Experimental investigation of non-Newtonian behavior of thermoplastic composites during forming process
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The transient elongational behavior of a highly-aligned 60% volume fraction long-discontinuous-fiber filled poly-ether-ketone-ketone melt was investigated with a computer controlled extensional rheometer at 370°C. Prior experiments at constant strain rate and constant stress produced transient elongational stress and creep response similar to a shear dominated flow of a nonlinear viscoelastic fluid. Increasing strain rate resulted in an increasing peak stress and in an increasing ratio between peak stress and plateau stress. In the present work, interrupted flow experiments show peak stresses under constant strain rate were a function of the stress decay period. Stress decay less than approximately 70 percent of the steady stress level greatly reduces the peak stress. The Giesekus nonlinear constitutive equation was fit to the steady elongation and then used to predict the response after an interrupted flow. Although suitable for elongation, a single parameter set Giesekus model could not account for the more complex relaxation in the filled fluid. Allowing the model to relax to the same stress level-rather than for the same rest time-produced reasonable prediction of system behavior after resumption of flow.
author list (cited authors)
Creasy, T. S., & Advani, S. G.