Criterion for neck-propagation stability in polymers

The conditions for the occurrence of instability during neck propagation in polymers are theoretically studied. The mechanism behind this phenomenon is concerned with the thermomechanical instability of neck propagation, which is provided by the transformation of a sample’s stored energy of elastic deformation into heat during tensile drawing of the plastic polymer. An analytical criterion for the occurrence of instability is formulated. The critical length of the sample, below which self-oscillations are not excited, is inversely proportional to strain rate and directly proportional to the coefficient of thermal conductivity and the elastic modulus of the material. Two principal distinctions between thermomechanical and mechanical instabilities are revealed. The first distinction is the existence of elastic energy (length), below which oscillations do not occur; the second is the fact that the interval of thermomechanical instability is wider than that of mechanical instability.