Constitutive model of adaptive links in viscoelastoplasticity

A new constitutive model for the nonlinear behavior of non-crosslinked polymers with infinitesimal strains is derived. Newly developed approach generalizes a model of adaptive links for the viscoelastic behavior of crosslinked polymers. According to it, a viscoelastic medium is treated as a set of elastic springs which replace each other. The springs model chemical links between polymer molecules, which arise and collapse due to the micro-Brownian motion, a law for replacing the springs determines the stress relaxation in a viscoelastic medium. Unlike crosslinked polymers, non-crosslinked polymers demonstrate steady-state creep flow after some transition period. In order to describe both the transition process and the steady creep, the model distinguishes two different types of adaptive links. Links of the first type are ruptured under loading, whereas links of the second type replace one another. Both these processes (destruction and replacement) are treated as kinetic, and equations of chemical kinetics are introduced for their description. The nonlinearity of the model arises due to a dependence of rates of kinetic processes on the stress intensity. The derived constitutive equations are verified by comparing theoretical results with the experimental data for polypropylene fibers. For this verification we use data presented by two independent references. The results demonstrate excellent agreement between experimental observations and our theoretical predictions.