Abstract
Polyvinyl chloride was oriented by hot drawing. The deformation was investigated under uniaxial tension at room temperature and at 50°C. The onset of yield was localised in deformation bands of which two kinds, referred to as type 1 and type 2, were observed. Type 2 bands formed before type 1 bands, but only type 1 bands developed into a running neck. The yield behaviour could be satisfactorily accounted for by a yield criterion based on the von Mises criterion, provided that a term representing an internal compressive stress in the molecular alignment direction was included. The internal stress was found to increase from zero with increasing prior extension ratio and birefringence, and was also equal to the true stress acting on the material during the initial hot drawing. The inclusion of a hydrostatic stress term in the criterion, the application to anisotropic materials of the Coulomb criterion and a critical strain form of the von Mises criterion are considered. Optical anisotropy changes are discussed in terms of an affine deformation model, which was found to apply for deformation at room temperature and 50°C, but not at 71°C and 90°C. The model has also been applied to the deformation, at 50°C, of oriented polyvinyl chloride, the molecular alignment of which was described by three different distribution functions. The agreement between theory and experiment was best for what is referred to as the Kuhn and Grun type 1 distribution. Stress whitening, which developed during hot stretching, is associated with changes in the room temperature deformation behaviour. There was a yield fracture transition at lambda[o] = 0° and a change in ductile fracture behavior at other values of lambda[o]. The ductile fracture direction in stress whitened material has been accounted for by a theory which proposed that the minimum energy was used in propagating the fracture. Two different kinds of type 1 bands were observed in oriented polyethylene. It is suggested that crystallinity may affect the position of the minimum yield stress.