Influence of temperature on viscoelastic–viscoplastic behavior of poly(lactic acid) under loading–unloading

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 50°C. 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. POLYM. ENG. SCI., 57:239–247, 2017.