The effect of pre-orientation on the fracture properties of glassy polymers

The paper is concerned with the effect of prior orientation on the fracture properties of perspex and polystyrene in directions both parallel and perpendicular to the axis of orientation. Different degrees of orientation, as measured by birefringence, were obtained by varying the temperature of drawing. A cleavage method was used to produce controlled crack propagation parallel to the direction of orientation. Application of the Griffith theory to the data obtained by this method enabled the fracture surface energy to be derived. This quantity showed a large decrease with increasing orientation. Satisfactory cleavage test data for the direction transverse to the orientation could not be obtained owing to the overriding tendency to crack development or splitting in the parallel direction. In highly oriented samples this tendency was so marked that specimens subjected to tensile stress in the axial direction failed by splitting or fibrillation rather than by transverse fracture. A comparable effect was observed in transverse impact tests, though in this case fracture occurred at an angle to the axis of orientation, producing a characteristic `saw-tooth' effect.