Microscopic mechanisms and mechanics of craze growth and fracture

Abstract

Isolated air crazes have been produced in thin films of polystyrene (PS) bonded to copper grids by straining these in tension. The craze thickness profile, r(x), was directly determined from a series of transmission electron micrographs taken along the craze. Local values of the craze fibril volume fraction v ₁ and fibril extension ratio λ were established at frequent intervals along the craze by optical densitometry of the micrographs. The craze surface displacement profile w(x), craze surface stress profile S(x) and the true stress a _t in the craze fibrils are computed from these parameters. The λ(x) profile provides conclusive evidence that the craze increases in thickness as it grows in length by drawing new polymer from the craze surface into the fibrils rather than by creep of the existing fibrils. The S profile exhibits a modest maximum at the craze tip but falls slowly over a distance of about 15 μm behind the craze tip to a value about 10% below the applied tensile stress. The λ of the drawn craze fibrils is a function of the surface stress S at the time they are drawn. The fibrils drawing at high S just behind the growing craze tip result in formation of the midrib, a layer of high λ in the craze midway between the craze surfaces. The highest values of λ observed, in the midrib and the craze tip, are nearly those required to completely extend PS molecules between molecular entanglements in the glass; further extension must result in chain disentanglement and fibril rupture. While chain disentanglement in the fibrils of the midrib is the probable first step in fracture, propagation of a crack in an existing craze results in a large increase in S ahead of the crack, and two layers of very-high-λ craze fibrils draw from the craze surfaces in response. The crack now prefers to propagate in these layers, rather than in the midrib, and jumps from one to the other causing the familiar mackerel or patch pattern on PS fracture surfaces.