Investigation of failure during elongational flow of polymer melts

Abstract

A basic study of the mechanisms of necking and ductile failure of polymer melts in uniaxial elongational flow has been carried out. A linear stability analysis was carried out using a White—Metzner convected Maxwell model with a deformation-rate-dependent relaxation time, which varies according to τ = τ o/(1 + aτ o[2tr d 2] 1 2 ). It was shown that filament stability and elongation to break depend upon τ o E, where E is the elongation rate, and a. At fixed τ o E, filament stability decreases with increasing a. At small a, stability increases with increasing τ o E while for a > 1 √3 , stability decreases with increasing τ o E. For a material with small a, ductile failure can occur for small τ o E, but cohesive fracture should be the cause of failure at larger τ o E. For a material with large a, however, ductile failure always dominates the failure mode. These results are used to interpret failure in elongational flow of low density and high density polyethylene and polypropylene melts and describe how the latter two melts exhibit ductile failure.