A multiprocess Eyring model is developed with a particular aim of predicting the localized instability occurring in “necking” polymers when cold-drawn. Differences from using single and multiple Eyring processes are examined using a published data-set for polypropylene test pieces; it is shown that a four Eyring process model can simultaneously fit both necking stretch ratio and draw force data for uniaxial stretching, whereas with a single process only one measurement could be fitted accurately. The multi process Eyring model is shown to give significantly more accurate predictions than a necking hyperelastic model. The multiprocess model is assessed against the same material undergoing a complex constant-width elongation. It is shown that agreement is quantitatively good for both drawing force and surface deformation, with some minor differences in transverse force and surface stretch. A pronounced intermittent stretching pattern that is seen on the experimental test piece is replicated by the multiprocess Eyring simulation, but is absent using the hyperelastic model. A method is described to deform a photograph of the original specimen according to a finite element solution. The method is shown to give a clear indication of the accuracy of the model in predicting final form. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011
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