NONLINEAR RESPONSE OF A LONG, DISCONTINUOUS FIBER MELT SYSTEM IN ELONGATIONAL FLOWS/

The authors investigated the transient elongational behavior of a high ly-aligned 60% volume fraction long, discontinuous fiber filled poly-e ther-ketone-ketone melt with a computer-controlled extensional rheomet er at 370 degrees C. Prior experiments at controlled strain rate and s tress produced tau(E)(+) (t,epsilon over dot) and epsilon over dot(-)( t, tau(E)) Similar to a shear dominated flow of a non-linear viscoelas tic fluid. Stress relaxation following steady extension showed nonline ar effects in the change in stress decay rate with increasing strain r ate. Continuous relaxation spectra showed a shift in the spectral peak to smaller values of lambda with increasing strain rate. The Giesekus nonlinear constitutive relation modeled the elongation and stress rel axation with shearing rate at the fiber surface set by a strain rate m agnification factor. Suitable for elongation, the model produced insuf ficient shift in the stress relaxation spectrum to account for the lar ge change in stress decay rate exhibited in the experiments.