Cleavage fracture of oriented poly(vinyl chloride)

Poly(vinyl chloride) (Cobex) sheet was oriented by hot drawing. Double cantilever beam cleavage specimens were then cut out parallel to the direction of molecular alignment. The fracture surface energy and fracture surface features were then investigated as a function of the birefringence, a measure of the degree of molecular alignment. Two different modes of crack propagation were observed. The first mode was by local plastic deformation at the crack tip coalescing voids present in the material and giving a dimpled fracture surface. This occurred up to a birefringence of 2.55 × 10−3 and was accompanied by a sharp fall in fracture surface energy with increasing birefringence. The second fracture mode was by smooth cleavage, this occurred above a birefringence of 2.13 × 10−3 and was accompanied by a slight decrease in fracture surface energy with increasing birefringence. There was a transition region at intermediate birefringence values where both modes of fracture were found.