  It has long been known for elastomers that the velocity of crack propagation
jumps as a function of strain. On the other hand, such a jump has not been
reported in the literature for polymers which do not exhibit a rubbery plateau
in the storage-modulus plot. Here, we report observation of jumps in crack
propagation for semi-crystalline polymer sheets without the rubbery plateau, as
a result of pulling the sheets at a constant speed. We discuss the advantages
of this crack-propagation test under constant-speed stretching and provide
physical interpretation of the velocity jump observed for non-elastomer sheets
on the basis of a recently proposed theory for the velocity jump in crack
propagation.
