Cold Drawing of Glassy Polystyrene under Dead Load

Abstract

The deformation of glassy polystyrene under a fixed tensile dead load can take place in three different modes: (a) drawing by formation and propagation of a neck, (b) drawing by proliferation of deformation bands without necking, and (c) homogeneous creep. Which mode is observed depends primarily on stress level and temperature. In homogeneous creep, deformation bands are not found; however, they are always observed in connection with the two types of drawing. At any given temperature a boundary stress is observed above which drawing takes place and below which only creep is observed. This critical stress decreases with increasing temperature. It is believed that this is the stress necessary either for the formation or propagation of deformation bands. It seems likely that drawing by shear band proliferation is the ideal mode which would be obtained in a perfectly homogeneous and uniform sample; however, drawing usually goes over into the necking mode because of nonuniformities in specimen geometry or uneven temperature. Neck initiation is delayed for a certain time interval after application of the dead load. The logarithm of the delay time is found to be a linear function of stress and, also, an approximately linear function of temperature.