Thermo-Mechanically Coupled Cyclic Deformation Of Shape Memory Polyurethane:Experiments And Constitutive Model

Shape memory polymers(SMPs)have attracted extensive attention as a new type of smart polymer materials.Among different SMPs,thermo-induced shape memory polymers(TSMPs)are the most widely studied and used.Since the shape memory effect of TSMPs involves the thermo-mechanical deformation process at the temperature range across the glassy transition temperature,the deformation behaviors in the glassy and rubbery states strongly depend on the temperature and loading rate,and also present a strong thermo-mechanically coupled effect.At present,there are still great challenges in the experiments and constitutive models of TSMPs,especially for the cyclic shape memory effect.Therefore,it is necessary to carry out the systematic experimental observation on the thermo-mechanically coupled cyclic deformation of TSMPs to reveal the influences of loading peak strain,loading rate and loading temperature on the deformation of glassy and rubbery states as well as the shape memory effect.Furthermore,a visco-hyperelastic-viscoplastic constitutive model is established to describe the thermo-mechanically coupled cyclic deformation of TSMPs.The following contents are included in this thesis:(1)The systematic experiments on the thermo-mechanically coupled cyclic deformation were carried out for shape memory polyurethane(TSMPU),a typical TSMPs.The uniaxial tensile behaviors of TSMPU at different loading rates under the conditions of natural and forced convections were observed in situ by using the non-contact digital image correlation(DIC)technology and infra-red(IR)thermometry.The coupling mechanism of strain localization and heat generation induced by the deformation was revealed.It is found that the competition between the hardening induced by the increasing loading rate and softening induced by the increasing temperature can change the strain localization mode.Then,the uniaxial tensile experiments of TSMPU at different temperatures and loading rates were performed,and the temperature-and loading rate-dependent mechanical behaviors of TSMPU were obtained.The experiments of shape memory behavior were carried out at different loading rates and with various loading peak strains,and the features of internal stress,shape fixity rate and irrecoverable strain were obtained in the process of shape memory.The results show that the viscosity at high temperature and internal stress after freezing are important factors affecting the shape memory effect of TSMPU.(2)Based on the finite deformation framework,the deformation gradient could be multiplicatively decomposed into thermal,elastic and viscoplastic parts.It was assumed that the internal energy of TSMPU could be decomposed into the energy caused by the deformation of molecular chains and the one caused by the slipping and friction of molecular chains.A competition mechanism between the hardening induced by the increasing loading rate and softening induced by the increasing temperature was introduced into the viscoplasticity flow rule,the logarithmic rate based constitutive equations and heat balance equations were derived,and then a thermo-mechanically coupled viscoplastic constitutive model of glassy TSMPU was established.Finally,the strain localization and heat generation in the tensile process were reproduced.(3)Based on the phase transition theory,it was assumed that the rubbery and glassy phases of TSMPU are in parallel.A three-element visco-hyperelastic model was adopted to describe the visco-hyperelastic behavior of rubbery phase at high temperature.The thermomechanically coupled viscoplastic constitutive model of TSMPU in(2)was used to describe the viscoplastic behavior of glassy phase at low temperature.The temperature hysteresis effect during the cooling and heating was considered by introducing a temperature hysteresis factor into the volume fraction of rubbery phase.The mechanism of strain storage and release during the cooling and heating was considered by introducing a storage rate related to the shape fixity rate.Then,a visco-hyperelastic-viscoplastic constitutive model of TSMPU was established.Moreover,a thermo-mechanically coupled cyclic constitutive model was established by introducing the influence of multi-relaxation mechanism on the cyclic shape memory behavior.Comparisons between the experimental and simulated results show that the proposed model can simultaneously describe the mechanical responses at different loading rates and temperatures,and cyclic shape memory behaviors at different loading rates and peak strains.