Mechanisms for different failure modes in startup uniaxial extension: Tensile (rupture-like) failure and necking

摘要

This work reports four different modes of failure during startup uniaxial extension of entangled polymer melts: Capillary, tensile decohesion, shear-yielding-induced necking, and rupture. Emphasis is placed on the identification of the critical condition separating tensile failure from necking and the molecular mechanisms for each type of failures. When Weissenberg number Wi is not vanishingly small, a startup extension terminates in a rupture-like failure where nonuniform extension takes place in a sharply localized manner. This decohesion event reflects the tensile yielding of the entanglement network that occurs due to insufficient intermolecular gripping force to balance the growing intrachain elastic retraction force. At higher rates, the failure mode switches from the tensile decohesion to necking as the entangled melts experience a Cauchy stress level in excess of twice the elastic plateau modulus (2 G N 0). Since the minimum stress to produce shear yielding is on the order of G N 0, in these high-rate extension tests the melts have the option to undergo shear yielding. Birefringence and particle-tracking velocimetric observations were carried out to reveal the first evidence for shear deformation as a precursor to the nonuniform extension: The necessarily localized shear yielding initiates nonuniformity of the stretched specimen sometimes termed "necking."