Delayed yielding of polycarbonate under constant load

The time‐dependent yielding of glassy polycarbonate subjected to constant tensile loads has been studied. Application of a constant stress of a magnitude between the yield stress and the stress required for propagation of a neck in constant strain tests results in inhomogeneous yielding after a well‐defined time lag. This delay time increases with decreasing stress and temperature. The critical stress for slowly cooled material is greater than that for quenched material in which the delay time is divided in two regions. The delay time is regarded as the time required for the initiation of inhomogeneous yielding at either edge of the specimen and growth over a certain distance across the specimen. Geometrical observations revealed that the inhomogeneous yielding is shear yielding which is initiated due to stress inhomogeneities caused by mechanical imperfections at the edge of the specimen. The Eyring treatment of delayed yielding can describe fairly well the stress and temperature dependence of the delay time.