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Influence of temperature on viscoelastic–viscoplastic behavior of poly(lactic acid) under loading–unloading
(Wiley, 2017) Düşünceli, Necmi; Drozdov, Aleksey D.; Theilgaard, Naseem
Experimental data are reported on poly(lactic acid) (PLA) in tensile loading-unloading tests and relaxation tests under stretching and retraction at temperatures ranging from room temperature up to 508C. Two characteristic features of the time-dependent response of PLA are revealed: (i) with a decrease in minimum stress under retraction at a fixed temperature, relaxation curves change their shape from monotonically decaying with time (simple relaxation), to non-monotonic (mixed relaxation) to monotonically increasing (inverse relaxation) and (ii) with an increase in temperature, inverse relaxation after unloading down to the zero stress evolves into mixed relaxation with a pronounced shift of the peak position to smaller relaxation times. Constitutive equations are derived for the mechanical behavior of PLA, and adjustable parameters in the stress-strain relations are found by fitting the observations. Ability of the model to predict the time-dependent response under cyclic deformation is confirmed by numerical simulation. (C) 2016 Society of Plastics Engineers