  Recent step strain experiments in well-entangled polymeric liquids
demonstrated a bulk fracture-like phenomenon. We have studied this instability
using a modern version of the Doi-Edwards theory for entangled polymers, and we
find close quantitative agreement with the experiments. The phenomenon occurs
because the viscoelastic liquid is sheared into a rubbery state that possesses
an elastic constitutive instability (Marrucci and Grizzuti, 1983). The fracture
is a transient manifestation of this instability, which relies on the
amplification of spatially inhomogeneous fluctuations. This mechanism differs
from fracture in glassy materials and dense suspensions.
