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FRACTURE OF POLYMERIC GLASSES

1972; Elsevier BV; Linguagem: Inglês

10.1016/b978-0-12-449707-8.50006-3

J. P. Berry,

Polymer crystallization and properties

Glassy polymers may fail in a brittle or a ductile manner depending on the experimental conditions of temperature and time scale. Although these extremes of behavior and the transition between them can be considered in terms of the Ludwik hypothesis, it is necessary to determine the mechanism of the failure process to elucidate the factors which govern the behavior displayed under any particular set of conditions. Application of the Griffith flaw theory to brittle fracture indicates that the fracture-surface energy and the inherent-flaw size are significant material parameters. The influence of changing experimental conditions and materials on the values of these parameters indicates that they are interdependent. A major contribution to the first arises from the energy required for the formation of a layer of modified structure at the fracture plane, while the second is related to the crazes that develop in these materials when they are stressed. Detailed examination of the structure of crazes reveals that they are planar regions formed by a hydrostatic tension and consist of oriented material containing about 50% by volume interconnecting voids. The fracture-surface layer is believed to possess a similar structure, and the process of brittle fracture in these materials involves the formation and rupture of craze material. Time-dependent effects are also important, and a large amount of experimental data can be systematized by a phenomenological theory which is of the same form as those obtained from molecular considerations. Unfortunately, these theories have tended to ignore structure effects, just as the structure theories have largely ignored time effects, since they are not readily accommodated within the Griffith approach. Consequently, there is not yet a completely satisfactory comprehensive theory of polymer fracture, and it is suggested that the elucidation of the structural factors is a necessary step in the formulation of such a theory.