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    Fading memory of deformation history in carbon black-filled thermoplastic elastomers
    (Elsevier, 2017) Düşünceli, Necmi; Drozdov, Aleksey D.
    Observations are reported on a carbon black reinforced thermoplastic elastomer in multistep uniaxial tensile cyclic tests with a mixed deformation program (oscillations between maximum elongation ratios k(max) and various minimum stresses Amin with km, monotonically increasing with number of cycles n). Fading memory of deformation history is demonstrated: when specimens are subjected to two loading programs that differ along the first n 1 cycles of deformation and coincide afterwards, their stress strain diagrams become identical starting from the nth cycle. A constitutive model is developed in cyclic viscoplasticity with finite deformations, and its adjustable parameters are found by fitting the observations. Ability of the stress strain relations to describe the fading memory phenomenon and to predict the mechanical response of polymer composites in multi-step cyclic tests with large strains is confirmed by numerical simulation. (C) 2016 Elsevier Ltd. All rights reserved.
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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