of polycarbonate in torsion under superposed hydrostatic-pressure. Brit. Polym. 10, 11. 

Modified Von Mises criterion provides reasonable predictions in regard to the macroscopic yield behavior of isotropic PP homopolymer, copolymers and blends despite the intrinsic nature of PP to deformed by craze under positive hydrostatic stresses. 

Amorphous polymers exhibit two mechanisms of localized plasticity crazing and shear yielding. These are generally thought of separately, with crazing corresponding to a brittle response while shear yielding is associated with ductile behavior and the development of noticeable plastic deformation prior... 

PET. The behavior of crystalline PET at plane strain can be explained if its yield locus is similar to that of PS and PMMA (9, 10) where a craze locus intercepts the shear yield locus. The transition at plane strain to a craze locus would account for the brittleness. This transition, which takes place quite sharply at W/t = 23 (W/b = 8), is probably the cause for the low impact strength (< 1 ft-lb/inch) of the Vs-inch thick notched bars. The plane strain brittleness can be avoided if the geometric constraints can be removed, such as making the notch less sharp or making the test bar thinner. In fact, unnotched bars of PET, equivalent to having an infinite notch radius, are quite tough. The notch sensitivity of PET is typical of crystalline polymers. 

Failure of unoriented SAN copolymers is dominated by crazing behavior. The total energy absorbed by the polymer during failure has been increased by optimizing both failure modes. Yielding can be enhanced by orienting the polymer [105], and crazing can be optimized by rubber modification. 

Tg—TJ and change with the size of the molecular segment involved in yield a single value corresponding to the Y of polycarbonate at 300 K is indicated on Fig. 20. For polymers with Ve above about 5 x 10 m" shear yield should oceur in preference to crazing. Because of individual variations in Y from polymer to polymer, in isolated instances crazing may be favored over shear even above this value this may be the reason for the anomalous behavior of PMMA. 

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