Semicrystalline thermoplastics, crazing fracture

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. 

Shear yielding in a form of homogeneous bulk process can contribute substantially to the crack resistance of a polymeric solid. On the other hand, however, localized shear yielding in the form of shear microbands is believed to be a precursor of brittle fracture in many semicrystalline thermoplastics. Localized shear yielding is thus a major contributor to the initiation of cracks along with crazing, which, however, is believed not to operate in semicrystalline thermoplastics. 

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