Theory of brittle fracture of polymers in surface active media

Abstract

Surface-active media, in contrast to chemically active, reduce the free surface energy of solids while not causing irreversible changes in molecular structure in them. In the failure of specimens of hard glasslike polymers surface adsorption interaction of the medium with the polymers occurs [2]. Molecules of the surface-active medium, penetrating by surface diffusion into the crack tip, decrease the free surface energy of the material and thereby promote acceleration of crack growth. The mechanism of fracture remains, as before, brittle, which is indicated by photomicrographs of the fracture crack growth [2] and also the relationships of the failure stress to strain of the specimen [2, 3]. In brittle fracture of a polymer separation of the specimen into pieces occurs as the result of propagation in it of the most dangerous microcrack [4]. The material fails in the small vicinity of the crack tip (fluctuation volume on the order of i0-28-i0"29 m 3) in which the local stresses o* activating rupture of the bonds significantly exceed the stresses in the remaining volume of the specimen [5, 6]. The crack grows as the result of fluctuation advance of portions of the crack front with a length of l~ by a distance of ~. The crack growth rate determined by the frequencies of rupture and restoration of the bonds at its tip may be represented as [7]