Amorphous polymer plastic deformation yielding behavior

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... 

It is well known that the mechanical behavior of glassy amorphous polymers is strongly influenced by hydrostatic pressure. A pronounced change is that polymers, which fracture in a brittle manner, can be made to yield by the application of hydrostatic pressure Additional experimental evidence for the role of a dilatational stress component in crazing in semicrystalline thermoplastics is obtainai by the tests in which hydrostatic pressure suppresses craze nucleation as a result, above a certain critical hydrostatic pressure the material can be plastically deformed. 

As with all other forms of plastic deformation in amorphous solids, we expect that in glassy polymers too the plastic deformation will consist of a series of discrete thermally assisted unit relaxation events on the atomic scale. Features of such discrete behavior have, e.g., been observed in deformation calorimetry by Oleinik (1991), who detected discrete unit inelastic events already in the pre-yield region in some glassy polymers. 

It was shown that for most crystalline polymers, including polypropylene and other polyolefins, the tensile drawing proceeds at a much lower stress than kinematically similar channel die compression [10,17]. Lower stress in tension was always associated with cavitation of the material. Usually a cavitating polymer is characterized by larger and more perfect lamellar crystals and cavities are formed in the amorphous phase before plastic yielding of crystals. If the lamellar crystals are thin and defected then the critical shear stress for crystal plastic deformation is resolved at a stress lower than the stress needed for cavitation. Then voiding is not activated. An example of such behavior is low density polyethylene [10]. 

The deformation behavior of amorphous polymers has been studied extensively, partly because the structure is rather simple as compared with semicrystalline polymers thus, the relationship between structure and properties can be established with relative ease. It is well known that two major micromechanisms are involved in the deformation and subsequent fracture of glassy polymers [1,2,13] (see Figs. 18.1 and 18.2). These are crazing and shear yielding, and both involve localized plastic deformation and some energy is dissipated during the deformation. In a craze, polymer chains are stretched along the stress direction and... 

Ductile deformation requires an adequate flexibility of polymer chain segments in order to ensure plastic flow on the molecular level. It has been long known that macromoleculai- chain mobility is a crucial factor decisive for either brittle or ductile behavior of a polymer [93-95]. An increase in the yield stress of a polymer with a decrease of the temperature is caused by the decrease of macromoleculai chain mobility, and vice versa the yield stress can serve as a qualitative measure of macromolecular chain mobility. It was shown that the temperature and strain rate dependencies of the yield stress are described in terms of relaxation processes, similarly as in linear viscoelasticity. Also, the kinetic elements taking pai-t in yielding and in viscoelastic response of a polymer are similar segments of chains, part of crystallites, fragments of amorphous phase. However, in crystalline polymei-s above their glass transition temperature the yield stress is determined by the yield stress required for crystal deformation... 

More recently there has been a strong interest in the deformation and fracture behavior of plastics under large hydrostatic pressures [31—32]. One should expect — and one observes — that the rigidity of a polymer increases with pressure. Sauer et al. [32] report that a pressure of 3.5 kbar raises the initial Young s moduli of amorphous thermoplastics (PC, PI, PSU, PVC, CA) by a factor of 1.2 to 1.9, that of crystalline polymers by 1.4 (POM) to 7.5 (PUR). Despite the increased rigidity, ductile fracture occurs. The effects are not yet understood in all generality. Following the two major review articles on this subject by Radcliffe [31] and Sauer and Pae [32] the Coulomb criterion corresponds best to most pressure-yield experiments. 

---